Citation: Sen, H.S. and Ambast, S.K. (2009). Improving drainage and irrigation through OFR in rainfed rice lowlands of Sundarbans delta. Paper presented in Seminar on "Drainage System in South Bengal: Its Problems and Management", held at Geology Department, Calcutta University, 10-11 October, 2009.
Improving Drainage and Irrigation through OFR in Rainfed Rice Lowlands of
Sundarbans Delta
H.S. Sen1 and S.K. Ambast2
1Ex-Director, Central Research Institute for Jute and Allied Fibres (ICAR), Barrackpore, WB, PIN 700 120, and 2Central Agricultural Research Institute, Port Blair, A&N Islands, PIN
(Email: hssen.india@gmail.com,hssen2000@hotmail.com; Tele: 9874189762)
Abstract
In regions depending entirely on rainfed agricultural operations, rainwater harvesting and its recycling is the most effective means of increasing food production. About a third of the world's rice cultivated land is under rainfed conditions, which is both drought and submergence prone. The Sundarbans, one of the largest deltas in the world, shared by India and Bangladesh is one such region. The region is criss crossed by innumerable tidal rivers and creeks forming 54 islands. The region is predominantly mono cropped with more than 80% area under kharif (June-November) rice cultivation and about 62% of the area is lowlying. The region is generally flat with some undulations having an average ground level 1.2 m above mean sea level, whereas the average high tide level is 2.75 m above mean sea level. The area is surrounded by protective earthen embankments provided with one way sluice gates for drainage of excess inland water and to prevent ingress of tidal water. The field level drainage network is inadequate in the region. In the years of high rainfall, it gets severely waterlogged or occasionally flooded during monsoon period due to impeded natural drainage. On the other hand in some years, due to saline ground water and lack of assured means of water supply, kharif crop suffers from prolonged drought spells in the later period of crop growth. This compels the farmer's to resort to more certain local low yielding (1-1.4 tonne/ha) tall varieties. The practice of rainwater storage in an on-farm reservoir (OFR) is quite common but its design is not based on scientific principles. Also, there is no information available on the extent of surface drainage improvement due to rainwater storage in an OFR. The use of stored water for cultivation of second crop during rabi (December-April) is not judicious. This often results in crop failure or low net returns during rainfall deficit years.
Simulation studies using soil water balance approach for the purpose of land and water resource management have been attempted by several researchers. A soil water balance model for rainfed rice lowland is used for estimating excess rainwater to design an on farm reservoir (OFR) and to assess surface drainage improvement due to storage in OFR. Weekly rainfall at 2 and 5 years return periods are used to optimize the size of OFR and to simulate surface drainage improvement, respectively. It is recommended to convert 20% of the farm area into OFR to harvest excess rainwater. It is also estimated that rainwater storage in OFR (in 20% of the farm area) reduces surface waterlogging to the extent of 75% and thus, the cultivation of high yielding dwarf rice varieties in lowlying areas can be made. Further, a simple linear programming model is used to propose optimal land allocation for rabi (winter) crop cultivation to increase the agricultural profit under various limitations of land and water.
Key words: Rainfed rice lowland; On farm reservoir; Surface drainage; Optimal land allocation
1. Introduction
In regions depending entirely on rainfed agricultural operations, rainwater harvesting and its recycling is the most effective means of increasing food production. About a third of the world's rice cultivated land is under rainfed conditions. In Asia, about 8% of the rice area is under rainfed humid condition which is both drought and submergence prone (Mackill, 1986). The Sundarbans, one of the largest deltas in the world, shared by India and Bangladesh is one such region. In India, it is located between 21o32' to 22o40' north latitude and 88o05' to 89o00' east longitude in the state of West Bengal. The region is criss crossed by innumerable tidal rivers and creeks forming 54 islands (Fig. 1).
The region is predominantly mono cropped with more than 80% area under kharif (June-November) rice cultivation and about 62% of the area is lowlying (Anonymous, 1988). The region is generally flat with some
undulations having an average ground level 1.2 m above mean sea level, whereas the average high tide level is 2.75 m above mean sea level. The area is surrounded by protective earthen embankments provided with one way sluice gates for drainage of excess inland water and to prevent ingress of tidal water. The field level drainage network is inadequate in the region. In the years of high rainfall, it gets severely waterlogged or occasionally flooded during monsoon period due to impeded natural drainage. On the other hand in some years, due to saline ground water and lack of assured means of water supply, kharif crop suffers from prolonged drought spells in the later period of crop growth. This compels the farmer's to resort to more certain local low yielding (1-1.4 tonne/ha) tall varieties. The practice of rainwater storage in an on-farm reservoir (OFR) is quite common but its design is not based on scientific principles. Also, there is no information available on the extent of surface drainage improvement due to rainwater storage in an OFR. The use of stored water for cultivation of second crop during rabi (December-April) is not judicious. This often results in crop failure or low net returns during rainfall deficit years.
Simulation studies using soil water balance approach for the purpose of land and water resource management have been attempted by several researchers (Harikrisna, 1981; Kemachandra and Murty, 1992). Jensen et al., (1993) reported the supplemental irrigation requirement using soil water balance technique and implications for small-scale water harvesting for rice lands in Bangladesh. In this study, a soil water balance model to optimize the size of an OFR on the basis of excess rainwater availability in rainfed rice lowlands is developed. Simulation study is made to evaluate reduction in weekly surface waterlogging by providing the optimum size of an OFR in a unit farm area. The need of supplemental irrigation during kharif season is also assessed. In order to improve the irrigation intensity through judicious use of stored water, an optimal land allocation for growing second crop under various constraints is proposed using linear programming model.
2. Materials and Methods
2.1 Analysis of rainfall and evaporation data
Historical daily rainfall (1963 92) and evaporation (1967-92) data for Canning, which is a representative block for Sundarbans delta, are collected and analyzed. The average monthly rainfall and evaporation are shown in Fig.2. The region receives an average annual rainfall of 1768 mm
ranging from 1030 mm to 2462 mm with a coefficient of variation of 21.2% (Fig.3). Of that, 82% rain occurs during monsoon season (June-October). Weekly rainfall and evaporation data are analyzed for different probability distributions, i.e., Normal, log Normal, Extreme value I and Log Pearson type III (Chow, 1964). The Extreme value I distribution is found suitable for the observed rainfall data, whereas Log Pearson type III is found closely fitting to the evaporation data, and therefore, used as input
parameter for further estimation. The weekly rainfall at 2 (50% probability) and 5 years (20% probability) return periods are given in Fig.4. It is estimated that on an average about five week
drought (no water stands on surface) for rice crop may occur during the season and about three week continuous
drought may be expected during the ripening stage compared to none during both vegetative and reproductive stages (Ambast et al., 1998).
2.2 Model formulation for optimizing OFR
A soil water balance model based on physical parameters is used to estimate the excess rainwater availability in lowland rice paddies. The schematic diagram for a typical rainfed rice lowland system is shown in Fig.5.
The soil water balance equation to estimate the depth of standing water in rainfed rice lowlands is expressed as follows:
SDWi = SDWi 1 + Ri – ETi DPLi + SIi (1)
where, i is the time index, week; SDW the depth of standing water at surface, mm; R the rainfall, mm; ET the crop evapotranspiration, mm; DPL the deep percolation loss, mm; SI the depth of supplemental irrigation, mm.
The following conditions are used to estimate the amount of excess rainwater in rainfed rice lowlands:
At the beginning of the computation, SDWi 1 is set to zero.
When SDWmin < SDWi < SDWmax, SDWi becomes actual water depth for the period.
When SDWi > SDWmax, the amount SDWi SDWmax is diverted to the OFR and SDWi is set to SDWmax.
When SDWi < SDWmin, the SIi (50 mm), if available in OFR is provided.
- When VOL > VOFR, the SDWi becomes actual water depth up to the height of field dyke (700 mm) and any excess rainwater beyond SDWi spills to drainage system.
SDWmin is the minimum depth of standing water for optimal growth of rice, mm; SDWmax the maximum depth of standing water for rice crop, mm; VOL the volume of rainwater storage in OFR, m3 and VOFR the volume of OFR, m3.
In Sundarbans region, the onset of monsoon occurs at 24th standard meteorological week (SMW) (Ambast and Sen, 1994a). It is assumed that the soil cracks may be filled during pre-monsoon rains. The weekly rainfall values at 2 and 5 years return periods are taken as R from 24th week onward to design OFR and to assess surface drainage improvement, respectively. As water availability is fairly well during kharif season weekly ET values are estimated using the pan evaporation method as proposed by Doorenbos and Pruitt (1977).
ETi = Kci Kp Evpi (2)
Where Kc is the dimensionless crop coefficient value for rice (Table 1); Kp the dimensionless pan coefficient value (0.8 valid for humid climate with moderate wind velocity) and Evp the open pan evaporation, mm.
The DPL beyond the rootzone occurs when soil moisture exceeds field capacity. The equivalent depth of soil moisture at field capacity is estimated by taking rootzone depth for rice and percentage moisture content at field capacity for silty clay loam soil (Michael, 1978). A maximum of 14 mm weekly DPL (subject to moisture availability) is taken on the basis of daily DPL (2 mm/day) measured in the farmer's field under puddled rice condition by ponded basin method (Rao and Dhruvanarayana, 1979). A simple book keeping approach is used to estimate the soil moisture storage in the rootzone. As rice requires an optimal submergence of 50±20 mm for potential production (Biswas et al., 1982), the SDWmax and SDWmin are taken as 100 mm and 0 mm (moisture at saturation), respectively. SI is provided only when water equivalent to a SI available in the OFR.
The low cost of construction has made OFR increasingly popular in the Sundarbans region. The design parameters are shape, size, depth, side slope, capacity and location. The prevailing dugout trapezoidal OFR with and without bund are considered for optimization. The length-width ratio of OFR is taken 1:1 as it is having minimum perimeter and therefore, attains maximum storage. As ground water is at shallow depth and saline in nature in this region, the depth of OFR is restricted to 3 m on the basis of observed water quality in the OFR at the end of rabi season. The standard side slope of 1:1 is taken for silty clay loam soil. The following equations are used to optimize the size of an OFR:
RA = WA (S / 100) (3)
W = (A / Y)½ (4)
L = W Y (5)
Ls = L - 2 ((2 Zb Hb) + TWb + Wb) (6)
Ws = W - 2 ((2 Zb Hb) + TWb + Wb) (7) ALs = [Ls + {Ls - (2 Zp D)}] / 2 (8)
AWs = [Ws + {Ws - (2 Zp D)}] / 2 (9)
VOFR = ALs AWs D (10)
Where RA is the surface area of OFR, m²; WA the farm area, m²; S the size of OFR, % WA; W the width of OFR at surface, m; Y the ratio of length to width, m/m; L the length of OFR, m; Ls the length of submergence at surface of the OFR, m; Zb the side slope of the bund, m/m; Hb the bund height, m; TWb the top width of bund, m; Wb the width of berm, m; Ws the width of submergence at surface of the OFR, m; ALs the average (at surface and bottom of OFR) length of submergence, m; AWs average width of submergence, m; Zp the side slope of OFR, m/m and VOFR the volume of OFR, m3.
The rainwater storage in OFR is computed using the equations expressed as:
Rexci = SDWi SDWmax (11)
n
VOLi = {(Rexci / 1000) (WA - RA) + ((Ri / 1000) RA)} (12) i=1
Where Rexc is the excess rainwater diverted from field to OFR, mm.
The evaporation and seepage losses from OFR are not considered during kharif season because shallow watertable (0.5-1.5 m below surface) contributes water to OFR whereas, losses takes place on account of evaporation. However, these losses are considered to compute available water storage in OFR for irrigation during rabi season due to deeper watertable (2-3 m).
A flow chart for development of computer program is given in Fig.6. The model is verified by comparing simulated and observed depth of standing water in rice lowlands without OFR. The weekly rainfall and evaporation data for kharif 1994 are used for generation of simulated depth of standing water. Two fields; one at the tail end (lowland) and the other at the head end (upland) in the catchment
of sluice no 2 of East Mograhat Drainage Outfall Division in Sundarbans delta, are selected. The maximum depth of standing water in lowland and upland are 600 mm and 100 mm, respectively
beyond which excess water spilled out to the drain. The depths of standing water are observed at different growth stages during kharif 1994. The regression analyses indicated a good agreement (r² =
0.97 for lowland and 0.95 for upland) between simulated and observed values (Fig.7) and therefore, used to simulate excess rainwater availability in rice paddies.
For computation purpose the initial size of OFR is assigned the maximum area (40% of farm area) which may be converted into OFR, whereas the location of OFR is varied to allow runoff from 10-100% of farm area. At the end of computation if storage in OFR is found less than the capacity, a decrement to the size of OFR is provided for further computation. When OFR gets filled the size is considered as an optimal and the process is terminated.
2.3 Model formulation for optimal land allocation during rabi
Rainwater harvesting through OFR can make available only limited quantity of water. To keep the economics of OFR in favorable range, it is imperative that water is used judiciously for crops having high water productivity in terms of economic returns. Linear programming formulations are normally used to arrive at the optimal land and water allocation amongst different crop activities (Palanisami, 1992). A simple linear programming model is used in the present case.
2.31 The objective function
The objective function maximizes returns from the irrigated area under different crops through OFR and cost of water stored in OFR (includes cost of OFR and a penalty cost for making OFR area out of production during kharif season). It is assumed that inputs other than land, water and labour are at fixed level.
n
Max NP = (( Ci Yi ) Pi ) Ai – Cs S (13)
i=1
Where i is the index for crop; NP the net profit, Rs/ha; A the area under crop, ha; C the unit sale cost of crop, Rs/tonne; P the unit cost of crop production, Rs/ha; Y the yield per unit area, tonne/ha; Cs the unit cost of water in OFR, Rs/m3/year and S the total stored volume in OFR, m3 .
2.32 Irrigation requirement constraint: The sum of water demand of crops should not exceed available water in OFR (Initial storage Seepage Evaporation) during the period of growth.
n
GIRi Ai (S WL) < 0 (14)
i=1
Where GIR is the gross irrigation requirement for crop, m3/ha; WL the water losses from OFR observed at the rate of 5 mm/day for 135 days, m3.
2.33 Land-use constraint: The percentage acreage under different crops is limited to 80% of the total available area as20 % area is used for OFR.
n
Ai – L < 0 (15)
i=1
Where L is the total available land area, ha
The acreage under chilli (A3), cotton (A4) and cucumber (A5) are restricted to a given value due to the risk of crop damage by heavy rainfall at the time of harvest. Rice is the most demanding and safe crop against the weather uncertainty, and therefore, a minimum acreage under rice (A1) crop is placed. The crop activities and the limits placed on crop area are given in Table 2.
A (3) < Amax (3) (16)
A (4) < Amax (4) (17)
A (5) < Amax (5) (18)
A (1) > Amin (1) (19)
Where A denotes the area under crop, Amax and Amin denote the maximum and minimum area under the crop, respectively.
2.34 Labour constraint: Most of the non rice crops are labour intensive because irrigation is often applied through pitcher, and the inter-culture operations also require considerable labour. However, labour availability in the area is low. Therefore, a constraint is placed on labour requirements.
n
Ai Lri – La < 0 (20)
i=1
Where Lr is the labour requirement for unit area for crops, man-days/ha; La the total labour availability, man-days.
Two OFR (with and without bund) at Nikarighata village in South 24 Parganas district of Sundarbans delta are selected for model application. The hydraulic and storage features of OFR are given in Table 3. The storage in OFR with and without bund is estimated 1100 and 5225 m3, respectively. Taking the surface area of OFR, crop duration and observed daily water losses into account, seepage and evaporation losses are estimated 270 and 1355 m3 for OFR with and without
bund, respectively. The water requirements of different crops are estimated using pan evaporation method (Doorenbos and Pruitt, 1977) and the prevailing water application methods. On the basis of 2 years (93 94 and 94 95) experiment in the farmer's field, the input on labour, cost of cultivation (includes pumping cost) and net profit per unit area for different crops are generated (Ambast and Sen, 1994b). The inputs on water use, labour requirement and net profit are given in Table 4.
3. Results and
Discussion
3.1 Design of an on-farm reservoir (OFR)
A nomograph is developed to determine the hydrologic and hydraulic features of OFR in unit farm area for various combinations (Fig.8). It represents three type of curves i.e. the design dimension curve,
OFR capacity curves and rainwater availability curves. Since OFR is considered square in shape, the design dimension curve represents the equal length and width of the selected size of OFR. The OFR capacity curves give the information of the total volume that can be stored in the OFR of different sizes. The capacity curves are plotted for OFR with and without
bund for 2 and 3 m depths. The rainwater availability curves for different size of OFR i.e. 10, 20, 30 and 40% of farm area are generated for construction of OFR at different location i.e. 10, 50 and 100% below farm area which contributes runoff from 10, 50 and 100% farm area, respectively.
The basic steps to use the nomograph are i) note the volume of a particular size of OFR by projecting vertical line to OFR capacity curves and extending it towards volume, ii) note the location of OFR to meet the rainwater availability curve for that particular size of OFR and for known deep percolation rate and iii) note the dimension of OFR by projecting vertical line to design dimension curve. For example, the dotted line in Fig.8 represents an optimal 3 m deep OFR with bund in 20% farm area in soils with percolation loss rate 2 mm/day to be placed at 75% below the farm area. Similarly the solid line represents an optimal size of 3 m deep OFR without bund in 17% farm area for such soils and placed at 83% below the farm area to meet rainwater to its designed capacity of 4268 m3. The length and width of OFR in 20% area of a hectare should be 44.7 x 44.7 m. Therefore, it is suggested to convert 20 or 17% of the farm area into OFR with or without bund, respectively. However, for recommendation purposes, it is suggested to convert 20% of the farm area into OFR. The design features of OFR for a unit farm area in Sundarbans delta is shown in Fig.9.
3.2 Surface drainage improvement due to OFR
In lowlying areas of the region, the scope for cultivation of high yielding dwarf rice varieties (HYV) is almost negligible due to surface waterlogging during kharif season. The simulation study is made to assess the reduction in weekly surface waterlogging by diverting excess rainwater into the OFR (with and without bund) in 20% of farm area over lowlying area without OFR.
The surface water depth hydrographs are generated with probable weekly rainfall at 5 years return period (Fig.10). It is estimated that in a lowland catchment with no drainage, the water depth may reach as high as 0.63 m and therefore the cultivation of HYV rice crop is not possible under existing condition. The water depth in case of OFR with bund, remains at required water depth for rice up to 33rd SMW till OFR gets filled. However, the water depth increases to 0.35 m at panicle initiation and flowering stages, which are critical to excess water and affect crop production adversely. It is estimated that surface waterlogging during the season may reduce to about 45% in case of OFR with bund. In case of OFR without bund, the depth of water remains at required for a major growth period up to 37th
SMW. The maximum depth of water may reach to 0.18 m at the beginning of grain formation stage but for a short period. Though, some reduction in crop yield may occur due to excess water at this stage but that may not be significant. In this case the reduction in surface waterlogging
is to the extent of 75%. Therefore, construction of OFR not only provides water for irrigation but also improves surface drainage and thus, makes it possible to grow HYV of rice in lowlands of Sundarbans delta.
3.3 Supplemental irrigation demand
In order to assess the requirement of supplemental irrigation (SI) during kharif rice and to formulate water availability constraint for optimal land allocation in rabi season, simulation study is made. The preconditions for supplemental irrigation (50 mm) are defined, as 1) storage in the OFR should be more than one SI for cultivated land and 2) soil moisture in subsurface should reach below saturation level. On the basis of weekly values at 80% annual rainfall (no rainfall after 37th week), it is estimated that at least one supplemental irrigation is needed at the time of grain formation stage in two out of ten years (Fig.11). This will stabilize the crop production against the weather uncertainty.
3.4 Optimal land allocation
In the existing cropping pattern, rice crop is cultivated through OFR that causes shortage of water or crop failure in rainfall deficit years. The optimal area allocation to different crops is shown in Table 5.
As per the allocation during rabi season in the years of normal rainfall, rice should be grown only in 20% of area and remaining area should be allocated to non rice crops in farms having OFR with bund. Under non rice crops 50% area is allocated to chilli crop, whereas 15% area allocated to each cucumber and watermelon. In case of OFR without bund, 40 and 60% area to rice and non-rice (chilli-50% and cucumber-10%) crops, respectively show an optimal allocation to maximize net profit. The increase in cost of rainwater storage in OFR does not change water allocation, and therefore, indicated the stability of the suggested crop allocation.
3.41 Effect of deficit rainfall on crop allocation: In rainfall deficit years it is stipulated that a SI of 50 mm is to be provided during kharif season and thus, water availability in OFR would reduce by same amount during rabi season. In case of OFR with bund, the optimal allocation indicates that the percentage rice area remains the same because of minimum area constraint but the irrigation intensity (percent area irrigated) reduced by 20%. In case of OFR without bund, the percentage acreage under rice crop reduces by 10% but the irrigation intensity remains same as 10% land now allocated to cucumber.
3.42 Effect of labour inadequacy on crop allocation: Labour is the major constraint in the region particularly when non rice crops are irrigated manually with water applied through pitcher. Therefore, the effect of labour availability on crop allocation is studied. Sufficiency of labour is estimated on the basis of average demand for chilli and wheat crop as the most and least labour intensive crop, respectively. In case of OFR with bund when labour sufficiency is reduced by 20%, the area allocated to rice remains the same due to minimum area constraint but the area allocated to chilli and cucumber crop reduced by 15 and 5%, respectively. However, the irrigation intensity remains the same as the area of watermelon is increased by 20%. In case of OFR without bund, the optimal land allocation to rice crop remains the same but the area under chilli crop reduces by 10%. However, now 10% area is allocated to wheat crop, as it requires minimum labour.
3.43 Effect of interaction of water and labour inadequacy: The effect of interaction between reduction in available water during rabi due to its use during kharif as SI and labour (by 20%) is studied. It is noticed that the rice area remains the same due to lower limit but the area under chilli and cucumber reduced equally by 10% in case of OFR with bund. Therefore, irrigation intensity reduced by 20%. In case of OFR without bund, the reduction in rice area is 5% and chilli and cucumber both are reduced equally by 10%. Since 20% area is allocated to watermelon, a reduction of 5% in irrigation intensity is observed.
3.5 Economic evaluation
To evaluate the economic feasibility of OFR, technical efficiency in terms of cost of water development (includes cost of OFR and a penalty cost for making OFR area out of production during kharif season) and productivity per unit water storage are estimated. The life of OFR is taken as 25 years. The annual cost of water harvesting and supply are estimated Rs 582/103 m3 and Rs 557/10 3 m3 for OFR with and without bund, respectively. The total agricultural profit per hectare area are estimated at Rs 1295/103 m 3 and Rs 1020/103 m3. Thus, the benefit cost ratio is around 2 in both the cases that justify investment in OFR.
4. Conclusions
The following conclusions can be drawn from this study:
1. It is suggested to convert 20% of the farm area into OFR to harvest excess rainwater during kharif season in Sundarbans delta.
2. Storage of excess rainwater in OFR may lead to reduction in surface waterlogging to the extent of 75% during kharif season. This provides scope for introduction of high yielding rice varieties in rainfed lowlands.
3. A supplemental irrigation at the time of grain formation in two out of ten years may be required during kharif season.
4. The judicious use of stored water in OFR through optimal land allocation indicated a cost benefit ratio around 2, which justifies investment in OFR.
5. The procedure suggested for optimal design of OFR may be used in similar agro-ecological conditions. It may also be used to assess the performance of a controlled surface drainage project through simulation study to draw policies for surface drainage system regulation in humid rice lowlands.
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“Improving Drainage and Irrigation through OFR in Rainfed Rice Lowlands of Sundarbans Delta” by H.S.Sen and S.K.Ambast
Recommendations
Background information
• The Sundarbans area comprises of large number of islands criss-crossed with rivers and tidal cannals/creeks. More than 50 islands of the Sundarbans is inhabited, is flat and low-lying with heavy textured soil. Average annual rainfall is 1768 mm, 82 % of which is received during 5-month long rainy season, and the rest during rest of the year. Average tidal level is 2.75 m above mean sea level (maximum recorded up to about 5 m above msl).
• The area is generally monocropped with tall indica low-yielding rainfed rice, while only about 20 % of the area under double cropping, mainly due to scarcity of good quality irrigation water needed for irrigation and presence of moderate to high soil salinity during dry period. The yield of rainfed rice during kharif is poor mainly due to high water congestion during majority of the crop growth period, while there is as much probability for drought (soil moisture below field capacity) during ripening stage of crop growth. High water depth on the soil surface is itself detrimental for good crop growth (tillering etc.), besides it does not permit scientific crop management including fertilizer, insecticide application, etc. Drought, on other hand, during flowering or ripening is likely to cause non-fulfillment of grains.
Existing practices on flood protection & drainage and suggested improvements
• Owing to siltation in the river beds water flows above the cultivable land during high tide in most of the areas, but the latter is protected from inundation by earthen embankments, and for draining of excess water from cultivable land especially during low tides manually operated one-way sluice gates are installed at strategic points. Neither the embankments nor the sluices are scientifically designed to cater to the needs, and areas experience frequent flooding due to breaching of the embankments especially during severe cyclones or depressions.
• Since majority of the embankments are in the process of reconstruction at present after the disastrous damage caused by Aila a couple of months back, they may be reconstructed with the following designs based on the detailed analyses at Central Soil Salinity Research Institute (ICAR) located at Canning Town: (i) earthen embankments with 3:1 slope on the river end and 2:1 slope at the country end with at least 1 m free board above the high tide level, (ii) the embankments may be brick-pitched at the river end to provide more stability, (iii) provision of suitable wind breaks (plants identified) at the river end, and if possible, at the country end also, (iv) provision of two rows of embankments with minimum 100–500 m no-activity gap in between in areas likely to be breached more frequently than others, (v) the sluice gates should be much better designed as per recommendations made and operated much more frequently during low tides than what is normally done at present for effective drainage, (vi) primary and secondary drains (earthen) should be provided in the cultivable field as per design suggested to cater to the needs for field drainage (drainage coefficient 37.5 mm per day).
• It has been suggested that in order to prevent frequent breaching of the embankments, and keeping particularly in mind the increasing frequency of storms observed world-wide in the wake of global warming, the design of the embankments should be made with minimum 500 year return period based on probability analyses of the weather data for minimum 35 years.
Micro-watershed or OFR approach for integrated water management
• As probably a better option for on-field water management along with flood protection measures with the objective to harvest excess rainwater and utilization of the same during long dry period, together called as integrated water management approach, it has been suggested, based on 35 year weather data analyses and detailed test conducted at the farm as well as in the farmers fields, by CSSRI at Canning Town to (i) create on-farm reservoir (pond, etc.) on over 20 % area within the total farm area, either on individual farm area or preferably on community basis to store about 400 mm estimated to be in excess of the optimal requirement for high yielding rainfed rice crop during kharif, (ii) and utilize the same for a second and partially a third crop during dry season, and (iii) grow fishes in the OFR or pond and suitable plantation/ horticulture crops on the bund around the pond for additional benefit. A number of derelict channels, not in effective use at present in Sundarbans, may be properly utilized after reshaping for community OFR. For lowlying cultivated areas the soils so excavated for creating OFR may be used to raise the remaining 80 % farm area by a minimum 15 cm in height which will thus be conducive to high yielding rice and other crops. The OFR, with or without bund, may preferably be of trapezoidal in shape, with length-width ratio as 1:1, side slope as 1:1, and depth as 3 m. For other details of the design and location of the OFR nomograph may be consulted. It has been recorded, for which computer simulation model has also been prepared and tested with sufficient success, that 45 and 75 % of the water depth above soil surface during kharif season, can be reduced for OFR with and without bund, respectively, thereby creating a much better atmosphere for rainfed rice. A complete crop calendar with details of every important event for cultivation for different topo-sequences has been prepared with scientific cultivation programme for each. Linear programming approach has been followed to identify crops and related cultivation practices depending upon nature of constraints, viz. water, labour, etc. A user-friendly software has been prepared for detailed design recommendations and cropping practices depending upon location-specific conditions related to soil, weather, availability of water, choice of crops, etc. for application to a wide variety of situations using this approach for multiple cropping under rainfed conditions.
Tuesday, April 27, 2010
Coastal soils:Nutrient management for SAARC countries
Citation: Sen, H.S. (2009). Nutrient management in coastal soils. Paper presented in SAARC Workshop on "Nutrient Use Efficiency in Agriculture", held at CSSRI, Karnal, 9-11 September, 2009.
Nutrient Management in Coastal Soils1
H.S.Sen2
Former Director, Central Research Institute for Jute & Allied Fibres (ICAR), P.O. Barrackpore, Nilganj, North 24 Parganas, West Bengal, PIN 700 120
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Locations of the SAARC countries are shown in the map (Fig. 1). The ratio between coastal length to total land area along with the corresponding data on percent share of agriculture to the total GDP show large variations among different SAARC countries (Table 1). Though it will be oversimplification to try to relate the above ratio to percent share of agriculture to country’s GDP it may not be out of place to give some cognizance, considering the two extremes (Maldives vis-à-vis Nepal and Bhutan), that higher the ratio lower may likely be the contribution of agriculture due to lower
productivity as a result of
several technological and other constraints (Yadav,2007) in the coastal areas,
as compared to the inland areas, in general, in most countries.
Coastal areas are diverse in function and form, dynamic, and do not lend themselves well to definition by strict spatial boundaries. Unlike watersheds, there are no exact natural boundaries that unambiguously delineate coastal areas at the global scale. The world coastline extends from 350,000-1,000,000 km in length, depending upon how finely _____________________________________________________________________________
1 Paper presented at the SAARC Workshop on “Nutrient Use Efficiency in Agriculture”, held at CSSRI, Karnal,
Haryana, India, 9-11 Sptember, 2009
2 Present address for communication: 2/74 Naktala, Kolkata, 700 047, West Bengal, India;
Email: hssen2000@hotmail.com, hssen.india@gmail.com
the ‘length’ is resolved. More comprehensively, the coastal ecosystem has been defined by Sen et al. (2000) as representing the transition from terrestrial to marine influences and vice versa. It comprises not only shoreline ecosystems, but also the upland watersheds draining into coastal waters, and the nearshore sub-littoral ecosystems influenced by land-based activities. Functionally, it is a broad interface between land and sea that is strongly influenced by both.
Soil characteristics of this region are unique and vary widely, depending on their physiographic and climatic conditions. Soil salinity is a major constraint for crop production in the coastal ecosystem. Plant growth is seriously constrained owing to, apart from other factors, various nutritional disorders in plants related to the problems in the coastal soils. This calls for a review of the nature and severity of the problems in soils and plants and to suggest improved nutrient management practices in order to alleviate the adverse situations and obtain higher productivity.
Salient characteristics of coastal soils
In India, of the two coastlines length of the East coast is higher than that of the West. The continental shelf is more stable than the coast. The continental shelf of 0-50 m depth spreads over 1,91,972 sq km and that of 0-200 depth over 4,52,060 sq km area. The shelf is wide (50-340 m) along the East coast. The Exclusive Economic Zone is estimated at 2.02 million sq km. Practically, no systematic study was earlier made to demarcate the coastal soils based on well-defined scientific indices valid for the different sub-ecosystems in this country, and possibly elsewhere. Notable among the past works was that of Yadav et al. (1983) who suggested 3.1 million hectare area (including mangrove forests), while Szabolcs (1979) suggested 23.8 million hectare under coastal salinity in India. In Bangladesh 1.4 million hectare is under coastal area. The coastal saline soil has been used by various workers almost synonymously with coastal soil per se which is not correct since coastal soils are not entirely saline in nature. None of the above estimates appears to have been made on sound scientific basis. However, the latest compilation made by Velayutham et al. (1998) on the soil resources and their potentials for different Agro-ecological Sub Regions (AESR) of India show total 10.78 million hectare area is under this ecosystem (including the islands) in India, which has been the first scientific approach for delineation of the coastal soils. Most of the areas have problematic soils, such as saline, alkaline, acid sulphate and marshy and waterlogged soils, situated in the low-lying flooded/ waterlogged areas, mainly along the deltas.
Saline and sodic soils
In case of the saline soil, the salinity status in soil varies widely from ECe 0.5 dS m-1 in monsoon to 50 dS m-1 in summer. Mostly NaCl followed by Na2SO4 are the dominant soluble salts, with abundance of soluble cations in the order Na>Mg>Ca>K, chloride as the predominant anion, and bicarbonate in traces. In India, the soils are, in general, free of sodicity except in a few pockets in the South and West coast. The saline and sodic soils may be defined as follows:
• Saline soils: Soils having pHs less than 8.5, ESP less than 15, and preponderance of chlorides and sulphates of sodium, calcium and magnesium. ECe should be more than 4 dSm-1 at 25C.
• Sodic soils: Soils having pHs more than 8.5, ESP 15, and preponderance of carbonates and bicarbonates of sodium. The ECe is limitless if originating from salts capable of alkali hydrolysis, otherwise it should be less than 4dSm-1 at 25C.
In coastal and delta regions of major rivers of the world the soils are rich in salts due to the presence of saline ground water table at shallow depth. The coastal delta regions are usually having low elevation and it may also be subjected to frequent brackish water inundation. The groundwater in the region is influenced by sea and the brackish water estuaries. The groundwater reaches the soil surface through capillary rise during dry season, evaporates from the soil leaving salts behind, finally making the soil saline and unproductive for agricultural crops. The soil salinity, thus, shows high temporal and spatial variation depending on the elevation, soil texture, climate (evapo-transpiration, precipitation, wind velocity, relative humidity, etc.), drainage, and other related factors. The ingress of seawater or brackish water, and salt-laden sands blown by sea winds are also greatly responsible for formation of coastal salt-affected soils.
Acid sulphate soils
Many of the coastal soils are developed under coastal swamps and mangrove forests leading to the environment of high organic matter and abundant supply of sulphate salts from the sea. In acid sulphate soils, the production of acid from oxidation of pyrites exceeds the neutralizing capacity of soil and the pH falls below 4. The soils are generally rich in organic matter and clay minerals. The total area of actual and potential acid sulphate soils is rather small: about 10 million hectares are known to occur in the tropics, and the world total probably does not exceed 14 million hectares in South and Southeast Asia, West and Southern Africa, and along the South American and Australian coastlines. In addition, some 20 million hectares of coastal peats, mainly in Indonesia, are underlain by potential acid sulphate soil.
Acid sulphate soils develop as a result of the drainage of soils that are rich in pyrites (FeS2), which on oxidation, produce sulphuric acid in presence of excess SO42— ions in soil.
4 FeS2 + 15 O2 + 14 H2O 4 Fe(OH)3 + 8SO4-- + 16H+
12FeS2 + 45O2 + 30 H2O + 4K+ 4KFe3(SO4)2.(OH)6 + 16 SO4-- + 36H+
(Soil pyrite) (Jarosite)
Pyrites accumulate in waterlogged soils that are both rich in organic matter and dissolved sulphates.
2Fe2O3 + 8 SO42- + 16CH2O + O2 4 FeS2 + 16 HCO3- + 8 H2O
(From (From (Soil org. (Diss- (Pyrites)
soil) sea water matter) olved
or soil) oxygen)
The growth of most dry land crops on acid sulphate soils is hampered by the toxic levels of iron and aluminium and the low availability of phosphorus. Toxic levels of dissolved iron plus low phosphorus are the most important adverse factors for wetland rice. In the near-neutral potential acid sulphate soils (Sulfaquents, Sulfic Fluvaquents) high salinity, poor bearing capacity, uneven land surface, and the risk of strong acidification during droughts are the main disadvantages. Young acid sulphate soils (Sulfaquepts), in which the pyritic substratum occurs, near the surface are often more acidic than those soils (Sulfic Tropaquepts, Sulfic Haplaquets) in which this horizon is found at greater depths.
Factors affecting nutrient availability and suggested management strategies
Additions of N and P generally increase the growth of plants grown in N- and P-deficient environments, provided that the plant is not experiencing severe salt stress. When salinity and nutrient deficiency are both factors limiting growth, relief of the most limiting factor will promote growth more than the relief of the less limiting factor. Therefore, addition of a limiting nutrient can either increase, decrease, or have no effect on relative plant tolerance to salinity, depending on the level of salt stress. Failure to account for the severity of salt stress when interpreting salinity x nutrient interactions has caused considerable confusion among researchers in the past.
Poor transformation and availability of nutrients are influenced by high soil and water salinity and other adverse soil properties including inhospitable biological environment, specific ion effects, ion antagonisms and toxicities, etc. In wet acid sulphate soils excess water soluble iron, aluminium, hydrogen sulphide and organic substances, especially fatty acids, are common examples to affect crop growth, particularly rice. Besides, there are other location-specific problems related to nutrient management, viz. highly leached low-fertility lateritic soils with severe erosion problem along with undulating topography, etc.
Physico-chemical factors
Saline and Sodic soils
Nitrogen: Most of the coastal saline soils are deficient in nitrogen. Besides lesser utilization of nitrogeneous fertilizers, especially in coastal areas, the mineralization of soil organic nitrogen, and thus the release of native soil nitrogen to the plant available form, is also slowed down in the salt-affected soils due to decrease in the population as well as activity of microbes with increase in soil salinity. It was revealed from a study at CSSRI, Regional Station Canning that the rates of both mineralization and immobilization of nitrogen in soil were considerably reduced at soil salinity of ECe 10 dSm-1 and above. The increasing loss of nitrogen through NH3 volatilization from applied nitrogeneous fertilizer with increase in soil salinity was studied under field condition at Canning. While comparing among different sources at two soil salinity levels, viz. ECe 3-4 and 7-8 dSm-1, the loss was found to be maximum under ammonium sulphate, followed by prilled urea, and minimum under placement of urea (in paper packet) at 5 cm depth, whereas the conventional slow-release sources as lac-coated urea, sulphur-coated urea, and placement of urea briquette occupied the intermediate positions. Reduction in the loss when compared with prilled urea broadcast under cropped condition was maximum (73.1%) for placement of urea.
Phosphorus: The level of phosphorus in the coastal saline soils is highly variable, and depends largely on the nature and degree of salinity. The availability of soil phosphorus largely depends on the pH of the soil developed after hydrolosis of salt. An increase in soil pH on hydrolysis reduces the availability of soil phosphorus. Very little work has been done on the transformation and availability of P to crops in coastal saline soils.
Potassium: The availability of potassium depends largely on the parent material, clay minerals and weathering conditions. It also depends on the nature and amount of salts in the soil. It was reported from CSSRI, Regional Station Canning that the coastal saline soils are rich in water soluble, exchangeable, non-exchangeable and available K.
Thus, with regard to soil fertility, the coastal soils are usually rich in available K and micronutrients (except Zn), low to medium in available N, and are variable in available P status (Bandyopadhyay et al., 1985, Bandyopadhyay, 1990, Maji and Bandyopadhyay, 1991). Major portion of the applied N fertilizer is lost through volatilization (Sen and Bandyopadhyay, 1987).
Micronutrients: Work done so far on the role of micronutrients in coastal saline soils is meagre. The soils are generally rich in micronutrients, such as Fe, Mn, Zn, Cu, B and Mo. In the coastal sands of Andhra Pradesh in India the soils are deficient in Zn as well as in N, P and K. Iron chlorosis is common to crops like sugarcane (Saccharum officinarum L), jowar (Sorghum bicolor L. Moench), rainfed rice (Oryza sativa L.), etc. As a remedial measure for iron deficiency common in upland rice nurseries, spray of iron salts has been suggested. Such symptoms are also common in groundnut (Arachis hypogaea L.), Cressandra undulaefolia Salisb., etc. In Andhra Pradesh salinity accentuated the zinc deficiency. Along with Zn, P deficiency is also reported from the red and deltaic black soils in this state. High amount of CaCO3 (up to 15%) is congenial to Zn and Fe deficiency disorders. High dose of Zn application is recommended in rice as foliar spray. Zn deficiency was noted from various laterite and lateritic soils in other coastal states in India also. Besides, Al and Fe toxicities too have been reported from a few acid lateritic soils along the coast.
Long term study: Long term field experiment in coastal saline soils in India showed that rice and wheat yield could be maintained with 50% NPK used in conjunction with FYM or green manure (DARE, 2003-04). In another detailed long term experiment conducted (CSSRI, 1990) in Sundarbans (India) it was observed that grain yield of crops in a rice-barley rotation increased significantly only due to the application of N. Application of P did not show any significant increase in the yield of crops in the initial 8 years, after which the yield of barley alone increased due to P application. Available K content was high in the soil. The experiments conducted so far showed that a basal dose of 11 kg P ha-1 for rice and 5.5 kg P ha-1 for barley or for similar upland crops should maintain the fertility status of the soil, whereas the K application may be omitted without any detrimental effect on soil fertility or crop growth. The K removal by the crop was compensated by K added through accumulation and release of non-exchangeable sources.
Acid sulphate soils
In Sri Lanka addition of Gliricidia muculata in combination with phosphate and a small dose of inorganic fertilizer was effective to secure high rice yields (Deturckl et al., http://www2.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub53/pub53-h8.pdf). In India, for the coastal acid sulphate soils of Sundarbans, application of lime, superphosphate and rock phosphate have been found beneficial in improving the soil properties and rice growth (Bandyopadhyay, 1989). Application of Ca-rich oystershell, which is available in plenty, was found beneficial, if applied in powdered form, as an inexpensive alternative soil ameliorating agent. In this soil continuous submergence for one year could not improve the soil properties substantially.
Mongia et al. (1989), while reporting for two soils in Andaman Islands observed that application of lime and phosphorus may be beneficial for lowland rice. In another study on mangrove (Avecenia marina) muds in this island, it has been reported that liming significantly depressed the concentrations of Al, Mn and Fe. Exchangeable Al content also decreased with lime application. The depression of exchangeable Al may be due to precipitation of trivalent Al and Al (OH)3 in the presence of high concentration of OH- ions. Lime application, in general, also reduced the exchangeable and extractable Fe contents of the soil.
In the rice-based systems of the Mekong Delta in Viet Nam, there is a trend towards replacing the traditional rice double cropping with a rotation of wet season rice and dry season upland crops (vegetables and tuber crops). However, under the prevailing acid sulphate soils, the build-up of excessive concentrations of exchangeable aluminium (Al3+) during the aerobic soil phase strongly limit upland cropping to few relatively Al-tolerant tuber crops that farmers grow on raised beds to enhance Al3+ leaching process. Study was conducted on the performance of major tuber crops (cassava, sweet potato and yam) in relation to soil exchangeable Al3+ concentration and as affected by the application of locally produced biogas sludge on farmers’ fields on a typical acid sulphate soil with observation plots laid out on raised beds and the same categorised based on the initial exchangeable Al3+ content of the topsoil in classes of <10, 10–15, and >15 meq Al3+ per 100g. Biogas sludge was applied at 3 Mg ha−1 (dry matter) to tuber crops and compared with an unamended control. It was found that biogas sludge tended to reduce soil exchangeable Al3+ concentrations but significantly increased the tolerance to given Al concentrations with higher tuber yield and P uptake in all tuber crops. However, Al-tolerant cassava showed stronger responses to amendment than Al-sensitive yam. It was concluded that in the absence of soil liming, the application of organic wastes can improve the performance of Al-tolerant while permitting the cultivation of more Al-sensitive crop on acid sulphate soils. Further research aims at identifying most appropriate substrate types and application rates for specific acid soil conditions and (Saleh et al., http://www.tropentag.de/2005/abstracts/links/Saleh_gFbcXBF1.pdf) crop tolerance levels.
Inundated and flooded soils
A flood is an overflow or accumulation of an expanse of water that submerges land. In the sense of ‘flowing water’, the word may also be applied to the inflow of the tide. ‘Coastal flood’ is caused by severe sea storms, or as a result of other hazards (e.g. tsunami or hurricane). The problems of maintaining these areas are accentuated by naturally rising sea levels due to global climate change. Very often the problem becomes much more severe with increase in salinity in the flood water caused by breaching or overflowing of the sea dykes, etc. Flooding thus causes significant change in soil properties depending on the soil, hydrological properties of the flood water, and duration of flood. Among others the most significant changes in soil properties of relevance to plant growth are silt deposition, accumulation of salts, erosion of top soil, organic C status in soil, depletion of soil oxygen ─ resulting in lack of plant metabolic activities and overall reduced soil atmosphere causing significant change in soil nutrient dynamics.
Soil-and plant-produced C2H4 were suggested as factors in reducing root permeability and increasing resistance to water uptake by tobacco (Hunt et al., 1981). It has been reported that soil flooding stress may severely reduce water and ion uptake directly by increasing the permeability of roots to water and indirectly by reducing the size and volume of the plant roots, as well as by decreasing the ion uptake per unit weight of roots. Such uptake implies a disruption in root metabolism and reducing the effective root surface area available for ion uptake. Many such evidences indicate that dysfunction in nutrient absorption by roots under flooding conditions is largely caused by lack of O2 and attendant deleterious metabolic effects. The accumulation of dioxygen in the earth’s atmosphere allowed for evolution of aerobic organisms that uses O2 as the terminal electron acceptor, thus providing a higher yield of energy compared with fermentation and anaerobic respiration. For example, in aerobic metabolism, the complete breakdown of one molecule of glucose yields a total of 38 molecules of adenosine triphosphate (ATP), whereas the anaerobic breakdown of this same glucose molecule to ethanol and CO2 yields only 8 molecules of ATP (Scandalios, 1993). Oxygen stress may inhibit guttation either by increasing the root resistance to water movement or by inhibition of ion transport to the xylem and arrest of vegetation and reproductive growth.
Flooding X Salinity interactions on H2S gas evolution: Bandyopadhyay et al. (1995) measured H2S production in coastal saline and flooded rice field in Sundarbans, West Bengal, India. They observed that sulphates constitute about 40-50 percent of salts in these soils undergoing reduction reactions resulting in transformation of sulphate salts into sulphides including H2S gas which is toxic to plants. It was estimated that during peak periods about 390 to 1180 cm3 of H2S gas per m2 per 24 hours (day time) escaped to the atmosphere from waterlogged rice field, extent of which depended on the soil salinity (ECe 5-9 dSm-1) values and dose of green manure (Sesbania aculeata) applied (0-5 Mg ha-1) in the experiment. Higher soil salinity and application of green manure increased the production of H2S. The concentration of P and micronutrients in both shoot and root of rice decreased considerably at higher concentration of H2S in irrigation water. The insoluble sulphides precipitated on root surfaces (Table 2) and possibly interfered with the growth and nutrient uptake by plants. The acid soluble metallic sulphide (FeS equivalent) content of soil increased from 221 mg per kg soil to 381 mg per kg soil as the H2S content of irrigation water increased from 0 to 80 ppm. The primary cationic constituent of the precipitate on root was Fe, the solubility of which increased considerably after submergence of soil accompanied by a sharp fall in the redox potential. It was inferred that besides the direct toxic effect of H2S gas the indirect effect of sulphate reduction in interfering with the nutrient uptake by plants was also responsible for decrease in yield of rice.
Table 2. Nutrient content in the extract of root coating (10 g fresh root per 20 ml 6N HCl) and the insoluble metallic sulphides (FeS equivalent) in soil at flowering stage of rice in pot culture experiment
H2S in irr. water
(ppm) Soil salinity, ECe 7.4 dSm-1 Soil salinity, ECe 3.8 dSm-1
Insol.
sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm) Insol. sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm)
Fe
Mn
Zn
Cu Fe
Mn
Zn
Cu
0 221 2913 63 68 13 204 2217 52 71 11
20 253 8492 121 142 22 244 7992 98 125 20
40 280 11948 144 181 24 265 10731 127 174 20
60 310 15316 175 218 26 309 14986 160 215 25
80 381 17723 225 268 32 355 17265 196 257 30
LSD
(p=0.05) 26 150 4 5 3 21 161 4 6 2
Adapted from Bandyopadhyay et al., 1995
Biological factors
It is not only the physico-chemical property of soil that is adversely affected by salinity and alkalinity, the soil biology is also highly affected by presence of excess salts in soil, and this, in turn, would affect nutrient availability.
Saline soils
In general, the biological activity of soil microorganisms is low in salt-affected soils due to high salt content. Carbon dioxide evolution and microbial count in soil decrease at higher concentration of salts. There is decrease in free living diazotroph and nitrogenase activity with increasing soil salinity. The inhibition effect also varied with kind of salt being more with Ca and Na type of salts than with K and Mg type of soils among the cations. Among the anions the adverse effect was more with CO3 and Cl salts than with SO4 type of salts. Initial total microbial count of heavy clay saline soil has been reported to have increased (Mahmoud et al., 1972) after reclamation (Table 3). Enzymatic activity in soil also decreases due to increase in soil salinity. The low biological activity of saline soil is largely due to the osmotic and ionic stress induced by salts. In saline soils organic amendments may improve the biological activity up to the salinity level of ECe 26 dSm-1. Thus, there is an urgent need of applying organic matter to saline and sodic soils for their efficient management. At higher salinity, the osmotic and specific ion effects predominate over organic matter in influencing the biological activities in soil. Thus, addition of organic matter alone may not be enough to improve the biological activity. It has been observed that desalinization immediately increases the biological activity, although not completely restored.
One of the most efficient ways of increasing the N status and sustaining crop production in the salt-affected soils is to exploit the benefit of biological N fixation, specially by cultivation of annual and perennial legumes in crop rotation, application of azolla and non-symbiotic N fixing bacteria, etc. Rhizobium spp. show considerable tolerance to high pH and can thrive well in sodic soils but these are considerably sensitive to salinity. Though Rhizobia could thrive and multiply in sodic soil with pH as high as 10.0 the effective contribution of the bacteria towards N-fixation reduces because of sensitivity of the host plants to salinity of soil. Nodulation may be reduced at soil salinity (ECe) as low as 5.0 dSm-1 in barseem (Trifolium alexandrnum), while nodulation was delayed and ineffective nodules were produced in the crop at ECe 15.0 dSm-1 (Bhardwaj, 1975). Legume crops vary in their nodulation performance on different soils (Table 4). Nodulation and N-fixation increase considerably after reclamation of the soils. No nodule may be formed on root when total soluble salt content of soil is about 5.16%. Application of phosphate fertilizer shows improvement in nodulation, and the phosphate x salinity interaction is found significant (Bajpai and Gupta, 1979). At moderate salinity level of ECe 5.0 dSm-1, application of 100 kg P2O5 ha-1 gave symbiotic performance similar to the application of 50 kg P2O5 ha-1 in normal soil. The performance of some forage legumes with Rhizobium spp. (Barseem with: Trifolium alexandrinum, Shaftal with: T. resupinatum, Lucerne with: Medicago sativa, and Senji with: Melilotus parviflora) tested under different soil salinity conditions shows that the mean nodule number, weight and yield of legumes reduced with increase in soil salinity (Table 5). There exists a considerable variation among the Rhizobium spp. in their tolerance to salinity. For example, there may be complete inhibition of barseem Rhizobia as has been obsreved by Pillai and Sen (1966) at 0.5- 0.7% NaCl in culture media, whereas strains from Dolichos showed highest growth at 0.7-1.2 % (Pillai and Sen, 1973). However, addition of organic material (green manure, FYM, etc.) or gypsum to soil may greatly improve the proliferation of Rhizobia.
Effect of salinity on the microbial and biochemical parameters of the salt-affected soils in Sundarbans (India) was studied at nine different sites. The study revealed that the average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating the adverse effect of soil salinity (Tripathi et al., 2006). It was suggested that integrated nutrient management should be very effective for increasing its use efficiency for higher and sustainable yield of crops (Bandyopadhyay et al., 2006, Tripathi et al., 2007). Bandyopadhyay and Rao (2001) were of the opinion to introduce systems approach involving organic, inorganic and biofertlizers compatible with the farmers’ practice. In coastal soil at Tamil Nadu (India), application of agro-industrial wastes significantly improved soil organic carbon, pH, EC and soil bacteria, fungus and actinomycetes population and enhanced the soil fertility status (macro and micro nutrients) and improved the crop productivity of finger millet. Application of pressmud @12.5 t ha-1 recorded better growth and yield of finger millet followed by composted coirpith @ 12.5 t ha-1 (Rangaraj et al., 2007).
Sodic soils
Sodic soils show low urease activity, but high pH does not affect the ammonifying or ureolytic bacteria as much as it affects the nitrifiers which are highly sensitive to both salinity and alkalinity. Production of ammonia from urea may not be affected although nitrification may be severely affected by NaCl, Na2SO4 or CaCl2. However, urease activity is also reduced at higher pH but enhanced with organic carbon content of soil. Among the nitrifiers Nitrobacter is more sensitive than Nitrosomonas. Both salinity and alkalinity do not show marked influence on the growth of Actinomycetes, but fungi and Azotobacter are very sensitive. Blue green algae are tolerant to high pH. Sulphur oxidizing bacteria, Thiobacillus thiooxidans and T. novellas, and iron-sulphur oxidizing bacteria, T. ferrooxidans, have been isolated from sodic soils (Rupela and Tauro, 1973), although their activities are poor. High concentration of sodium chloride inhibits nitrogen fixation by Azotobacter, and the strains isolated from saline soil are less inhibited than others. Negative correlation exists between the Diazotrophic bacteria and salinity (Hassouna et al., 1995). On the other hand, Azospirillum, a non-symbiotic root associative nitrogen fixing bacteria may be recommended as a bio-fertilizer up to total soluble salt content of 2500 ppm in soil. Growth of non-symbiotic bacteria and their N fixation may be improved by application of chemical amendments and phosphatic fertilizers to sodic soils.
Blue Green Algae (BGA) can be used to reclaim the sodic soils due to their ability to secrete organic acids and immobilize Na in the biomass. Though some contradictory reports are available it is true that blue green algae increase the organic matter and N content of saline and sodic soils. Azolla, the water fern, having symbiotic association with BGA, can be grown as a pioneer plant to improve the health, N status, and reclamation of salt-affected soils. By cultivating azolla for a few successive years, the salt content of saline soils may be decreased considerably. The desalinization rate is 1.8 times higher than leaching through water, or 2.1 times than desalinization through Sesbania incorporation (Shang et al.,1987).
Phosphate solubulizing bacteria do not show much effect on the available P status of sodic soil, whereas VAM (vesicular-arbuscular micorrhizae) can grow with their extensive hyphal network up to a salinity level of 12 dSm-1. It may also help the plants in scavenging phosphorus, nitrogen, sulphur and micronutrients in salt-affected soils. The data in Table 6 show that inoculation with VAM resulted in significant increase in VAM colonization of wheat roots and VAM spore count in sodic soil (pH 8.8) over uninoculated controls. The increase in Olsen’s P, as a result P application, inhibits VAM colonization and spores. A positive relation (r=0.74) exists between the VAM colonization and soil available P which indicates mobilization of P by VAM. The inoculation with Glomus mossae and native VAM results in increase in wheat yields. These was no significant increase in P content of both straw and grain as a result of inoculation with VAM in treatments without P application. The inoculation with VAM results in significant increase in Zn content of grain and a positive relation has been reported between the Zn content and VAM in roots.
Table 6. Effect of inoculation with VAM cultures and P application on wheat yield,
available P, VAM spores and % VAM colonization of roots
P level VAM Culture Available P Wheat yield (g pot-1) VAM spores %VAM P Zn
(mg kg-1) Straw Grain (10 g-1) Colonisation - (kg ha-1)-
0 None 4.8 19.3 13.6 17.0 9.7 2.2 53.3
0 Glomus mossae
6.3
24.4
17.6
22.4
76.1
2.1
48.5
0 Native VAM 6.0 24.6 13.6 19.9 59.2 2.2 51.8
40 None 21.5 24.6 17.5 18.7 2.0 3.0 21.5
40 Glomus mossae
19.4
25.6
15.3
10.7
44.5
3.5
36.6
40 Native VAM
17.1 24.3 17.3 15.5 18.4 3.4 36.8
CD (P=0.05) P 1.2 0.7 1.0 NS 5.7 0.2 4.5
VAM NS 0.9 NS NS 7.0 NS 5.5
P X VAM 2.2 1.3 1.8 NS 9.9 NS NS
Source: Sharma and Swarup (1996)
Use of green manure crops like Sesbania spp. (S. aculeata) has been proved very suitable for saline sodic soils (Keating and Fisher, 1985, Rao, 1986, Evans and Rotar, 1986). In favourable climate with proper management, these crops accumulate well over 100 kg N ha-1, mostly through biological N fixation, in 50-55 days, thereby increasing the yield of the following rice crop significantly (Singh et al., 1991). Likewise, application of green leaves of Glyricidia maculata @ 10 tonne ha-1 to the puddled soil before rice transplanting has been found comparable in terms of yield with inorganic fertilizer application of N, P and K at 100, 50 and 50 kg ha-1, respectively (Chavan and Dongale, 1994). Buresh and De Datta (1991), however, were of the view that leguminous green manure and its residue normally are able to meet only partially the N requirement for the following high yielding rice variety. They focused attention on the high loss of N under the anaerobic-aerobic soil cycles typical of legume-lowland rice sequence, as well as under higher production of methane and nitrous oxide from the lowland rice field. However, in view of rising energy cost and limited input availability, recycling of organic wastes and use of renewable sources of biofertilizers, viz. rhizobium cultures for pulse or legume, and blue-green algae for waterlogged rice field may play a significant role in terms of integrated nutrient management for rice in coastal saline soils (Kundu and Pillai, 1992).
Soil quality
The importance of improved soil quality in the coastal plains through higher SOC level of the soils was highlighted by Mandal et al. (2008). Very recently, IRRI characterized lowland rice soils (excluding deepwater rice) in Asia in respect of soil quality (Haefele and Hijmans, 2009), and the study included large areas under coastal plains (Fig. 2). They grouped soil qualities into four categories. These were: Good, Poor, Very Poor and Problem soils. ‘Good’ and ‘Poor soils’ represent those with different degrees of weathering but without major constraints; ‘Very Poor’ represents soils with multiple chemical constraints (acidity, deficiency of phosphorous, or toxicities of iron and aluminum); while ‘Problem soils’ represent those with the most frequently cited soil problems, including acid sulphate, peat, saline, and alkaline soils, which partly cause low fertility, and partly soil chemical toxicity.
Nutritional disorder in plants
Nitrogen: Under salt-stress conditions, the uptake of N by crop plants is generally affected (Alam, 1999). A substantial number of laboratory and greenhouse studies have shown that salinity reduces N accumulation in plants. It has been reported that an increase in Cl- uptake and accumulation is accompanied by a decrease in shoot nitrate concentration. In their experiment, Aslam et al. (1984) have reported that Cl- inhibited NO3- uptake more than SO42- when these anions were present on an equal osmolarity basis. In contrast to the effect of Cl- on NO3- uptake, reported data indicated that increased NO3- in the substrate decreased Cl- uptake and accumulation. The possible decrease in N uptake by increasing salinity has been partly attributed to a probable substitution of Cl- for NO3-. Both the chloride salts of Na and K inhibited the nitrate uptake similarly, suggesting that the process was more sensitive to anionic salinity than to cationic salinity (Aslam et al.,1984). Although, Cl- salts were primarily responsible for reduced NO3- uptake by plants, NO3- reduction in plants was not affected by salinity in studies with barley (Aslam et al.,1984).
Salinity also stimulated nitrate reductase activity in peanut plants but decreased the nitrate reductase activity in tomato and cucumber (Cucumis sativus L.) plants. Reduction in NRA may be due to inhibition of NO3-uptake by Cl- in plant species (Abdul-Kadir and Paulsen, 1982) .
Potassium, Sodium, Calcium & Magnesium: The higher K/Na ratio in shoots of barley cultivars compared with that in root medium solution indicated selective uptake of K, which seems to be among the processes involved in tolerance of cultivars to salinity stress (Alam, 1999, Niazi et al., 1992). Addition of K suppressed the uptake of other cations by rice and tomato plants in the order of Na>Mg>Ca. The depression of Na uptake by K could be due to the antagonism between the two cations. It is widely recognized that a high Na concentration inhibits K uptake by plants. On the other hand, Na appeared to stimulate the K uptake by plants.
Salinity stress has significant inhibitory effects on the concentrations of K, Ca, and Mg as well as stimulatory effects of these nutrient elements on different crop plants (Alam,1999). With the increasing concentration of NaCl salts, K concentration decreased in the leaves, stems and roots, and was accompanied by a substantial increase of Na in the organs.
In lowland rice plants adapt to saline conditions and avoid dehydration by reducing the osmotic potential of plant cells. Antagonistic effects on nutrient uptake may occur, causing deficiencies, particularly of K and Ca under conditions of excessive Na content. For example, Na is antagonistic to K uptake in sodic soils with moderate to high available K, resulting in high Na:K ratio in the rice plant and reduced K transport rates. Sodium-induced inhibition of Ca uptake and transport limits shoot growth. Increasing salinity inhibits nitrate reductase activity, decreases chlorophyll content and photosynthetic rate, and increases the respiration rate and N content in the plant. Plant K and Ca contents decrease but the concentrations of NO3-N, Na, S, and Cl in shoot tissue increase.
The factors related to nutrient disorder that affect different rice varieties are:
• Differences in nutrient uptake under Na stress: Tolerant cultivars have a narrower Na:K ratio (higher K uptake) and greater leaf Ca2+ content than susceptible cultivars.
• Efficient exclusion of Na+ and Cl-: Salt-tolerant rice varieties have a reduced Na+ and Cl- uptake compared with less tolerant cultivars.
Calcium plays a vital nutritional and physiological role in plant metabolism. Calcium, which like K also is an essential mineral nutrient, helps in maintaining membrane integrity, is important in senescence processes, and is known to counteract the harmful effects of Na on crops (Lahaye and Epstein, 1971). Plant growth is dependent on Ca2+, and both cell division and cell elongation processes are affected by the Ca2+ ion concentration.
The presence of Ca2+ as the dominant cation in agricultural soils generally ensures that the absolute Ca2+ level is not a primary growth-limiting factor. As salinity increases, the requirement of plants for Ca2+ increases. The uptake of Ca2+ from the soil solution may decrease because of ion interactions, precipitation, and increases in ionic strength that reduce the activity of Ca2+. In citrus, Ca was found to be effective in reducing the transport of both Na and Cl from the roots to leaves. Maintaining an adequate supply of Ca2+ in the soil solution is an important factor in controlling the severity of specific ion toxicities.
The magnesium content of the leaves of saline-treated bean plants increased, whereas it decreased in the root. Hodson et al. (1982) found potentially toxic concentrations of Mg in salt-marsh soil solution samples and demonstrated that a salt-marsh clone, Agrostis stolonifera, was considerably more tolerant to Mg2+ than was an inland clone. Magnesium concentration of avocado leaves was decreased with an increase in the exchangeable Na in the soil. In rice, Mg transport to the tops was suppressed by Na compared with Mg uptake (Song and Fujiwara, 1996). The Mg content in the roots revealed the competition between Mg and Na uptake and transport to the tops (Alam, 1999, Alfocea et al., 1993).
Phosphorus: Phosphorus, which has a crucial role in the energy metabolism of cells, is involved in a number of anabolic and catabolic pathways (Alam, 1999). A recent study indicates that salinity may increase the P requirement of certain plants. Awad et al. (1990) found that when NaCl increased in the substrate from 10 to 50 or 100 mM, the P content in the tomato leaf increased from 58 to 70 and 97 mmol kg-1 dry weight. The influence of salinity on P accumulation in crop plants is variable and depends on the plant and experimental conditions. In many cases, salinity decreased the P concentration in plant tissue (Sharpley et al., 1992). It is unlikely that Cl and H2PO4 ions are competitive in terms of plant uptake. However, it has also been observed that Cl may have a suppressing effect on P uptake in tomato shoots (Papadopoulos and Rendig, 1983). The presence of Cl as well as SO4 reduced P uptake in barley and sunflower plants. In other cases, a reduction in plant P concentration by salinity may result from the reduced activity of P in the soil solution due to the high ionic strength of the growth media (Awad et al.,1990). Phosphate solubility and its availability are reduced in saline soils not only because of ionic strength effects that reduce the activity of phosphate but also because the P concentration in soil solution is tightly controlled by sorption processes and by the low solubility of Ca-P minerals. It is, therefore, understandable that P concentrations in field-grown agronomic crops decreased as salinity increased in the media. When plants are P-deficient, they may be more sensitive to salinity (Sharpley et al., 1992).
Gibson (1988) observed that adequate phosphorus nutrition was essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed wheat.
Micronutrients: The concentrations of micronutrients in the soil solutions, with the exception of Cl, seem to be low and depend on the physical and chemical characteristics of the soil bodies. The availability of most micronutrients depends on the pH of the soil solution as well as the nature of binding sites on organic and inorganic particle surfaces. In saline soils, the solubility of micronutrients such as Fe, Mn, Zn and Cl is particularly low and plants grown in these soils often experience deficiencies in these elements (Alam, 1999, Page et al., 1990). Nevertheless, the micronutrient concentration in plant shoots may increase, decrease, or have no effect depending on the type of plant tissue, salt tolerance of plant species, salinity, micronutrient concentration, environmental conditions, and/ or abrupt changes in the permeability of the crop cell membranes.
Both Fe and Mn contents were reported to increase in all parts of the salt-treated peanut plants (Chavan and Karadge, 1980). The increase in Fe contents was more prominent than that of Mn. Salinity increased the Fe concentration in the shoots of pea and rice and decreased its concentration in the shoots of barley and corn (Hassan et al., 1970).
A range of soil conditions have been associated with binding of Zn in less plant available forms: high pH (7.0), prolonged submergence and low redox potential common in coastal soils, high organic matter and bicarbonate content, high Mg:Ca ratio, and high available P. Perennial wetness and low redox potential are the major causes of Zn deficiency in peat and coastal saline soils (Neue and Lantin, 1994; Quijano-Guerta et al., 2002). One remarkable feature of Zn-deficiency tolerance in rice is that the trait seems to be associated with tolerance of other abiotic stresses. Rice germplasm identified with tolerance of Zn deficiency from large-scale screening at IRRI often showed cross-tolerance of salinity, P deficiency, and peat soils (Quijano-Guerta et al., 2002). The reasons behind this cross-tolerance are still unknown and could be attributed to better Zn acquisition when Zn is the most limiting factor.
Boron is highly toxic to almost all plants. Sensitive plants may be affected at concentrations > 0.50 gm-3 in soil saturation extract (Maas, 1986).
Future research options for improved nutrient management
A. Adequate phosphorus nutrition has been found essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed plants. P translocation from roots to young shoots should increase in the presence of an additional supply of Ca2+. An increased Ca2+ supply to the plant could be more efficient than P fertilization itself in restoring the P supply to young tissues under saline conditions.
B. Elevated Ca2+ levels may protect the plant from NaCl toxicity by reducing the displacement of membrane-associated Ca2+ by reducing Na+ uptake and transport to the shoots or by a combination of these effects. Ca2+ also improves K+ uptake under NaCl salinity, effectively improving on the Na/K value in the tissues. An increase in the Cl- concentration, on the other hand, in the nutrient media may lead to a reduction in the NO3 content of plants, observed in case of tomato. Under saline conditions, a high Ca2+ supply should alleviate the inhibition of NO3 uptake and increase Na/K selectivity.
C. Supplementation of Ca2+ may also improve the growth rate of the plants in the NO3 treatment based on which it may be suggested that NO3- is possibly a better N source than NH4+.
D. In coastal flooded saline soils measures should be taken to reduce volatilization loss, in particular, either through placement of N-source (urea) at sub-surface depth, through application of slow release source, through use of urease inhibitor, or by adjusting the time of application coinciding with the plants’ active growth stage, for higher N-uptake.
E. Under flooded condition, soil organic matter contributes to Fe and Mn availability through the formation of metallo-organic complexes with organic substances. This phenomenon may be attributed to the production of chelating agents from compost that generally keep the micronutrient elements soluble and, consequently, more available to crop plants. Increased Fe and Mn solubility in flooded soils benefits rice, which has a higher requirement for these elements, than other plants. There is concomitant increase in pH, CO32-, and DTPA extractable Fe and Mn on the submergence of a lowland rice plants.
F. Under flooded conditions, the production of organic complexing compounds and reductions of Fe and Mn tend to enhance the solubility of Zn and Cu in the growth media. Contradictory results are also, however, available. When a soil undergoes reduction by flooding, the breakdown of Fe and Mn oxides may provide an increased surface area with a high adsorptive capacity onto which Cu and Zn may be firmly adsorbed in some soils. Thus, long-term flooding of noncalcareous soils though generally tends to increase the availability of Cu and Mo may depress that of Zn. Deficiency may also occur primarily during early growth of the crop due to immobilization of Zn in roots by bicarbonate ions that are produced in alkaline soils soon after submergence with increase in CO2 concentration in soil.
G. Plant roots and their interaction with the different abiotic and biotic soil components represent a key point in the acquisition of water and essential nutrients. However, anthropogenic effects on the environment – including soil and water deterioration and contamination – could alter these relationships. In addition to these, vegetable production presents diverse problems, which could be mitigated by the use of plant-growth promoting microorganisms (PGPMs). On the soil, PGPMs could contribute to solubilize and/ or acquire essential minerals, making scarce nutrients more available to the plant. On the host, they stimulate several physiological changes that could lead to better growth and render the plant more tolerant to abiotic stresses. Amongst PGPMs, Azospirillum is one of the most studied genera. Even though it colonizes different plant species in an ample variety of soils, its favourable effects on vegetable germination, emergence and growth have not been thoroughly studied. The review (Barrasi et al., 2007) describes the beneficial effects PGPM inoculation could have on vegetables growing either under normal or stressful conditions including salinity, with an emphasis on the use of Azospirillum. Focus may be made on the recent advances on Azospirillum-plant interactions and the bacterial mechanisms of plant growth promotion.
H. For sustainable soil health in order to ensure improved plant nutrient status and its use by the plants the importance of improved soil quality in the coastal plains through higher SOC level of the soils, for which C sequestration is one of the important pathways, may be emphasized since lowlying coastal soils may be a useful sink for higher organic carbon pool for the terrestrial system.
I. Based on earlier observations on acid sulphate soils attempt may be made, in the absence of soil liming, to explore the utility of organic wastes which may improve the performance of Al-tolerant crops while permitting the cultivation of more Al-sensitive crops on these soils. Further research should aim at identifying most appropriate substrate types and application rates for specific acid soil conditions and crop tolerance levels.
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Nutrient Management in Coastal Soils1
H.S.Sen2
Former Director, Central Research Institute for Jute & Allied Fibres (ICAR), P.O. Barrackpore, Nilganj, North 24 Parganas, West Bengal, PIN 700 120
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Locations of the SAARC countries are shown in the map (Fig. 1). The ratio between coastal length to total land area along with the corresponding data on percent share of agriculture to the total GDP show large variations among different SAARC countries (Table 1). Though it will be oversimplification to try to relate the above ratio to percent share of agriculture to country’s GDP it may not be out of place to give some cognizance, considering the two extremes (Maldives vis-à-vis Nepal and Bhutan), that higher the ratio lower may likely be the contribution of agriculture due to lower
productivity as a result of
several technological and other constraints (Yadav,2007) in the coastal areas,
as compared to the inland areas, in general, in most countries.
Coastal areas are diverse in function and form, dynamic, and do not lend themselves well to definition by strict spatial boundaries. Unlike watersheds, there are no exact natural boundaries that unambiguously delineate coastal areas at the global scale. The world coastline extends from 350,000-1,000,000 km in length, depending upon how finely _____________________________________________________________________________
1 Paper presented at the SAARC Workshop on “Nutrient Use Efficiency in Agriculture”, held at CSSRI, Karnal,
Haryana, India, 9-11 Sptember, 2009
2 Present address for communication: 2/74 Naktala, Kolkata, 700 047, West Bengal, India;
Email: hssen2000@hotmail.com, hssen.india@gmail.com
the ‘length’ is resolved. More comprehensively, the coastal ecosystem has been defined by Sen et al. (2000) as representing the transition from terrestrial to marine influences and vice versa. It comprises not only shoreline ecosystems, but also the upland watersheds draining into coastal waters, and the nearshore sub-littoral ecosystems influenced by land-based activities. Functionally, it is a broad interface between land and sea that is strongly influenced by both.
Soil characteristics of this region are unique and vary widely, depending on their physiographic and climatic conditions. Soil salinity is a major constraint for crop production in the coastal ecosystem. Plant growth is seriously constrained owing to, apart from other factors, various nutritional disorders in plants related to the problems in the coastal soils. This calls for a review of the nature and severity of the problems in soils and plants and to suggest improved nutrient management practices in order to alleviate the adverse situations and obtain higher productivity.
Salient characteristics of coastal soils
In India, of the two coastlines length of the East coast is higher than that of the West. The continental shelf is more stable than the coast. The continental shelf of 0-50 m depth spreads over 1,91,972 sq km and that of 0-200 depth over 4,52,060 sq km area. The shelf is wide (50-340 m) along the East coast. The Exclusive Economic Zone is estimated at 2.02 million sq km. Practically, no systematic study was earlier made to demarcate the coastal soils based on well-defined scientific indices valid for the different sub-ecosystems in this country, and possibly elsewhere. Notable among the past works was that of Yadav et al. (1983) who suggested 3.1 million hectare area (including mangrove forests), while Szabolcs (1979) suggested 23.8 million hectare under coastal salinity in India. In Bangladesh 1.4 million hectare is under coastal area. The coastal saline soil has been used by various workers almost synonymously with coastal soil per se which is not correct since coastal soils are not entirely saline in nature. None of the above estimates appears to have been made on sound scientific basis. However, the latest compilation made by Velayutham et al. (1998) on the soil resources and their potentials for different Agro-ecological Sub Regions (AESR) of India show total 10.78 million hectare area is under this ecosystem (including the islands) in India, which has been the first scientific approach for delineation of the coastal soils. Most of the areas have problematic soils, such as saline, alkaline, acid sulphate and marshy and waterlogged soils, situated in the low-lying flooded/ waterlogged areas, mainly along the deltas.
Saline and sodic soils
In case of the saline soil, the salinity status in soil varies widely from ECe 0.5 dS m-1 in monsoon to 50 dS m-1 in summer. Mostly NaCl followed by Na2SO4 are the dominant soluble salts, with abundance of soluble cations in the order Na>Mg>Ca>K, chloride as the predominant anion, and bicarbonate in traces. In India, the soils are, in general, free of sodicity except in a few pockets in the South and West coast. The saline and sodic soils may be defined as follows:
• Saline soils: Soils having pHs less than 8.5, ESP less than 15, and preponderance of chlorides and sulphates of sodium, calcium and magnesium. ECe should be more than 4 dSm-1 at 25C.
• Sodic soils: Soils having pHs more than 8.5, ESP 15, and preponderance of carbonates and bicarbonates of sodium. The ECe is limitless if originating from salts capable of alkali hydrolysis, otherwise it should be less than 4dSm-1 at 25C.
In coastal and delta regions of major rivers of the world the soils are rich in salts due to the presence of saline ground water table at shallow depth. The coastal delta regions are usually having low elevation and it may also be subjected to frequent brackish water inundation. The groundwater in the region is influenced by sea and the brackish water estuaries. The groundwater reaches the soil surface through capillary rise during dry season, evaporates from the soil leaving salts behind, finally making the soil saline and unproductive for agricultural crops. The soil salinity, thus, shows high temporal and spatial variation depending on the elevation, soil texture, climate (evapo-transpiration, precipitation, wind velocity, relative humidity, etc.), drainage, and other related factors. The ingress of seawater or brackish water, and salt-laden sands blown by sea winds are also greatly responsible for formation of coastal salt-affected soils.
Acid sulphate soils
Many of the coastal soils are developed under coastal swamps and mangrove forests leading to the environment of high organic matter and abundant supply of sulphate salts from the sea. In acid sulphate soils, the production of acid from oxidation of pyrites exceeds the neutralizing capacity of soil and the pH falls below 4. The soils are generally rich in organic matter and clay minerals. The total area of actual and potential acid sulphate soils is rather small: about 10 million hectares are known to occur in the tropics, and the world total probably does not exceed 14 million hectares in South and Southeast Asia, West and Southern Africa, and along the South American and Australian coastlines. In addition, some 20 million hectares of coastal peats, mainly in Indonesia, are underlain by potential acid sulphate soil.
Acid sulphate soils develop as a result of the drainage of soils that are rich in pyrites (FeS2), which on oxidation, produce sulphuric acid in presence of excess SO42— ions in soil.
4 FeS2 + 15 O2 + 14 H2O 4 Fe(OH)3 + 8SO4-- + 16H+
12FeS2 + 45O2 + 30 H2O + 4K+ 4KFe3(SO4)2.(OH)6 + 16 SO4-- + 36H+
(Soil pyrite) (Jarosite)
Pyrites accumulate in waterlogged soils that are both rich in organic matter and dissolved sulphates.
2Fe2O3 + 8 SO42- + 16CH2O + O2 4 FeS2 + 16 HCO3- + 8 H2O
(From (From (Soil org. (Diss- (Pyrites)
soil) sea water matter) olved
or soil) oxygen)
The growth of most dry land crops on acid sulphate soils is hampered by the toxic levels of iron and aluminium and the low availability of phosphorus. Toxic levels of dissolved iron plus low phosphorus are the most important adverse factors for wetland rice. In the near-neutral potential acid sulphate soils (Sulfaquents, Sulfic Fluvaquents) high salinity, poor bearing capacity, uneven land surface, and the risk of strong acidification during droughts are the main disadvantages. Young acid sulphate soils (Sulfaquepts), in which the pyritic substratum occurs, near the surface are often more acidic than those soils (Sulfic Tropaquepts, Sulfic Haplaquets) in which this horizon is found at greater depths.
Factors affecting nutrient availability and suggested management strategies
Additions of N and P generally increase the growth of plants grown in N- and P-deficient environments, provided that the plant is not experiencing severe salt stress. When salinity and nutrient deficiency are both factors limiting growth, relief of the most limiting factor will promote growth more than the relief of the less limiting factor. Therefore, addition of a limiting nutrient can either increase, decrease, or have no effect on relative plant tolerance to salinity, depending on the level of salt stress. Failure to account for the severity of salt stress when interpreting salinity x nutrient interactions has caused considerable confusion among researchers in the past.
Poor transformation and availability of nutrients are influenced by high soil and water salinity and other adverse soil properties including inhospitable biological environment, specific ion effects, ion antagonisms and toxicities, etc. In wet acid sulphate soils excess water soluble iron, aluminium, hydrogen sulphide and organic substances, especially fatty acids, are common examples to affect crop growth, particularly rice. Besides, there are other location-specific problems related to nutrient management, viz. highly leached low-fertility lateritic soils with severe erosion problem along with undulating topography, etc.
Physico-chemical factors
Saline and Sodic soils
Nitrogen: Most of the coastal saline soils are deficient in nitrogen. Besides lesser utilization of nitrogeneous fertilizers, especially in coastal areas, the mineralization of soil organic nitrogen, and thus the release of native soil nitrogen to the plant available form, is also slowed down in the salt-affected soils due to decrease in the population as well as activity of microbes with increase in soil salinity. It was revealed from a study at CSSRI, Regional Station Canning that the rates of both mineralization and immobilization of nitrogen in soil were considerably reduced at soil salinity of ECe 10 dSm-1 and above. The increasing loss of nitrogen through NH3 volatilization from applied nitrogeneous fertilizer with increase in soil salinity was studied under field condition at Canning. While comparing among different sources at two soil salinity levels, viz. ECe 3-4 and 7-8 dSm-1, the loss was found to be maximum under ammonium sulphate, followed by prilled urea, and minimum under placement of urea (in paper packet) at 5 cm depth, whereas the conventional slow-release sources as lac-coated urea, sulphur-coated urea, and placement of urea briquette occupied the intermediate positions. Reduction in the loss when compared with prilled urea broadcast under cropped condition was maximum (73.1%) for placement of urea.
Phosphorus: The level of phosphorus in the coastal saline soils is highly variable, and depends largely on the nature and degree of salinity. The availability of soil phosphorus largely depends on the pH of the soil developed after hydrolosis of salt. An increase in soil pH on hydrolysis reduces the availability of soil phosphorus. Very little work has been done on the transformation and availability of P to crops in coastal saline soils.
Potassium: The availability of potassium depends largely on the parent material, clay minerals and weathering conditions. It also depends on the nature and amount of salts in the soil. It was reported from CSSRI, Regional Station Canning that the coastal saline soils are rich in water soluble, exchangeable, non-exchangeable and available K.
Thus, with regard to soil fertility, the coastal soils are usually rich in available K and micronutrients (except Zn), low to medium in available N, and are variable in available P status (Bandyopadhyay et al., 1985, Bandyopadhyay, 1990, Maji and Bandyopadhyay, 1991). Major portion of the applied N fertilizer is lost through volatilization (Sen and Bandyopadhyay, 1987).
Micronutrients: Work done so far on the role of micronutrients in coastal saline soils is meagre. The soils are generally rich in micronutrients, such as Fe, Mn, Zn, Cu, B and Mo. In the coastal sands of Andhra Pradesh in India the soils are deficient in Zn as well as in N, P and K. Iron chlorosis is common to crops like sugarcane (Saccharum officinarum L), jowar (Sorghum bicolor L. Moench), rainfed rice (Oryza sativa L.), etc. As a remedial measure for iron deficiency common in upland rice nurseries, spray of iron salts has been suggested. Such symptoms are also common in groundnut (Arachis hypogaea L.), Cressandra undulaefolia Salisb., etc. In Andhra Pradesh salinity accentuated the zinc deficiency. Along with Zn, P deficiency is also reported from the red and deltaic black soils in this state. High amount of CaCO3 (up to 15%) is congenial to Zn and Fe deficiency disorders. High dose of Zn application is recommended in rice as foliar spray. Zn deficiency was noted from various laterite and lateritic soils in other coastal states in India also. Besides, Al and Fe toxicities too have been reported from a few acid lateritic soils along the coast.
Long term study: Long term field experiment in coastal saline soils in India showed that rice and wheat yield could be maintained with 50% NPK used in conjunction with FYM or green manure (DARE, 2003-04). In another detailed long term experiment conducted (CSSRI, 1990) in Sundarbans (India) it was observed that grain yield of crops in a rice-barley rotation increased significantly only due to the application of N. Application of P did not show any significant increase in the yield of crops in the initial 8 years, after which the yield of barley alone increased due to P application. Available K content was high in the soil. The experiments conducted so far showed that a basal dose of 11 kg P ha-1 for rice and 5.5 kg P ha-1 for barley or for similar upland crops should maintain the fertility status of the soil, whereas the K application may be omitted without any detrimental effect on soil fertility or crop growth. The K removal by the crop was compensated by K added through accumulation and release of non-exchangeable sources.
Acid sulphate soils
In Sri Lanka addition of Gliricidia muculata in combination with phosphate and a small dose of inorganic fertilizer was effective to secure high rice yields (Deturckl et al., http://www2.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub53/pub53-h8.pdf). In India, for the coastal acid sulphate soils of Sundarbans, application of lime, superphosphate and rock phosphate have been found beneficial in improving the soil properties and rice growth (Bandyopadhyay, 1989). Application of Ca-rich oystershell, which is available in plenty, was found beneficial, if applied in powdered form, as an inexpensive alternative soil ameliorating agent. In this soil continuous submergence for one year could not improve the soil properties substantially.
Mongia et al. (1989), while reporting for two soils in Andaman Islands observed that application of lime and phosphorus may be beneficial for lowland rice. In another study on mangrove (Avecenia marina) muds in this island, it has been reported that liming significantly depressed the concentrations of Al, Mn and Fe. Exchangeable Al content also decreased with lime application. The depression of exchangeable Al may be due to precipitation of trivalent Al and Al (OH)3 in the presence of high concentration of OH- ions. Lime application, in general, also reduced the exchangeable and extractable Fe contents of the soil.
In the rice-based systems of the Mekong Delta in Viet Nam, there is a trend towards replacing the traditional rice double cropping with a rotation of wet season rice and dry season upland crops (vegetables and tuber crops). However, under the prevailing acid sulphate soils, the build-up of excessive concentrations of exchangeable aluminium (Al3+) during the aerobic soil phase strongly limit upland cropping to few relatively Al-tolerant tuber crops that farmers grow on raised beds to enhance Al3+ leaching process. Study was conducted on the performance of major tuber crops (cassava, sweet potato and yam) in relation to soil exchangeable Al3+ concentration and as affected by the application of locally produced biogas sludge on farmers’ fields on a typical acid sulphate soil with observation plots laid out on raised beds and the same categorised based on the initial exchangeable Al3+ content of the topsoil in classes of <10, 10–15, and >15 meq Al3+ per 100g. Biogas sludge was applied at 3 Mg ha−1 (dry matter) to tuber crops and compared with an unamended control. It was found that biogas sludge tended to reduce soil exchangeable Al3+ concentrations but significantly increased the tolerance to given Al concentrations with higher tuber yield and P uptake in all tuber crops. However, Al-tolerant cassava showed stronger responses to amendment than Al-sensitive yam. It was concluded that in the absence of soil liming, the application of organic wastes can improve the performance of Al-tolerant while permitting the cultivation of more Al-sensitive crop on acid sulphate soils. Further research aims at identifying most appropriate substrate types and application rates for specific acid soil conditions and (Saleh et al., http://www.tropentag.de/2005/abstracts/links/Saleh_gFbcXBF1.pdf) crop tolerance levels.
Inundated and flooded soils
A flood is an overflow or accumulation of an expanse of water that submerges land. In the sense of ‘flowing water’, the word may also be applied to the inflow of the tide. ‘Coastal flood’ is caused by severe sea storms, or as a result of other hazards (e.g. tsunami or hurricane). The problems of maintaining these areas are accentuated by naturally rising sea levels due to global climate change. Very often the problem becomes much more severe with increase in salinity in the flood water caused by breaching or overflowing of the sea dykes, etc. Flooding thus causes significant change in soil properties depending on the soil, hydrological properties of the flood water, and duration of flood. Among others the most significant changes in soil properties of relevance to plant growth are silt deposition, accumulation of salts, erosion of top soil, organic C status in soil, depletion of soil oxygen ─ resulting in lack of plant metabolic activities and overall reduced soil atmosphere causing significant change in soil nutrient dynamics.
Soil-and plant-produced C2H4 were suggested as factors in reducing root permeability and increasing resistance to water uptake by tobacco (Hunt et al., 1981). It has been reported that soil flooding stress may severely reduce water and ion uptake directly by increasing the permeability of roots to water and indirectly by reducing the size and volume of the plant roots, as well as by decreasing the ion uptake per unit weight of roots. Such uptake implies a disruption in root metabolism and reducing the effective root surface area available for ion uptake. Many such evidences indicate that dysfunction in nutrient absorption by roots under flooding conditions is largely caused by lack of O2 and attendant deleterious metabolic effects. The accumulation of dioxygen in the earth’s atmosphere allowed for evolution of aerobic organisms that uses O2 as the terminal electron acceptor, thus providing a higher yield of energy compared with fermentation and anaerobic respiration. For example, in aerobic metabolism, the complete breakdown of one molecule of glucose yields a total of 38 molecules of adenosine triphosphate (ATP), whereas the anaerobic breakdown of this same glucose molecule to ethanol and CO2 yields only 8 molecules of ATP (Scandalios, 1993). Oxygen stress may inhibit guttation either by increasing the root resistance to water movement or by inhibition of ion transport to the xylem and arrest of vegetation and reproductive growth.
Flooding X Salinity interactions on H2S gas evolution: Bandyopadhyay et al. (1995) measured H2S production in coastal saline and flooded rice field in Sundarbans, West Bengal, India. They observed that sulphates constitute about 40-50 percent of salts in these soils undergoing reduction reactions resulting in transformation of sulphate salts into sulphides including H2S gas which is toxic to plants. It was estimated that during peak periods about 390 to 1180 cm3 of H2S gas per m2 per 24 hours (day time) escaped to the atmosphere from waterlogged rice field, extent of which depended on the soil salinity (ECe 5-9 dSm-1) values and dose of green manure (Sesbania aculeata) applied (0-5 Mg ha-1) in the experiment. Higher soil salinity and application of green manure increased the production of H2S. The concentration of P and micronutrients in both shoot and root of rice decreased considerably at higher concentration of H2S in irrigation water. The insoluble sulphides precipitated on root surfaces (Table 2) and possibly interfered with the growth and nutrient uptake by plants. The acid soluble metallic sulphide (FeS equivalent) content of soil increased from 221 mg per kg soil to 381 mg per kg soil as the H2S content of irrigation water increased from 0 to 80 ppm. The primary cationic constituent of the precipitate on root was Fe, the solubility of which increased considerably after submergence of soil accompanied by a sharp fall in the redox potential. It was inferred that besides the direct toxic effect of H2S gas the indirect effect of sulphate reduction in interfering with the nutrient uptake by plants was also responsible for decrease in yield of rice.
Table 2. Nutrient content in the extract of root coating (10 g fresh root per 20 ml 6N HCl) and the insoluble metallic sulphides (FeS equivalent) in soil at flowering stage of rice in pot culture experiment
H2S in irr. water
(ppm) Soil salinity, ECe 7.4 dSm-1 Soil salinity, ECe 3.8 dSm-1
Insol.
sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm) Insol. sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm)
Fe
Mn
Zn
Cu Fe
Mn
Zn
Cu
0 221 2913 63 68 13 204 2217 52 71 11
20 253 8492 121 142 22 244 7992 98 125 20
40 280 11948 144 181 24 265 10731 127 174 20
60 310 15316 175 218 26 309 14986 160 215 25
80 381 17723 225 268 32 355 17265 196 257 30
LSD
(p=0.05) 26 150 4 5 3 21 161 4 6 2
Adapted from Bandyopadhyay et al., 1995
Biological factors
It is not only the physico-chemical property of soil that is adversely affected by salinity and alkalinity, the soil biology is also highly affected by presence of excess salts in soil, and this, in turn, would affect nutrient availability.
Saline soils
In general, the biological activity of soil microorganisms is low in salt-affected soils due to high salt content. Carbon dioxide evolution and microbial count in soil decrease at higher concentration of salts. There is decrease in free living diazotroph and nitrogenase activity with increasing soil salinity. The inhibition effect also varied with kind of salt being more with Ca and Na type of salts than with K and Mg type of soils among the cations. Among the anions the adverse effect was more with CO3 and Cl salts than with SO4 type of salts. Initial total microbial count of heavy clay saline soil has been reported to have increased (Mahmoud et al., 1972) after reclamation (Table 3). Enzymatic activity in soil also decreases due to increase in soil salinity. The low biological activity of saline soil is largely due to the osmotic and ionic stress induced by salts. In saline soils organic amendments may improve the biological activity up to the salinity level of ECe 26 dSm-1. Thus, there is an urgent need of applying organic matter to saline and sodic soils for their efficient management. At higher salinity, the osmotic and specific ion effects predominate over organic matter in influencing the biological activities in soil. Thus, addition of organic matter alone may not be enough to improve the biological activity. It has been observed that desalinization immediately increases the biological activity, although not completely restored.
One of the most efficient ways of increasing the N status and sustaining crop production in the salt-affected soils is to exploit the benefit of biological N fixation, specially by cultivation of annual and perennial legumes in crop rotation, application of azolla and non-symbiotic N fixing bacteria, etc. Rhizobium spp. show considerable tolerance to high pH and can thrive well in sodic soils but these are considerably sensitive to salinity. Though Rhizobia could thrive and multiply in sodic soil with pH as high as 10.0 the effective contribution of the bacteria towards N-fixation reduces because of sensitivity of the host plants to salinity of soil. Nodulation may be reduced at soil salinity (ECe) as low as 5.0 dSm-1 in barseem (Trifolium alexandrnum), while nodulation was delayed and ineffective nodules were produced in the crop at ECe 15.0 dSm-1 (Bhardwaj, 1975). Legume crops vary in their nodulation performance on different soils (Table 4). Nodulation and N-fixation increase considerably after reclamation of the soils. No nodule may be formed on root when total soluble salt content of soil is about 5.16%. Application of phosphate fertilizer shows improvement in nodulation, and the phosphate x salinity interaction is found significant (Bajpai and Gupta, 1979). At moderate salinity level of ECe 5.0 dSm-1, application of 100 kg P2O5 ha-1 gave symbiotic performance similar to the application of 50 kg P2O5 ha-1 in normal soil. The performance of some forage legumes with Rhizobium spp. (Barseem with: Trifolium alexandrinum, Shaftal with: T. resupinatum, Lucerne with: Medicago sativa, and Senji with: Melilotus parviflora) tested under different soil salinity conditions shows that the mean nodule number, weight and yield of legumes reduced with increase in soil salinity (Table 5). There exists a considerable variation among the Rhizobium spp. in their tolerance to salinity. For example, there may be complete inhibition of barseem Rhizobia as has been obsreved by Pillai and Sen (1966) at 0.5- 0.7% NaCl in culture media, whereas strains from Dolichos showed highest growth at 0.7-1.2 % (Pillai and Sen, 1973). However, addition of organic material (green manure, FYM, etc.) or gypsum to soil may greatly improve the proliferation of Rhizobia.
Effect of salinity on the microbial and biochemical parameters of the salt-affected soils in Sundarbans (India) was studied at nine different sites. The study revealed that the average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating the adverse effect of soil salinity (Tripathi et al., 2006). It was suggested that integrated nutrient management should be very effective for increasing its use efficiency for higher and sustainable yield of crops (Bandyopadhyay et al., 2006, Tripathi et al., 2007). Bandyopadhyay and Rao (2001) were of the opinion to introduce systems approach involving organic, inorganic and biofertlizers compatible with the farmers’ practice. In coastal soil at Tamil Nadu (India), application of agro-industrial wastes significantly improved soil organic carbon, pH, EC and soil bacteria, fungus and actinomycetes population and enhanced the soil fertility status (macro and micro nutrients) and improved the crop productivity of finger millet. Application of pressmud @12.5 t ha-1 recorded better growth and yield of finger millet followed by composted coirpith @ 12.5 t ha-1 (Rangaraj et al., 2007).
Sodic soils
Sodic soils show low urease activity, but high pH does not affect the ammonifying or ureolytic bacteria as much as it affects the nitrifiers which are highly sensitive to both salinity and alkalinity. Production of ammonia from urea may not be affected although nitrification may be severely affected by NaCl, Na2SO4 or CaCl2. However, urease activity is also reduced at higher pH but enhanced with organic carbon content of soil. Among the nitrifiers Nitrobacter is more sensitive than Nitrosomonas. Both salinity and alkalinity do not show marked influence on the growth of Actinomycetes, but fungi and Azotobacter are very sensitive. Blue green algae are tolerant to high pH. Sulphur oxidizing bacteria, Thiobacillus thiooxidans and T. novellas, and iron-sulphur oxidizing bacteria, T. ferrooxidans, have been isolated from sodic soils (Rupela and Tauro, 1973), although their activities are poor. High concentration of sodium chloride inhibits nitrogen fixation by Azotobacter, and the strains isolated from saline soil are less inhibited than others. Negative correlation exists between the Diazotrophic bacteria and salinity (Hassouna et al., 1995). On the other hand, Azospirillum, a non-symbiotic root associative nitrogen fixing bacteria may be recommended as a bio-fertilizer up to total soluble salt content of 2500 ppm in soil. Growth of non-symbiotic bacteria and their N fixation may be improved by application of chemical amendments and phosphatic fertilizers to sodic soils.
Blue Green Algae (BGA) can be used to reclaim the sodic soils due to their ability to secrete organic acids and immobilize Na in the biomass. Though some contradictory reports are available it is true that blue green algae increase the organic matter and N content of saline and sodic soils. Azolla, the water fern, having symbiotic association with BGA, can be grown as a pioneer plant to improve the health, N status, and reclamation of salt-affected soils. By cultivating azolla for a few successive years, the salt content of saline soils may be decreased considerably. The desalinization rate is 1.8 times higher than leaching through water, or 2.1 times than desalinization through Sesbania incorporation (Shang et al.,1987).
Phosphate solubulizing bacteria do not show much effect on the available P status of sodic soil, whereas VAM (vesicular-arbuscular micorrhizae) can grow with their extensive hyphal network up to a salinity level of 12 dSm-1. It may also help the plants in scavenging phosphorus, nitrogen, sulphur and micronutrients in salt-affected soils. The data in Table 6 show that inoculation with VAM resulted in significant increase in VAM colonization of wheat roots and VAM spore count in sodic soil (pH 8.8) over uninoculated controls. The increase in Olsen’s P, as a result P application, inhibits VAM colonization and spores. A positive relation (r=0.74) exists between the VAM colonization and soil available P which indicates mobilization of P by VAM. The inoculation with Glomus mossae and native VAM results in increase in wheat yields. These was no significant increase in P content of both straw and grain as a result of inoculation with VAM in treatments without P application. The inoculation with VAM results in significant increase in Zn content of grain and a positive relation has been reported between the Zn content and VAM in roots.
Table 6. Effect of inoculation with VAM cultures and P application on wheat yield,
available P, VAM spores and % VAM colonization of roots
P level VAM Culture Available P Wheat yield (g pot-1) VAM spores %VAM P Zn
(mg kg-1) Straw Grain (10 g-1) Colonisation - (kg ha-1)-
0 None 4.8 19.3 13.6 17.0 9.7 2.2 53.3
0 Glomus mossae
6.3
24.4
17.6
22.4
76.1
2.1
48.5
0 Native VAM 6.0 24.6 13.6 19.9 59.2 2.2 51.8
40 None 21.5 24.6 17.5 18.7 2.0 3.0 21.5
40 Glomus mossae
19.4
25.6
15.3
10.7
44.5
3.5
36.6
40 Native VAM
17.1 24.3 17.3 15.5 18.4 3.4 36.8
CD (P=0.05) P 1.2 0.7 1.0 NS 5.7 0.2 4.5
VAM NS 0.9 NS NS 7.0 NS 5.5
P X VAM 2.2 1.3 1.8 NS 9.9 NS NS
Source: Sharma and Swarup (1996)
Use of green manure crops like Sesbania spp. (S. aculeata) has been proved very suitable for saline sodic soils (Keating and Fisher, 1985, Rao, 1986, Evans and Rotar, 1986). In favourable climate with proper management, these crops accumulate well over 100 kg N ha-1, mostly through biological N fixation, in 50-55 days, thereby increasing the yield of the following rice crop significantly (Singh et al., 1991). Likewise, application of green leaves of Glyricidia maculata @ 10 tonne ha-1 to the puddled soil before rice transplanting has been found comparable in terms of yield with inorganic fertilizer application of N, P and K at 100, 50 and 50 kg ha-1, respectively (Chavan and Dongale, 1994). Buresh and De Datta (1991), however, were of the view that leguminous green manure and its residue normally are able to meet only partially the N requirement for the following high yielding rice variety. They focused attention on the high loss of N under the anaerobic-aerobic soil cycles typical of legume-lowland rice sequence, as well as under higher production of methane and nitrous oxide from the lowland rice field. However, in view of rising energy cost and limited input availability, recycling of organic wastes and use of renewable sources of biofertilizers, viz. rhizobium cultures for pulse or legume, and blue-green algae for waterlogged rice field may play a significant role in terms of integrated nutrient management for rice in coastal saline soils (Kundu and Pillai, 1992).
Soil quality
The importance of improved soil quality in the coastal plains through higher SOC level of the soils was highlighted by Mandal et al. (2008). Very recently, IRRI characterized lowland rice soils (excluding deepwater rice) in Asia in respect of soil quality (Haefele and Hijmans, 2009), and the study included large areas under coastal plains (Fig. 2). They grouped soil qualities into four categories. These were: Good, Poor, Very Poor and Problem soils. ‘Good’ and ‘Poor soils’ represent those with different degrees of weathering but without major constraints; ‘Very Poor’ represents soils with multiple chemical constraints (acidity, deficiency of phosphorous, or toxicities of iron and aluminum); while ‘Problem soils’ represent those with the most frequently cited soil problems, including acid sulphate, peat, saline, and alkaline soils, which partly cause low fertility, and partly soil chemical toxicity.
Nutritional disorder in plants
Nitrogen: Under salt-stress conditions, the uptake of N by crop plants is generally affected (Alam, 1999). A substantial number of laboratory and greenhouse studies have shown that salinity reduces N accumulation in plants. It has been reported that an increase in Cl- uptake and accumulation is accompanied by a decrease in shoot nitrate concentration. In their experiment, Aslam et al. (1984) have reported that Cl- inhibited NO3- uptake more than SO42- when these anions were present on an equal osmolarity basis. In contrast to the effect of Cl- on NO3- uptake, reported data indicated that increased NO3- in the substrate decreased Cl- uptake and accumulation. The possible decrease in N uptake by increasing salinity has been partly attributed to a probable substitution of Cl- for NO3-. Both the chloride salts of Na and K inhibited the nitrate uptake similarly, suggesting that the process was more sensitive to anionic salinity than to cationic salinity (Aslam et al.,1984). Although, Cl- salts were primarily responsible for reduced NO3- uptake by plants, NO3- reduction in plants was not affected by salinity in studies with barley (Aslam et al.,1984).
Salinity also stimulated nitrate reductase activity in peanut plants but decreased the nitrate reductase activity in tomato and cucumber (Cucumis sativus L.) plants. Reduction in NRA may be due to inhibition of NO3-uptake by Cl- in plant species (Abdul-Kadir and Paulsen, 1982) .
Potassium, Sodium, Calcium & Magnesium: The higher K/Na ratio in shoots of barley cultivars compared with that in root medium solution indicated selective uptake of K, which seems to be among the processes involved in tolerance of cultivars to salinity stress (Alam, 1999, Niazi et al., 1992). Addition of K suppressed the uptake of other cations by rice and tomato plants in the order of Na>Mg>Ca. The depression of Na uptake by K could be due to the antagonism between the two cations. It is widely recognized that a high Na concentration inhibits K uptake by plants. On the other hand, Na appeared to stimulate the K uptake by plants.
Salinity stress has significant inhibitory effects on the concentrations of K, Ca, and Mg as well as stimulatory effects of these nutrient elements on different crop plants (Alam,1999). With the increasing concentration of NaCl salts, K concentration decreased in the leaves, stems and roots, and was accompanied by a substantial increase of Na in the organs.
In lowland rice plants adapt to saline conditions and avoid dehydration by reducing the osmotic potential of plant cells. Antagonistic effects on nutrient uptake may occur, causing deficiencies, particularly of K and Ca under conditions of excessive Na content. For example, Na is antagonistic to K uptake in sodic soils with moderate to high available K, resulting in high Na:K ratio in the rice plant and reduced K transport rates. Sodium-induced inhibition of Ca uptake and transport limits shoot growth. Increasing salinity inhibits nitrate reductase activity, decreases chlorophyll content and photosynthetic rate, and increases the respiration rate and N content in the plant. Plant K and Ca contents decrease but the concentrations of NO3-N, Na, S, and Cl in shoot tissue increase.
The factors related to nutrient disorder that affect different rice varieties are:
• Differences in nutrient uptake under Na stress: Tolerant cultivars have a narrower Na:K ratio (higher K uptake) and greater leaf Ca2+ content than susceptible cultivars.
• Efficient exclusion of Na+ and Cl-: Salt-tolerant rice varieties have a reduced Na+ and Cl- uptake compared with less tolerant cultivars.
Calcium plays a vital nutritional and physiological role in plant metabolism. Calcium, which like K also is an essential mineral nutrient, helps in maintaining membrane integrity, is important in senescence processes, and is known to counteract the harmful effects of Na on crops (Lahaye and Epstein, 1971). Plant growth is dependent on Ca2+, and both cell division and cell elongation processes are affected by the Ca2+ ion concentration.
The presence of Ca2+ as the dominant cation in agricultural soils generally ensures that the absolute Ca2+ level is not a primary growth-limiting factor. As salinity increases, the requirement of plants for Ca2+ increases. The uptake of Ca2+ from the soil solution may decrease because of ion interactions, precipitation, and increases in ionic strength that reduce the activity of Ca2+. In citrus, Ca was found to be effective in reducing the transport of both Na and Cl from the roots to leaves. Maintaining an adequate supply of Ca2+ in the soil solution is an important factor in controlling the severity of specific ion toxicities.
The magnesium content of the leaves of saline-treated bean plants increased, whereas it decreased in the root. Hodson et al. (1982) found potentially toxic concentrations of Mg in salt-marsh soil solution samples and demonstrated that a salt-marsh clone, Agrostis stolonifera, was considerably more tolerant to Mg2+ than was an inland clone. Magnesium concentration of avocado leaves was decreased with an increase in the exchangeable Na in the soil. In rice, Mg transport to the tops was suppressed by Na compared with Mg uptake (Song and Fujiwara, 1996). The Mg content in the roots revealed the competition between Mg and Na uptake and transport to the tops (Alam, 1999, Alfocea et al., 1993).
Phosphorus: Phosphorus, which has a crucial role in the energy metabolism of cells, is involved in a number of anabolic and catabolic pathways (Alam, 1999). A recent study indicates that salinity may increase the P requirement of certain plants. Awad et al. (1990) found that when NaCl increased in the substrate from 10 to 50 or 100 mM, the P content in the tomato leaf increased from 58 to 70 and 97 mmol kg-1 dry weight. The influence of salinity on P accumulation in crop plants is variable and depends on the plant and experimental conditions. In many cases, salinity decreased the P concentration in plant tissue (Sharpley et al., 1992). It is unlikely that Cl and H2PO4 ions are competitive in terms of plant uptake. However, it has also been observed that Cl may have a suppressing effect on P uptake in tomato shoots (Papadopoulos and Rendig, 1983). The presence of Cl as well as SO4 reduced P uptake in barley and sunflower plants. In other cases, a reduction in plant P concentration by salinity may result from the reduced activity of P in the soil solution due to the high ionic strength of the growth media (Awad et al.,1990). Phosphate solubility and its availability are reduced in saline soils not only because of ionic strength effects that reduce the activity of phosphate but also because the P concentration in soil solution is tightly controlled by sorption processes and by the low solubility of Ca-P minerals. It is, therefore, understandable that P concentrations in field-grown agronomic crops decreased as salinity increased in the media. When plants are P-deficient, they may be more sensitive to salinity (Sharpley et al., 1992).
Gibson (1988) observed that adequate phosphorus nutrition was essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed wheat.
Micronutrients: The concentrations of micronutrients in the soil solutions, with the exception of Cl, seem to be low and depend on the physical and chemical characteristics of the soil bodies. The availability of most micronutrients depends on the pH of the soil solution as well as the nature of binding sites on organic and inorganic particle surfaces. In saline soils, the solubility of micronutrients such as Fe, Mn, Zn and Cl is particularly low and plants grown in these soils often experience deficiencies in these elements (Alam, 1999, Page et al., 1990). Nevertheless, the micronutrient concentration in plant shoots may increase, decrease, or have no effect depending on the type of plant tissue, salt tolerance of plant species, salinity, micronutrient concentration, environmental conditions, and/ or abrupt changes in the permeability of the crop cell membranes.
Both Fe and Mn contents were reported to increase in all parts of the salt-treated peanut plants (Chavan and Karadge, 1980). The increase in Fe contents was more prominent than that of Mn. Salinity increased the Fe concentration in the shoots of pea and rice and decreased its concentration in the shoots of barley and corn (Hassan et al., 1970).
A range of soil conditions have been associated with binding of Zn in less plant available forms: high pH (7.0), prolonged submergence and low redox potential common in coastal soils, high organic matter and bicarbonate content, high Mg:Ca ratio, and high available P. Perennial wetness and low redox potential are the major causes of Zn deficiency in peat and coastal saline soils (Neue and Lantin, 1994; Quijano-Guerta et al., 2002). One remarkable feature of Zn-deficiency tolerance in rice is that the trait seems to be associated with tolerance of other abiotic stresses. Rice germplasm identified with tolerance of Zn deficiency from large-scale screening at IRRI often showed cross-tolerance of salinity, P deficiency, and peat soils (Quijano-Guerta et al., 2002). The reasons behind this cross-tolerance are still unknown and could be attributed to better Zn acquisition when Zn is the most limiting factor.
Boron is highly toxic to almost all plants. Sensitive plants may be affected at concentrations > 0.50 gm-3 in soil saturation extract (Maas, 1986).
Future research options for improved nutrient management
A. Adequate phosphorus nutrition has been found essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed plants. P translocation from roots to young shoots should increase in the presence of an additional supply of Ca2+. An increased Ca2+ supply to the plant could be more efficient than P fertilization itself in restoring the P supply to young tissues under saline conditions.
B. Elevated Ca2+ levels may protect the plant from NaCl toxicity by reducing the displacement of membrane-associated Ca2+ by reducing Na+ uptake and transport to the shoots or by a combination of these effects. Ca2+ also improves K+ uptake under NaCl salinity, effectively improving on the Na/K value in the tissues. An increase in the Cl- concentration, on the other hand, in the nutrient media may lead to a reduction in the NO3 content of plants, observed in case of tomato. Under saline conditions, a high Ca2+ supply should alleviate the inhibition of NO3 uptake and increase Na/K selectivity.
C. Supplementation of Ca2+ may also improve the growth rate of the plants in the NO3 treatment based on which it may be suggested that NO3- is possibly a better N source than NH4+.
D. In coastal flooded saline soils measures should be taken to reduce volatilization loss, in particular, either through placement of N-source (urea) at sub-surface depth, through application of slow release source, through use of urease inhibitor, or by adjusting the time of application coinciding with the plants’ active growth stage, for higher N-uptake.
E. Under flooded condition, soil organic matter contributes to Fe and Mn availability through the formation of metallo-organic complexes with organic substances. This phenomenon may be attributed to the production of chelating agents from compost that generally keep the micronutrient elements soluble and, consequently, more available to crop plants. Increased Fe and Mn solubility in flooded soils benefits rice, which has a higher requirement for these elements, than other plants. There is concomitant increase in pH, CO32-, and DTPA extractable Fe and Mn on the submergence of a lowland rice plants.
F. Under flooded conditions, the production of organic complexing compounds and reductions of Fe and Mn tend to enhance the solubility of Zn and Cu in the growth media. Contradictory results are also, however, available. When a soil undergoes reduction by flooding, the breakdown of Fe and Mn oxides may provide an increased surface area with a high adsorptive capacity onto which Cu and Zn may be firmly adsorbed in some soils. Thus, long-term flooding of noncalcareous soils though generally tends to increase the availability of Cu and Mo may depress that of Zn. Deficiency may also occur primarily during early growth of the crop due to immobilization of Zn in roots by bicarbonate ions that are produced in alkaline soils soon after submergence with increase in CO2 concentration in soil.
G. Plant roots and their interaction with the different abiotic and biotic soil components represent a key point in the acquisition of water and essential nutrients. However, anthropogenic effects on the environment – including soil and water deterioration and contamination – could alter these relationships. In addition to these, vegetable production presents diverse problems, which could be mitigated by the use of plant-growth promoting microorganisms (PGPMs). On the soil, PGPMs could contribute to solubilize and/ or acquire essential minerals, making scarce nutrients more available to the plant. On the host, they stimulate several physiological changes that could lead to better growth and render the plant more tolerant to abiotic stresses. Amongst PGPMs, Azospirillum is one of the most studied genera. Even though it colonizes different plant species in an ample variety of soils, its favourable effects on vegetable germination, emergence and growth have not been thoroughly studied. The review (Barrasi et al., 2007) describes the beneficial effects PGPM inoculation could have on vegetables growing either under normal or stressful conditions including salinity, with an emphasis on the use of Azospirillum. Focus may be made on the recent advances on Azospirillum-plant interactions and the bacterial mechanisms of plant growth promotion.
H. For sustainable soil health in order to ensure improved plant nutrient status and its use by the plants the importance of improved soil quality in the coastal plains through higher SOC level of the soils, for which C sequestration is one of the important pathways, may be emphasized since lowlying coastal soils may be a useful sink for higher organic carbon pool for the terrestrial system.
I. Based on earlier observations on acid sulphate soils attempt may be made, in the absence of soil liming, to explore the utility of organic wastes which may improve the performance of Al-tolerant crops while permitting the cultivation of more Al-sensitive crops on these soils. Further research should aim at identifying most appropriate substrate types and application rates for specific acid soil conditions and crop tolerance levels.
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Nutrient Management in Coastal Soils1
H.S.Sen2
Former Director, Central Research Institute for Jute & Allied Fibres (ICAR), P.O. Barrackpore, Nilganj, North 24 Parganas, West Bengal, PIN 700 120
--------------------
Locations of the SAARC countries are shown in the map (Fig. 1). The ratio between coastal length to total land area along with the corresponding data on percent share of agriculture to the total GDP show large variations among different SAARC countries (Table 1). Though it will be oversimplification to try to relate the above ratio to percent share of agriculture to country’s GDP it may not be out of place to give some cognizance, considering the two extremes (Maldives vis-à-vis Nepal and Bhutan), that higher the ratio lower may likely be the contribution of agriculture due to lower
productivity as a result of
several technological and other constraints (Yadav,2007) in the coastal areas,
as compared to the inland areas, in general, in most countries.
Coastal areas are diverse in function and form, dynamic, and do not lend themselves well to definition by strict spatial boundaries. Unlike watersheds, there are no exact natural boundaries that unambiguously delineate coastal areas at the global scale. The world coastline extends from 350,000-1,000,000 km in length, depending upon how finely _____________________________________________________________________________
1 Paper presented at the SAARC Workshop on “Nutrient Use Efficiency in Agriculture”, held at CSSRI, Karnal,
Haryana, India, 9-11 Sptember, 2009
2 Present address for communication: 2/74 Naktala, Kolkata, 700 047, West Bengal, India;
Email: hssen2000@hotmail.com, hssen.india@gmail.com
the ‘length’ is resolved. More comprehensively, the coastal ecosystem has been defined by Sen et al. (2000) as representing the transition from terrestrial to marine influences and vice versa. It comprises not only shoreline ecosystems, but also the upland watersheds draining into coastal waters, and the nearshore sub-littoral ecosystems influenced by land-based activities. Functionally, it is a broad interface between land and sea that is strongly influenced by both.
Soil characteristics of this region are unique and vary widely, depending on their physiographic and climatic conditions. Soil salinity is a major constraint for crop production in the coastal ecosystem. Plant growth is seriously constrained owing to, apart from other factors, various nutritional disorders in plants related to the problems in the coastal soils. This calls for a review of the nature and severity of the problems in soils and plants and to suggest improved nutrient management practices in order to alleviate the adverse situations and obtain higher productivity.
Salient characteristics of coastal soils
In India, of the two coastlines length of the East coast is higher than that of the West. The continental shelf is more stable than the coast. The continental shelf of 0-50 m depth spreads over 1,91,972 sq km and that of 0-200 depth over 4,52,060 sq km area. The shelf is wide (50-340 m) along the East coast. The Exclusive Economic Zone is estimated at 2.02 million sq km. Practically, no systematic study was earlier made to demarcate the coastal soils based on well-defined scientific indices valid for the different sub-ecosystems in this country, and possibly elsewhere. Notable among the past works was that of Yadav et al. (1983) who suggested 3.1 million hectare area (including mangrove forests), while Szabolcs (1979) suggested 23.8 million hectare under coastal salinity in India. In Bangladesh 1.4 million hectare is under coastal area. The coastal saline soil has been used by various workers almost synonymously with coastal soil per se which is not correct since coastal soils are not entirely saline in nature. None of the above estimates appears to have been made on sound scientific basis. However, the latest compilation made by Velayutham et al. (1998) on the soil resources and their potentials for different Agro-ecological Sub Regions (AESR) of India show total 10.78 million hectare area is under this ecosystem (including the islands) in India, which has been the first scientific approach for delineation of the coastal soils. Most of the areas have problematic soils, such as saline, alkaline, acid sulphate and marshy and waterlogged soils, situated in the low-lying flooded/ waterlogged areas, mainly along the deltas.
Saline and sodic soils
In case of the saline soil, the salinity status in soil varies widely from ECe 0.5 dS m-1 in monsoon to 50 dS m-1 in summer. Mostly NaCl followed by Na2SO4 are the dominant soluble salts, with abundance of soluble cations in the order Na>Mg>Ca>K, chloride as the predominant anion, and bicarbonate in traces. In India, the soils are, in general, free of sodicity except in a few pockets in the South and West coast. The saline and sodic soils may be defined as follows:
• Saline soils: Soils having pHs less than 8.5, ESP less than 15, and preponderance of chlorides and sulphates of sodium, calcium and magnesium. ECe should be more than 4 dSm-1 at 25C.
• Sodic soils: Soils having pHs more than 8.5, ESP 15, and preponderance of carbonates and bicarbonates of sodium. The ECe is limitless if originating from salts capable of alkali hydrolysis, otherwise it should be less than 4dSm-1 at 25C.
In coastal and delta regions of major rivers of the world the soils are rich in salts due to the presence of saline ground water table at shallow depth. The coastal delta regions are usually having low elevation and it may also be subjected to frequent brackish water inundation. The groundwater in the region is influenced by sea and the brackish water estuaries. The groundwater reaches the soil surface through capillary rise during dry season, evaporates from the soil leaving salts behind, finally making the soil saline and unproductive for agricultural crops. The soil salinity, thus, shows high temporal and spatial variation depending on the elevation, soil texture, climate (evapo-transpiration, precipitation, wind velocity, relative humidity, etc.), drainage, and other related factors. The ingress of seawater or brackish water, and salt-laden sands blown by sea winds are also greatly responsible for formation of coastal salt-affected soils.
Acid sulphate soils
Many of the coastal soils are developed under coastal swamps and mangrove forests leading to the environment of high organic matter and abundant supply of sulphate salts from the sea. In acid sulphate soils, the production of acid from oxidation of pyrites exceeds the neutralizing capacity of soil and the pH falls below 4. The soils are generally rich in organic matter and clay minerals. The total area of actual and potential acid sulphate soils is rather small: about 10 million hectares are known to occur in the tropics, and the world total probably does not exceed 14 million hectares in South and Southeast Asia, West and Southern Africa, and along the South American and Australian coastlines. In addition, some 20 million hectares of coastal peats, mainly in Indonesia, are underlain by potential acid sulphate soil.
Acid sulphate soils develop as a result of the drainage of soils that are rich in pyrites (FeS2), which on oxidation, produce sulphuric acid in presence of excess SO42— ions in soil.
4 FeS2 + 15 O2 + 14 H2O 4 Fe(OH)3 + 8SO4-- + 16H+
12FeS2 + 45O2 + 30 H2O + 4K+ 4KFe3(SO4)2.(OH)6 + 16 SO4-- + 36H+
(Soil pyrite) (Jarosite)
Pyrites accumulate in waterlogged soils that are both rich in organic matter and dissolved sulphates.
2Fe2O3 + 8 SO42- + 16CH2O + O2 4 FeS2 + 16 HCO3- + 8 H2O
(From (From (Soil org. (Diss- (Pyrites)
soil) sea water matter) olved
or soil) oxygen)
The growth of most dry land crops on acid sulphate soils is hampered by the toxic levels of iron and aluminium and the low availability of phosphorus. Toxic levels of dissolved iron plus low phosphorus are the most important adverse factors for wetland rice. In the near-neutral potential acid sulphate soils (Sulfaquents, Sulfic Fluvaquents) high salinity, poor bearing capacity, uneven land surface, and the risk of strong acidification during droughts are the main disadvantages. Young acid sulphate soils (Sulfaquepts), in which the pyritic substratum occurs, near the surface are often more acidic than those soils (Sulfic Tropaquepts, Sulfic Haplaquets) in which this horizon is found at greater depths.
Factors affecting nutrient availability and suggested management strategies
Additions of N and P generally increase the growth of plants grown in N- and P-deficient environments, provided that the plant is not experiencing severe salt stress. When salinity and nutrient deficiency are both factors limiting growth, relief of the most limiting factor will promote growth more than the relief of the less limiting factor. Therefore, addition of a limiting nutrient can either increase, decrease, or have no effect on relative plant tolerance to salinity, depending on the level of salt stress. Failure to account for the severity of salt stress when interpreting salinity x nutrient interactions has caused considerable confusion among researchers in the past.
Poor transformation and availability of nutrients are influenced by high soil and water salinity and other adverse soil properties including inhospitable biological environment, specific ion effects, ion antagonisms and toxicities, etc. In wet acid sulphate soils excess water soluble iron, aluminium, hydrogen sulphide and organic substances, especially fatty acids, are common examples to affect crop growth, particularly rice. Besides, there are other location-specific problems related to nutrient management, viz. highly leached low-fertility lateritic soils with severe erosion problem along with undulating topography, etc.
Physico-chemical factors
Saline and Sodic soils
Nitrogen: Most of the coastal saline soils are deficient in nitrogen. Besides lesser utilization of nitrogeneous fertilizers, especially in coastal areas, the mineralization of soil organic nitrogen, and thus the release of native soil nitrogen to the plant available form, is also slowed down in the salt-affected soils due to decrease in the population as well as activity of microbes with increase in soil salinity. It was revealed from a study at CSSRI, Regional Station Canning that the rates of both mineralization and immobilization of nitrogen in soil were considerably reduced at soil salinity of ECe 10 dSm-1 and above. The increasing loss of nitrogen through NH3 volatilization from applied nitrogeneous fertilizer with increase in soil salinity was studied under field condition at Canning. While comparing among different sources at two soil salinity levels, viz. ECe 3-4 and 7-8 dSm-1, the loss was found to be maximum under ammonium sulphate, followed by prilled urea, and minimum under placement of urea (in paper packet) at 5 cm depth, whereas the conventional slow-release sources as lac-coated urea, sulphur-coated urea, and placement of urea briquette occupied the intermediate positions. Reduction in the loss when compared with prilled urea broadcast under cropped condition was maximum (73.1%) for placement of urea.
Phosphorus: The level of phosphorus in the coastal saline soils is highly variable, and depends largely on the nature and degree of salinity. The availability of soil phosphorus largely depends on the pH of the soil developed after hydrolosis of salt. An increase in soil pH on hydrolysis reduces the availability of soil phosphorus. Very little work has been done on the transformation and availability of P to crops in coastal saline soils.
Potassium: The availability of potassium depends largely on the parent material, clay minerals and weathering conditions. It also depends on the nature and amount of salts in the soil. It was reported from CSSRI, Regional Station Canning that the coastal saline soils are rich in water soluble, exchangeable, non-exchangeable and available K.
Thus, with regard to soil fertility, the coastal soils are usually rich in available K and micronutrients (except Zn), low to medium in available N, and are variable in available P status (Bandyopadhyay et al., 1985, Bandyopadhyay, 1990, Maji and Bandyopadhyay, 1991). Major portion of the applied N fertilizer is lost through volatilization (Sen and Bandyopadhyay, 1987).
Micronutrients: Work done so far on the role of micronutrients in coastal saline soils is meagre. The soils are generally rich in micronutrients, such as Fe, Mn, Zn, Cu, B and Mo. In the coastal sands of Andhra Pradesh in India the soils are deficient in Zn as well as in N, P and K. Iron chlorosis is common to crops like sugarcane (Saccharum officinarum L), jowar (Sorghum bicolor L. Moench), rainfed rice (Oryza sativa L.), etc. As a remedial measure for iron deficiency common in upland rice nurseries, spray of iron salts has been suggested. Such symptoms are also common in groundnut (Arachis hypogaea L.), Cressandra undulaefolia Salisb., etc. In Andhra Pradesh salinity accentuated the zinc deficiency. Along with Zn, P deficiency is also reported from the red and deltaic black soils in this state. High amount of CaCO3 (up to 15%) is congenial to Zn and Fe deficiency disorders. High dose of Zn application is recommended in rice as foliar spray. Zn deficiency was noted from various laterite and lateritic soils in other coastal states in India also. Besides, Al and Fe toxicities too have been reported from a few acid lateritic soils along the coast.
Long term study: Long term field experiment in coastal saline soils in India showed that rice and wheat yield could be maintained with 50% NPK used in conjunction with FYM or green manure (DARE, 2003-04). In another detailed long term experiment conducted (CSSRI, 1990) in Sundarbans (India) it was observed that grain yield of crops in a rice-barley rotation increased significantly only due to the application of N. Application of P did not show any significant increase in the yield of crops in the initial 8 years, after which the yield of barley alone increased due to P application. Available K content was high in the soil. The experiments conducted so far showed that a basal dose of 11 kg P ha-1 for rice and 5.5 kg P ha-1 for barley or for similar upland crops should maintain the fertility status of the soil, whereas the K application may be omitted without any detrimental effect on soil fertility or crop growth. The K removal by the crop was compensated by K added through accumulation and release of non-exchangeable sources.
Acid sulphate soils
In Sri Lanka addition of Gliricidia muculata in combination with phosphate and a small dose of inorganic fertilizer was effective to secure high rice yields (Deturckl et al., http://www2.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub53/pub53-h8.pdf). In India, for the coastal acid sulphate soils of Sundarbans, application of lime, superphosphate and rock phosphate have been found beneficial in improving the soil properties and rice growth (Bandyopadhyay, 1989). Application of Ca-rich oystershell, which is available in plenty, was found beneficial, if applied in powdered form, as an inexpensive alternative soil ameliorating agent. In this soil continuous submergence for one year could not improve the soil properties substantially.
Mongia et al. (1989), while reporting for two soils in Andaman Islands observed that application of lime and phosphorus may be beneficial for lowland rice. In another study on mangrove (Avecenia marina) muds in this island, it has been reported that liming significantly depressed the concentrations of Al, Mn and Fe. Exchangeable Al content also decreased with lime application. The depression of exchangeable Al may be due to precipitation of trivalent Al and Al (OH)3 in the presence of high concentration of OH- ions. Lime application, in general, also reduced the exchangeable and extractable Fe contents of the soil.
In the rice-based systems of the Mekong Delta in Viet Nam, there is a trend towards replacing the traditional rice double cropping with a rotation of wet season rice and dry season upland crops (vegetables and tuber crops). However, under the prevailing acid sulphate soils, the build-up of excessive concentrations of exchangeable aluminium (Al3+) during the aerobic soil phase strongly limit upland cropping to few relatively Al-tolerant tuber crops that farmers grow on raised beds to enhance Al3+ leaching process. Study was conducted on the performance of major tuber crops (cassava, sweet potato and yam) in relation to soil exchangeable Al3+ concentration and as affected by the application of locally produced biogas sludge on farmers’ fields on a typical acid sulphate soil with observation plots laid out on raised beds and the same categorised based on the initial exchangeable Al3+ content of the topsoil in classes of <10, 10–15, and >15 meq Al3+ per 100g. Biogas sludge was applied at 3 Mg ha−1 (dry matter) to tuber crops and compared with an unamended control. It was found that biogas sludge tended to reduce soil exchangeable Al3+ concentrations but significantly increased the tolerance to given Al concentrations with higher tuber yield and P uptake in all tuber crops. However, Al-tolerant cassava showed stronger responses to amendment than Al-sensitive yam. It was concluded that in the absence of soil liming, the application of organic wastes can improve the performance of Al-tolerant while permitting the cultivation of more Al-sensitive crop on acid sulphate soils. Further research aims at identifying most appropriate substrate types and application rates for specific acid soil conditions and (Saleh et al., http://www.tropentag.de/2005/abstracts/links/Saleh_gFbcXBF1.pdf) crop tolerance levels.
Inundated and flooded soils
A flood is an overflow or accumulation of an expanse of water that submerges land. In the sense of ‘flowing water’, the word may also be applied to the inflow of the tide. ‘Coastal flood’ is caused by severe sea storms, or as a result of other hazards (e.g. tsunami or hurricane). The problems of maintaining these areas are accentuated by naturally rising sea levels due to global climate change. Very often the problem becomes much more severe with increase in salinity in the flood water caused by breaching or overflowing of the sea dykes, etc. Flooding thus causes significant change in soil properties depending on the soil, hydrological properties of the flood water, and duration of flood. Among others the most significant changes in soil properties of relevance to plant growth are silt deposition, accumulation of salts, erosion of top soil, organic C status in soil, depletion of soil oxygen ─ resulting in lack of plant metabolic activities and overall reduced soil atmosphere causing significant change in soil nutrient dynamics.
Soil-and plant-produced C2H4 were suggested as factors in reducing root permeability and increasing resistance to water uptake by tobacco (Hunt et al., 1981). It has been reported that soil flooding stress may severely reduce water and ion uptake directly by increasing the permeability of roots to water and indirectly by reducing the size and volume of the plant roots, as well as by decreasing the ion uptake per unit weight of roots. Such uptake implies a disruption in root metabolism and reducing the effective root surface area available for ion uptake. Many such evidences indicate that dysfunction in nutrient absorption by roots under flooding conditions is largely caused by lack of O2 and attendant deleterious metabolic effects. The accumulation of dioxygen in the earth’s atmosphere allowed for evolution of aerobic organisms that uses O2 as the terminal electron acceptor, thus providing a higher yield of energy compared with fermentation and anaerobic respiration. For example, in aerobic metabolism, the complete breakdown of one molecule of glucose yields a total of 38 molecules of adenosine triphosphate (ATP), whereas the anaerobic breakdown of this same glucose molecule to ethanol and CO2 yields only 8 molecules of ATP (Scandalios, 1993). Oxygen stress may inhibit guttation either by increasing the root resistance to water movement or by inhibition of ion transport to the xylem and arrest of vegetation and reproductive growth.
Flooding X Salinity interactions on H2S gas evolution: Bandyopadhyay et al. (1995) measured H2S production in coastal saline and flooded rice field in Sundarbans, West Bengal, India. They observed that sulphates constitute about 40-50 percent of salts in these soils undergoing reduction reactions resulting in transformation of sulphate salts into sulphides including H2S gas which is toxic to plants. It was estimated that during peak periods about 390 to 1180 cm3 of H2S gas per m2 per 24 hours (day time) escaped to the atmosphere from waterlogged rice field, extent of which depended on the soil salinity (ECe 5-9 dSm-1) values and dose of green manure (Sesbania aculeata) applied (0-5 Mg ha-1) in the experiment. Higher soil salinity and application of green manure increased the production of H2S. The concentration of P and micronutrients in both shoot and root of rice decreased considerably at higher concentration of H2S in irrigation water. The insoluble sulphides precipitated on root surfaces (Table 2) and possibly interfered with the growth and nutrient uptake by plants. The acid soluble metallic sulphide (FeS equivalent) content of soil increased from 221 mg per kg soil to 381 mg per kg soil as the H2S content of irrigation water increased from 0 to 80 ppm. The primary cationic constituent of the precipitate on root was Fe, the solubility of which increased considerably after submergence of soil accompanied by a sharp fall in the redox potential. It was inferred that besides the direct toxic effect of H2S gas the indirect effect of sulphate reduction in interfering with the nutrient uptake by plants was also responsible for decrease in yield of rice.
Table 2. Nutrient content in the extract of root coating (10 g fresh root per 20 ml 6N HCl) and the insoluble metallic sulphides (FeS equivalent) in soil at flowering stage of rice in pot culture experiment
H2S in irr. water
(ppm) Soil salinity, ECe 7.4 dSm-1 Soil salinity, ECe 3.8 dSm-1
Insol.
sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm) Insol. sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm)
Fe
Mn
Zn
Cu Fe
Mn
Zn
Cu
0 221 2913 63 68 13 204 2217 52 71 11
20 253 8492 121 142 22 244 7992 98 125 20
40 280 11948 144 181 24 265 10731 127 174 20
60 310 15316 175 218 26 309 14986 160 215 25
80 381 17723 225 268 32 355 17265 196 257 30
LSD
(p=0.05) 26 150 4 5 3 21 161 4 6 2
Adapted from Bandyopadhyay et al., 1995
Biological factors
It is not only the physico-chemical property of soil that is adversely affected by salinity and alkalinity, the soil biology is also highly affected by presence of excess salts in soil, and this, in turn, would affect nutrient availability.
Saline soils
In general, the biological activity of soil microorganisms is low in salt-affected soils due to high salt content. Carbon dioxide evolution and microbial count in soil decrease at higher concentration of salts. There is decrease in free living diazotroph and nitrogenase activity with increasing soil salinity. The inhibition effect also varied with kind of salt being more with Ca and Na type of salts than with K and Mg type of soils among the cations. Among the anions the adverse effect was more with CO3 and Cl salts than with SO4 type of salts. Initial total microbial count of heavy clay saline soil has been reported to have increased (Mahmoud et al., 1972) after reclamation (Table 3). Enzymatic activity in soil also decreases due to increase in soil salinity. The low biological activity of saline soil is largely due to the osmotic and ionic stress induced by salts. In saline soils organic amendments may improve the biological activity up to the salinity level of ECe 26 dSm-1. Thus, there is an urgent need of applying organic matter to saline and sodic soils for their efficient management. At higher salinity, the osmotic and specific ion effects predominate over organic matter in influencing the biological activities in soil. Thus, addition of organic matter alone may not be enough to improve the biological activity. It has been observed that desalinization immediately increases the biological activity, although not completely restored.
One of the most efficient ways of increasing the N status and sustaining crop production in the salt-affected soils is to exploit the benefit of biological N fixation, specially by cultivation of annual and perennial legumes in crop rotation, application of azolla and non-symbiotic N fixing bacteria, etc. Rhizobium spp. show considerable tolerance to high pH and can thrive well in sodic soils but these are considerably sensitive to salinity. Though Rhizobia could thrive and multiply in sodic soil with pH as high as 10.0 the effective contribution of the bacteria towards N-fixation reduces because of sensitivity of the host plants to salinity of soil. Nodulation may be reduced at soil salinity (ECe) as low as 5.0 dSm-1 in barseem (Trifolium alexandrnum), while nodulation was delayed and ineffective nodules were produced in the crop at ECe 15.0 dSm-1 (Bhardwaj, 1975). Legume crops vary in their nodulation performance on different soils (Table 4). Nodulation and N-fixation increase considerably after reclamation of the soils. No nodule may be formed on root when total soluble salt content of soil is about 5.16%. Application of phosphate fertilizer shows improvement in nodulation, and the phosphate x salinity interaction is found significant (Bajpai and Gupta, 1979). At moderate salinity level of ECe 5.0 dSm-1, application of 100 kg P2O5 ha-1 gave symbiotic performance similar to the application of 50 kg P2O5 ha-1 in normal soil. The performance of some forage legumes with Rhizobium spp. (Barseem with: Trifolium alexandrinum, Shaftal with: T. resupinatum, Lucerne with: Medicago sativa, and Senji with: Melilotus parviflora) tested under different soil salinity conditions shows that the mean nodule number, weight and yield of legumes reduced with increase in soil salinity (Table 5). There exists a considerable variation among the Rhizobium spp. in their tolerance to salinity. For example, there may be complete inhibition of barseem Rhizobia as has been obsreved by Pillai and Sen (1966) at 0.5- 0.7% NaCl in culture media, whereas strains from Dolichos showed highest growth at 0.7-1.2 % (Pillai and Sen, 1973). However, addition of organic material (green manure, FYM, etc.) or gypsum to soil may greatly improve the proliferation of Rhizobia.
Effect of salinity on the microbial and biochemical parameters of the salt-affected soils in Sundarbans (India) was studied at nine different sites. The study revealed that the average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating the adverse effect of soil salinity (Tripathi et al., 2006). It was suggested that integrated nutrient management should be very effective for increasing its use efficiency for higher and sustainable yield of crops (Bandyopadhyay et al., 2006, Tripathi et al., 2007). Bandyopadhyay and Rao (2001) were of the opinion to introduce systems approach involving organic, inorganic and biofertlizers compatible with the farmers’ practice. In coastal soil at Tamil Nadu (India), application of agro-industrial wastes significantly improved soil organic carbon, pH, EC and soil bacteria, fungus and actinomycetes population and enhanced the soil fertility status (macro and micro nutrients) and improved the crop productivity of finger millet. Application of pressmud @12.5 t ha-1 recorded better growth and yield of finger millet followed by composted coirpith @ 12.5 t ha-1 (Rangaraj et al., 2007).
Sodic soils
Sodic soils show low urease activity, but high pH does not affect the ammonifying or ureolytic bacteria as much as it affects the nitrifiers which are highly sensitive to both salinity and alkalinity. Production of ammonia from urea may not be affected although nitrification may be severely affected by NaCl, Na2SO4 or CaCl2. However, urease activity is also reduced at higher pH but enhanced with organic carbon content of soil. Among the nitrifiers Nitrobacter is more sensitive than Nitrosomonas. Both salinity and alkalinity do not show marked influence on the growth of Actinomycetes, but fungi and Azotobacter are very sensitive. Blue green algae are tolerant to high pH. Sulphur oxidizing bacteria, Thiobacillus thiooxidans and T. novellas, and iron-sulphur oxidizing bacteria, T. ferrooxidans, have been isolated from sodic soils (Rupela and Tauro, 1973), although their activities are poor. High concentration of sodium chloride inhibits nitrogen fixation by Azotobacter, and the strains isolated from saline soil are less inhibited than others. Negative correlation exists between the Diazotrophic bacteria and salinity (Hassouna et al., 1995). On the other hand, Azospirillum, a non-symbiotic root associative nitrogen fixing bacteria may be recommended as a bio-fertilizer up to total soluble salt content of 2500 ppm in soil. Growth of non-symbiotic bacteria and their N fixation may be improved by application of chemical amendments and phosphatic fertilizers to sodic soils.
Blue Green Algae (BGA) can be used to reclaim the sodic soils due to their ability to secrete organic acids and immobilize Na in the biomass. Though some contradictory reports are available it is true that blue green algae increase the organic matter and N content of saline and sodic soils. Azolla, the water fern, having symbiotic association with BGA, can be grown as a pioneer plant to improve the health, N status, and reclamation of salt-affected soils. By cultivating azolla for a few successive years, the salt content of saline soils may be decreased considerably. The desalinization rate is 1.8 times higher than leaching through water, or 2.1 times than desalinization through Sesbania incorporation (Shang et al.,1987).
Phosphate solubulizing bacteria do not show much effect on the available P status of sodic soil, whereas VAM (vesicular-arbuscular micorrhizae) can grow with their extensive hyphal network up to a salinity level of 12 dSm-1. It may also help the plants in scavenging phosphorus, nitrogen, sulphur and micronutrients in salt-affected soils. The data in Table 6 show that inoculation with VAM resulted in significant increase in VAM colonization of wheat roots and VAM spore count in sodic soil (pH 8.8) over uninoculated controls. The increase in Olsen’s P, as a result P application, inhibits VAM colonization and spores. A positive relation (r=0.74) exists between the VAM colonization and soil available P which indicates mobilization of P by VAM. The inoculation with Glomus mossae and native VAM results in increase in wheat yields. These was no significant increase in P content of both straw and grain as a result of inoculation with VAM in treatments without P application. The inoculation with VAM results in significant increase in Zn content of grain and a positive relation has been reported between the Zn content and VAM in roots.
Table 6. Effect of inoculation with VAM cultures and P application on wheat yield,
available P, VAM spores and % VAM colonization of roots
P level VAM Culture Available P Wheat yield (g pot-1) VAM spores %VAM P Zn
(mg kg-1) Straw Grain (10 g-1) Colonisation - (kg ha-1)-
0 None 4.8 19.3 13.6 17.0 9.7 2.2 53.3
0 Glomus mossae
6.3
24.4
17.6
22.4
76.1
2.1
48.5
0 Native VAM 6.0 24.6 13.6 19.9 59.2 2.2 51.8
40 None 21.5 24.6 17.5 18.7 2.0 3.0 21.5
40 Glomus mossae
19.4
25.6
15.3
10.7
44.5
3.5
36.6
40 Native VAM
17.1 24.3 17.3 15.5 18.4 3.4 36.8
CD (P=0.05) P 1.2 0.7 1.0 NS 5.7 0.2 4.5
VAM NS 0.9 NS NS 7.0 NS 5.5
P X VAM 2.2 1.3 1.8 NS 9.9 NS NS
Source: Sharma and Swarup (1996)
Use of green manure crops like Sesbania spp. (S. aculeata) has been proved very suitable for saline sodic soils (Keating and Fisher, 1985, Rao, 1986, Evans and Rotar, 1986). In favourable climate with proper management, these crops accumulate well over 100 kg N ha-1, mostly through biological N fixation, in 50-55 days, thereby increasing the yield of the following rice crop significantly (Singh et al., 1991). Likewise, application of green leaves of Glyricidia maculata @ 10 tonne ha-1 to the puddled soil before rice transplanting has been found comparable in terms of yield with inorganic fertilizer application of N, P and K at 100, 50 and 50 kg ha-1, respectively (Chavan and Dongale, 1994). Buresh and De Datta (1991), however, were of the view that leguminous green manure and its residue normally are able to meet only partially the N requirement for the following high yielding rice variety. They focused attention on the high loss of N under the anaerobic-aerobic soil cycles typical of legume-lowland rice sequence, as well as under higher production of methane and nitrous oxide from the lowland rice field. However, in view of rising energy cost and limited input availability, recycling of organic wastes and use of renewable sources of biofertilizers, viz. rhizobium cultures for pulse or legume, and blue-green algae for waterlogged rice field may play a significant role in terms of integrated nutrient management for rice in coastal saline soils (Kundu and Pillai, 1992).
Soil quality
The importance of improved soil quality in the coastal plains through higher SOC level of the soils was highlighted by Mandal et al. (2008). Very recently, IRRI characterized lowland rice soils (excluding deepwater rice) in Asia in respect of soil quality (Haefele and Hijmans, 2009), and the study included large areas under coastal plains (Fig. 2). They grouped soil qualities into four categories. These were: Good, Poor, Very Poor and Problem soils. ‘Good’ and ‘Poor soils’ represent those with different degrees of weathering but without major constraints; ‘Very Poor’ represents soils with multiple chemical constraints (acidity, deficiency of phosphorous, or toxicities of iron and aluminum); while ‘Problem soils’ represent those with the most frequently cited soil problems, including acid sulphate, peat, saline, and alkaline soils, which partly cause low fertility, and partly soil chemical toxicity.
Nutritional disorder in plants
Nitrogen: Under salt-stress conditions, the uptake of N by crop plants is generally affected (Alam, 1999). A substantial number of laboratory and greenhouse studies have shown that salinity reduces N accumulation in plants. It has been reported that an increase in Cl- uptake and accumulation is accompanied by a decrease in shoot nitrate concentration. In their experiment, Aslam et al. (1984) have reported that Cl- inhibited NO3- uptake more than SO42- when these anions were present on an equal osmolarity basis. In contrast to the effect of Cl- on NO3- uptake, reported data indicated that increased NO3- in the substrate decreased Cl- uptake and accumulation. The possible decrease in N uptake by increasing salinity has been partly attributed to a probable substitution of Cl- for NO3-. Both the chloride salts of Na and K inhibited the nitrate uptake similarly, suggesting that the process was more sensitive to anionic salinity than to cationic salinity (Aslam et al.,1984). Although, Cl- salts were primarily responsible for reduced NO3- uptake by plants, NO3- reduction in plants was not affected by salinity in studies with barley (Aslam et al.,1984).
Salinity also stimulated nitrate reductase activity in peanut plants but decreased the nitrate reductase activity in tomato and cucumber (Cucumis sativus L.) plants. Reduction in NRA may be due to inhibition of NO3-uptake by Cl- in plant species (Abdul-Kadir and Paulsen, 1982) .
Potassium, Sodium, Calcium & Magnesium: The higher K/Na ratio in shoots of barley cultivars compared with that in root medium solution indicated selective uptake of K, which seems to be among the processes involved in tolerance of cultivars to salinity stress (Alam, 1999, Niazi et al., 1992). Addition of K suppressed the uptake of other cations by rice and tomato plants in the order of Na>Mg>Ca. The depression of Na uptake by K could be due to the antagonism between the two cations. It is widely recognized that a high Na concentration inhibits K uptake by plants. On the other hand, Na appeared to stimulate the K uptake by plants.
Salinity stress has significant inhibitory effects on the concentrations of K, Ca, and Mg as well as stimulatory effects of these nutrient elements on different crop plants (Alam,1999). With the increasing concentration of NaCl salts, K concentration decreased in the leaves, stems and roots, and was accompanied by a substantial increase of Na in the organs.
In lowland rice plants adapt to saline conditions and avoid dehydration by reducing the osmotic potential of plant cells. Antagonistic effects on nutrient uptake may occur, causing deficiencies, particularly of K and Ca under conditions of excessive Na content. For example, Na is antagonistic to K uptake in sodic soils with moderate to high available K, resulting in high Na:K ratio in the rice plant and reduced K transport rates. Sodium-induced inhibition of Ca uptake and transport limits shoot growth. Increasing salinity inhibits nitrate reductase activity, decreases chlorophyll content and photosynthetic rate, and increases the respiration rate and N content in the plant. Plant K and Ca contents decrease but the concentrations of NO3-N, Na, S, and Cl in shoot tissue increase.
The factors related to nutrient disorder that affect different rice varieties are:
• Differences in nutrient uptake under Na stress: Tolerant cultivars have a narrower Na:K ratio (higher K uptake) and greater leaf Ca2+ content than susceptible cultivars.
• Efficient exclusion of Na+ and Cl-: Salt-tolerant rice varieties have a reduced Na+ and Cl- uptake compared with less tolerant cultivars.
Calcium plays a vital nutritional and physiological role in plant metabolism. Calcium, which like K also is an essential mineral nutrient, helps in maintaining membrane integrity, is important in senescence processes, and is known to counteract the harmful effects of Na on crops (Lahaye and Epstein, 1971). Plant growth is dependent on Ca2+, and both cell division and cell elongation processes are affected by the Ca2+ ion concentration.
The presence of Ca2+ as the dominant cation in agricultural soils generally ensures that the absolute Ca2+ level is not a primary growth-limiting factor. As salinity increases, the requirement of plants for Ca2+ increases. The uptake of Ca2+ from the soil solution may decrease because of ion interactions, precipitation, and increases in ionic strength that reduce the activity of Ca2+. In citrus, Ca was found to be effective in reducing the transport of both Na and Cl from the roots to leaves. Maintaining an adequate supply of Ca2+ in the soil solution is an important factor in controlling the severity of specific ion toxicities.
The magnesium content of the leaves of saline-treated bean plants increased, whereas it decreased in the root. Hodson et al. (1982) found potentially toxic concentrations of Mg in salt-marsh soil solution samples and demonstrated that a salt-marsh clone, Agrostis stolonifera, was considerably more tolerant to Mg2+ than was an inland clone. Magnesium concentration of avocado leaves was decreased with an increase in the exchangeable Na in the soil. In rice, Mg transport to the tops was suppressed by Na compared with Mg uptake (Song and Fujiwara, 1996). The Mg content in the roots revealed the competition between Mg and Na uptake and transport to the tops (Alam, 1999, Alfocea et al., 1993).
Phosphorus: Phosphorus, which has a crucial role in the energy metabolism of cells, is involved in a number of anabolic and catabolic pathways (Alam, 1999). A recent study indicates that salinity may increase the P requirement of certain plants. Awad et al. (1990) found that when NaCl increased in the substrate from 10 to 50 or 100 mM, the P content in the tomato leaf increased from 58 to 70 and 97 mmol kg-1 dry weight. The influence of salinity on P accumulation in crop plants is variable and depends on the plant and experimental conditions. In many cases, salinity decreased the P concentration in plant tissue (Sharpley et al., 1992). It is unlikely that Cl and H2PO4 ions are competitive in terms of plant uptake. However, it has also been observed that Cl may have a suppressing effect on P uptake in tomato shoots (Papadopoulos and Rendig, 1983). The presence of Cl as well as SO4 reduced P uptake in barley and sunflower plants. In other cases, a reduction in plant P concentration by salinity may result from the reduced activity of P in the soil solution due to the high ionic strength of the growth media (Awad et al.,1990). Phosphate solubility and its availability are reduced in saline soils not only because of ionic strength effects that reduce the activity of phosphate but also because the P concentration in soil solution is tightly controlled by sorption processes and by the low solubility of Ca-P minerals. It is, therefore, understandable that P concentrations in field-grown agronomic crops decreased as salinity increased in the media. When plants are P-deficient, they may be more sensitive to salinity (Sharpley et al., 1992).
Gibson (1988) observed that adequate phosphorus nutrition was essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed wheat.
Micronutrients: The concentrations of micronutrients in the soil solutions, with the exception of Cl, seem to be low and depend on the physical and chemical characteristics of the soil bodies. The availability of most micronutrients depends on the pH of the soil solution as well as the nature of binding sites on organic and inorganic particle surfaces. In saline soils, the solubility of micronutrients such as Fe, Mn, Zn and Cl is particularly low and plants grown in these soils often experience deficiencies in these elements (Alam, 1999, Page et al., 1990). Nevertheless, the micronutrient concentration in plant shoots may increase, decrease, or have no effect depending on the type of plant tissue, salt tolerance of plant species, salinity, micronutrient concentration, environmental conditions, and/ or abrupt changes in the permeability of the crop cell membranes.
Both Fe and Mn contents were reported to increase in all parts of the salt-treated peanut plants (Chavan and Karadge, 1980). The increase in Fe contents was more prominent than that of Mn. Salinity increased the Fe concentration in the shoots of pea and rice and decreased its concentration in the shoots of barley and corn (Hassan et al., 1970).
A range of soil conditions have been associated with binding of Zn in less plant available forms: high pH (7.0), prolonged submergence and low redox potential common in coastal soils, high organic matter and bicarbonate content, high Mg:Ca ratio, and high available P. Perennial wetness and low redox potential are the major causes of Zn deficiency in peat and coastal saline soils (Neue and Lantin, 1994; Quijano-Guerta et al., 2002). One remarkable feature of Zn-deficiency tolerance in rice is that the trait seems to be associated with tolerance of other abiotic stresses. Rice germplasm identified with tolerance of Zn deficiency from large-scale screening at IRRI often showed cross-tolerance of salinity, P deficiency, and peat soils (Quijano-Guerta et al., 2002). The reasons behind this cross-tolerance are still unknown and could be attributed to better Zn acquisition when Zn is the most limiting factor.
Boron is highly toxic to almost all plants. Sensitive plants may be affected at concentrations > 0.50 gm-3 in soil saturation extract (Maas, 1986).
Future research options for improved nutrient management
A. Adequate phosphorus nutrition has been found essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed plants. P translocation from roots to young shoots should increase in the presence of an additional supply of Ca2+. An increased Ca2+ supply to the plant could be more efficient than P fertilization itself in restoring the P supply to young tissues under saline conditions.
B. Elevated Ca2+ levels may protect the plant from NaCl toxicity by reducing the displacement of membrane-associated Ca2+ by reducing Na+ uptake and transport to the shoots or by a combination of these effects. Ca2+ also improves K+ uptake under NaCl salinity, effectively improving on the Na/K value in the tissues. An increase in the Cl- concentration, on the other hand, in the nutrient media may lead to a reduction in the NO3 content of plants, observed in case of tomato. Under saline conditions, a high Ca2+ supply should alleviate the inhibition of NO3 uptake and increase Na/K selectivity.
C. Supplementation of Ca2+ may also improve the growth rate of the plants in the NO3 treatment based on which it may be suggested that NO3- is possibly a better N source than NH4+.
D. In coastal flooded saline soils measures should be taken to reduce volatilization loss, in particular, either through placement of N-source (urea) at sub-surface depth, through application of slow release source, through use of urease inhibitor, or by adjusting the time of application coinciding with the plants’ active growth stage, for higher N-uptake.
E. Under flooded condition, soil organic matter contributes to Fe and Mn availability through the formation of metallo-organic complexes with organic substances. This phenomenon may be attributed to the production of chelating agents from compost that generally keep the micronutrient elements soluble and, consequently, more available to crop plants. Increased Fe and Mn solubility in flooded soils benefits rice, which has a higher requirement for these elements, than other plants. There is concomitant increase in pH, CO32-, and DTPA extractable Fe and Mn on the submergence of a lowland rice plants.
F. Under flooded conditions, the production of organic complexing compounds and reductions of Fe and Mn tend to enhance the solubility of Zn and Cu in the growth media. Contradictory results are also, however, available. When a soil undergoes reduction by flooding, the breakdown of Fe and Mn oxides may provide an increased surface area with a high adsorptive capacity onto which Cu and Zn may be firmly adsorbed in some soils. Thus, long-term flooding of noncalcareous soils though generally tends to increase the availability of Cu and Mo may depress that of Zn. Deficiency may also occur primarily during early growth of the crop due to immobilization of Zn in roots by bicarbonate ions that are produced in alkaline soils soon after submergence with increase in CO2 concentration in soil.
G. Plant roots and their interaction with the different abiotic and biotic soil components represent a key point in the acquisition of water and essential nutrients. However, anthropogenic effects on the environment – including soil and water deterioration and contamination – could alter these relationships. In addition to these, vegetable production presents diverse problems, which could be mitigated by the use of plant-growth promoting microorganisms (PGPMs). On the soil, PGPMs could contribute to solubilize and/ or acquire essential minerals, making scarce nutrients more available to the plant. On the host, they stimulate several physiological changes that could lead to better growth and render the plant more tolerant to abiotic stresses. Amongst PGPMs, Azospirillum is one of the most studied genera. Even though it colonizes different plant species in an ample variety of soils, its favourable effects on vegetable germination, emergence and growth have not been thoroughly studied. The review (Barrasi et al., 2007) describes the beneficial effects PGPM inoculation could have on vegetables growing either under normal or stressful conditions including salinity, with an emphasis on the use of Azospirillum. Focus may be made on the recent advances on Azospirillum-plant interactions and the bacterial mechanisms of plant growth promotion.
H. For sustainable soil health in order to ensure improved plant nutrient status and its use by the plants the importance of improved soil quality in the coastal plains through higher SOC level of the soils, for which C sequestration is one of the important pathways, may be emphasized since lowlying coastal soils may be a useful sink for higher organic carbon pool for the terrestrial system.
I. Based on earlier observations on acid sulphate soils attempt may be made, in the absence of soil liming, to explore the utility of organic wastes which may improve the performance of Al-tolerant crops while permitting the cultivation of more Al-sensitive crops on these soils. Further research should aim at identifying most appropriate substrate types and application rates for specific acid soil conditions and crop tolerance levels.
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H.S.Sen2
Former Director, Central Research Institute for Jute & Allied Fibres (ICAR), P.O. Barrackpore, Nilganj, North 24 Parganas, West Bengal, PIN 700 120
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Locations of the SAARC countries are shown in the map (Fig. 1). The ratio between coastal length to total land area along with the corresponding data on percent share of agriculture to the total GDP show large variations among different SAARC countries (Table 1). Though it will be oversimplification to try to relate the above ratio to percent share of agriculture to country’s GDP it may not be out of place to give some cognizance, considering the two extremes (Maldives vis-à-vis Nepal and Bhutan), that higher the ratio lower may likely be the contribution of agriculture due to lower
productivity as a result of
several technological and other constraints (Yadav,2007) in the coastal areas,
as compared to the inland areas, in general, in most countries.
Coastal areas are diverse in function and form, dynamic, and do not lend themselves well to definition by strict spatial boundaries. Unlike watersheds, there are no exact natural boundaries that unambiguously delineate coastal areas at the global scale. The world coastline extends from 350,000-1,000,000 km in length, depending upon how finely _____________________________________________________________________________
1 Paper presented at the SAARC Workshop on “Nutrient Use Efficiency in Agriculture”, held at CSSRI, Karnal,
Haryana, India, 9-11 Sptember, 2009
2 Present address for communication: 2/74 Naktala, Kolkata, 700 047, West Bengal, India;
Email: hssen2000@hotmail.com, hssen.india@gmail.com
the ‘length’ is resolved. More comprehensively, the coastal ecosystem has been defined by Sen et al. (2000) as representing the transition from terrestrial to marine influences and vice versa. It comprises not only shoreline ecosystems, but also the upland watersheds draining into coastal waters, and the nearshore sub-littoral ecosystems influenced by land-based activities. Functionally, it is a broad interface between land and sea that is strongly influenced by both.
Soil characteristics of this region are unique and vary widely, depending on their physiographic and climatic conditions. Soil salinity is a major constraint for crop production in the coastal ecosystem. Plant growth is seriously constrained owing to, apart from other factors, various nutritional disorders in plants related to the problems in the coastal soils. This calls for a review of the nature and severity of the problems in soils and plants and to suggest improved nutrient management practices in order to alleviate the adverse situations and obtain higher productivity.
Salient characteristics of coastal soils
In India, of the two coastlines length of the East coast is higher than that of the West. The continental shelf is more stable than the coast. The continental shelf of 0-50 m depth spreads over 1,91,972 sq km and that of 0-200 depth over 4,52,060 sq km area. The shelf is wide (50-340 m) along the East coast. The Exclusive Economic Zone is estimated at 2.02 million sq km. Practically, no systematic study was earlier made to demarcate the coastal soils based on well-defined scientific indices valid for the different sub-ecosystems in this country, and possibly elsewhere. Notable among the past works was that of Yadav et al. (1983) who suggested 3.1 million hectare area (including mangrove forests), while Szabolcs (1979) suggested 23.8 million hectare under coastal salinity in India. In Bangladesh 1.4 million hectare is under coastal area. The coastal saline soil has been used by various workers almost synonymously with coastal soil per se which is not correct since coastal soils are not entirely saline in nature. None of the above estimates appears to have been made on sound scientific basis. However, the latest compilation made by Velayutham et al. (1998) on the soil resources and their potentials for different Agro-ecological Sub Regions (AESR) of India show total 10.78 million hectare area is under this ecosystem (including the islands) in India, which has been the first scientific approach for delineation of the coastal soils. Most of the areas have problematic soils, such as saline, alkaline, acid sulphate and marshy and waterlogged soils, situated in the low-lying flooded/ waterlogged areas, mainly along the deltas.
Saline and sodic soils
In case of the saline soil, the salinity status in soil varies widely from ECe 0.5 dS m-1 in monsoon to 50 dS m-1 in summer. Mostly NaCl followed by Na2SO4 are the dominant soluble salts, with abundance of soluble cations in the order Na>Mg>Ca>K, chloride as the predominant anion, and bicarbonate in traces. In India, the soils are, in general, free of sodicity except in a few pockets in the South and West coast. The saline and sodic soils may be defined as follows:
• Saline soils: Soils having pHs less than 8.5, ESP less than 15, and preponderance of chlorides and sulphates of sodium, calcium and magnesium. ECe should be more than 4 dSm-1 at 25C.
• Sodic soils: Soils having pHs more than 8.5, ESP 15, and preponderance of carbonates and bicarbonates of sodium. The ECe is limitless if originating from salts capable of alkali hydrolysis, otherwise it should be less than 4dSm-1 at 25C.
In coastal and delta regions of major rivers of the world the soils are rich in salts due to the presence of saline ground water table at shallow depth. The coastal delta regions are usually having low elevation and it may also be subjected to frequent brackish water inundation. The groundwater in the region is influenced by sea and the brackish water estuaries. The groundwater reaches the soil surface through capillary rise during dry season, evaporates from the soil leaving salts behind, finally making the soil saline and unproductive for agricultural crops. The soil salinity, thus, shows high temporal and spatial variation depending on the elevation, soil texture, climate (evapo-transpiration, precipitation, wind velocity, relative humidity, etc.), drainage, and other related factors. The ingress of seawater or brackish water, and salt-laden sands blown by sea winds are also greatly responsible for formation of coastal salt-affected soils.
Acid sulphate soils
Many of the coastal soils are developed under coastal swamps and mangrove forests leading to the environment of high organic matter and abundant supply of sulphate salts from the sea. In acid sulphate soils, the production of acid from oxidation of pyrites exceeds the neutralizing capacity of soil and the pH falls below 4. The soils are generally rich in organic matter and clay minerals. The total area of actual and potential acid sulphate soils is rather small: about 10 million hectares are known to occur in the tropics, and the world total probably does not exceed 14 million hectares in South and Southeast Asia, West and Southern Africa, and along the South American and Australian coastlines. In addition, some 20 million hectares of coastal peats, mainly in Indonesia, are underlain by potential acid sulphate soil.
Acid sulphate soils develop as a result of the drainage of soils that are rich in pyrites (FeS2), which on oxidation, produce sulphuric acid in presence of excess SO42— ions in soil.
4 FeS2 + 15 O2 + 14 H2O 4 Fe(OH)3 + 8SO4-- + 16H+
12FeS2 + 45O2 + 30 H2O + 4K+ 4KFe3(SO4)2.(OH)6 + 16 SO4-- + 36H+
(Soil pyrite) (Jarosite)
Pyrites accumulate in waterlogged soils that are both rich in organic matter and dissolved sulphates.
2Fe2O3 + 8 SO42- + 16CH2O + O2 4 FeS2 + 16 HCO3- + 8 H2O
(From (From (Soil org. (Diss- (Pyrites)
soil) sea water matter) olved
or soil) oxygen)
The growth of most dry land crops on acid sulphate soils is hampered by the toxic levels of iron and aluminium and the low availability of phosphorus. Toxic levels of dissolved iron plus low phosphorus are the most important adverse factors for wetland rice. In the near-neutral potential acid sulphate soils (Sulfaquents, Sulfic Fluvaquents) high salinity, poor bearing capacity, uneven land surface, and the risk of strong acidification during droughts are the main disadvantages. Young acid sulphate soils (Sulfaquepts), in which the pyritic substratum occurs, near the surface are often more acidic than those soils (Sulfic Tropaquepts, Sulfic Haplaquets) in which this horizon is found at greater depths.
Factors affecting nutrient availability and suggested management strategies
Additions of N and P generally increase the growth of plants grown in N- and P-deficient environments, provided that the plant is not experiencing severe salt stress. When salinity and nutrient deficiency are both factors limiting growth, relief of the most limiting factor will promote growth more than the relief of the less limiting factor. Therefore, addition of a limiting nutrient can either increase, decrease, or have no effect on relative plant tolerance to salinity, depending on the level of salt stress. Failure to account for the severity of salt stress when interpreting salinity x nutrient interactions has caused considerable confusion among researchers in the past.
Poor transformation and availability of nutrients are influenced by high soil and water salinity and other adverse soil properties including inhospitable biological environment, specific ion effects, ion antagonisms and toxicities, etc. In wet acid sulphate soils excess water soluble iron, aluminium, hydrogen sulphide and organic substances, especially fatty acids, are common examples to affect crop growth, particularly rice. Besides, there are other location-specific problems related to nutrient management, viz. highly leached low-fertility lateritic soils with severe erosion problem along with undulating topography, etc.
Physico-chemical factors
Saline and Sodic soils
Nitrogen: Most of the coastal saline soils are deficient in nitrogen. Besides lesser utilization of nitrogeneous fertilizers, especially in coastal areas, the mineralization of soil organic nitrogen, and thus the release of native soil nitrogen to the plant available form, is also slowed down in the salt-affected soils due to decrease in the population as well as activity of microbes with increase in soil salinity. It was revealed from a study at CSSRI, Regional Station Canning that the rates of both mineralization and immobilization of nitrogen in soil were considerably reduced at soil salinity of ECe 10 dSm-1 and above. The increasing loss of nitrogen through NH3 volatilization from applied nitrogeneous fertilizer with increase in soil salinity was studied under field condition at Canning. While comparing among different sources at two soil salinity levels, viz. ECe 3-4 and 7-8 dSm-1, the loss was found to be maximum under ammonium sulphate, followed by prilled urea, and minimum under placement of urea (in paper packet) at 5 cm depth, whereas the conventional slow-release sources as lac-coated urea, sulphur-coated urea, and placement of urea briquette occupied the intermediate positions. Reduction in the loss when compared with prilled urea broadcast under cropped condition was maximum (73.1%) for placement of urea.
Phosphorus: The level of phosphorus in the coastal saline soils is highly variable, and depends largely on the nature and degree of salinity. The availability of soil phosphorus largely depends on the pH of the soil developed after hydrolosis of salt. An increase in soil pH on hydrolysis reduces the availability of soil phosphorus. Very little work has been done on the transformation and availability of P to crops in coastal saline soils.
Potassium: The availability of potassium depends largely on the parent material, clay minerals and weathering conditions. It also depends on the nature and amount of salts in the soil. It was reported from CSSRI, Regional Station Canning that the coastal saline soils are rich in water soluble, exchangeable, non-exchangeable and available K.
Thus, with regard to soil fertility, the coastal soils are usually rich in available K and micronutrients (except Zn), low to medium in available N, and are variable in available P status (Bandyopadhyay et al., 1985, Bandyopadhyay, 1990, Maji and Bandyopadhyay, 1991). Major portion of the applied N fertilizer is lost through volatilization (Sen and Bandyopadhyay, 1987).
Micronutrients: Work done so far on the role of micronutrients in coastal saline soils is meagre. The soils are generally rich in micronutrients, such as Fe, Mn, Zn, Cu, B and Mo. In the coastal sands of Andhra Pradesh in India the soils are deficient in Zn as well as in N, P and K. Iron chlorosis is common to crops like sugarcane (Saccharum officinarum L), jowar (Sorghum bicolor L. Moench), rainfed rice (Oryza sativa L.), etc. As a remedial measure for iron deficiency common in upland rice nurseries, spray of iron salts has been suggested. Such symptoms are also common in groundnut (Arachis hypogaea L.), Cressandra undulaefolia Salisb., etc. In Andhra Pradesh salinity accentuated the zinc deficiency. Along with Zn, P deficiency is also reported from the red and deltaic black soils in this state. High amount of CaCO3 (up to 15%) is congenial to Zn and Fe deficiency disorders. High dose of Zn application is recommended in rice as foliar spray. Zn deficiency was noted from various laterite and lateritic soils in other coastal states in India also. Besides, Al and Fe toxicities too have been reported from a few acid lateritic soils along the coast.
Long term study: Long term field experiment in coastal saline soils in India showed that rice and wheat yield could be maintained with 50% NPK used in conjunction with FYM or green manure (DARE, 2003-04). In another detailed long term experiment conducted (CSSRI, 1990) in Sundarbans (India) it was observed that grain yield of crops in a rice-barley rotation increased significantly only due to the application of N. Application of P did not show any significant increase in the yield of crops in the initial 8 years, after which the yield of barley alone increased due to P application. Available K content was high in the soil. The experiments conducted so far showed that a basal dose of 11 kg P ha-1 for rice and 5.5 kg P ha-1 for barley or for similar upland crops should maintain the fertility status of the soil, whereas the K application may be omitted without any detrimental effect on soil fertility or crop growth. The K removal by the crop was compensated by K added through accumulation and release of non-exchangeable sources.
Acid sulphate soils
In Sri Lanka addition of Gliricidia muculata in combination with phosphate and a small dose of inorganic fertilizer was effective to secure high rice yields (Deturckl et al., http://www2.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub53/pub53-h8.pdf). In India, for the coastal acid sulphate soils of Sundarbans, application of lime, superphosphate and rock phosphate have been found beneficial in improving the soil properties and rice growth (Bandyopadhyay, 1989). Application of Ca-rich oystershell, which is available in plenty, was found beneficial, if applied in powdered form, as an inexpensive alternative soil ameliorating agent. In this soil continuous submergence for one year could not improve the soil properties substantially.
Mongia et al. (1989), while reporting for two soils in Andaman Islands observed that application of lime and phosphorus may be beneficial for lowland rice. In another study on mangrove (Avecenia marina) muds in this island, it has been reported that liming significantly depressed the concentrations of Al, Mn and Fe. Exchangeable Al content also decreased with lime application. The depression of exchangeable Al may be due to precipitation of trivalent Al and Al (OH)3 in the presence of high concentration of OH- ions. Lime application, in general, also reduced the exchangeable and extractable Fe contents of the soil.
In the rice-based systems of the Mekong Delta in Viet Nam, there is a trend towards replacing the traditional rice double cropping with a rotation of wet season rice and dry season upland crops (vegetables and tuber crops). However, under the prevailing acid sulphate soils, the build-up of excessive concentrations of exchangeable aluminium (Al3+) during the aerobic soil phase strongly limit upland cropping to few relatively Al-tolerant tuber crops that farmers grow on raised beds to enhance Al3+ leaching process. Study was conducted on the performance of major tuber crops (cassava, sweet potato and yam) in relation to soil exchangeable Al3+ concentration and as affected by the application of locally produced biogas sludge on farmers’ fields on a typical acid sulphate soil with observation plots laid out on raised beds and the same categorised based on the initial exchangeable Al3+ content of the topsoil in classes of <10, 10–15, and >15 meq Al3+ per 100g. Biogas sludge was applied at 3 Mg ha−1 (dry matter) to tuber crops and compared with an unamended control. It was found that biogas sludge tended to reduce soil exchangeable Al3+ concentrations but significantly increased the tolerance to given Al concentrations with higher tuber yield and P uptake in all tuber crops. However, Al-tolerant cassava showed stronger responses to amendment than Al-sensitive yam. It was concluded that in the absence of soil liming, the application of organic wastes can improve the performance of Al-tolerant while permitting the cultivation of more Al-sensitive crop on acid sulphate soils. Further research aims at identifying most appropriate substrate types and application rates for specific acid soil conditions and (Saleh et al., http://www.tropentag.de/2005/abstracts/links/Saleh_gFbcXBF1.pdf) crop tolerance levels.
Inundated and flooded soils
A flood is an overflow or accumulation of an expanse of water that submerges land. In the sense of ‘flowing water’, the word may also be applied to the inflow of the tide. ‘Coastal flood’ is caused by severe sea storms, or as a result of other hazards (e.g. tsunami or hurricane). The problems of maintaining these areas are accentuated by naturally rising sea levels due to global climate change. Very often the problem becomes much more severe with increase in salinity in the flood water caused by breaching or overflowing of the sea dykes, etc. Flooding thus causes significant change in soil properties depending on the soil, hydrological properties of the flood water, and duration of flood. Among others the most significant changes in soil properties of relevance to plant growth are silt deposition, accumulation of salts, erosion of top soil, organic C status in soil, depletion of soil oxygen ─ resulting in lack of plant metabolic activities and overall reduced soil atmosphere causing significant change in soil nutrient dynamics.
Soil-and plant-produced C2H4 were suggested as factors in reducing root permeability and increasing resistance to water uptake by tobacco (Hunt et al., 1981). It has been reported that soil flooding stress may severely reduce water and ion uptake directly by increasing the permeability of roots to water and indirectly by reducing the size and volume of the plant roots, as well as by decreasing the ion uptake per unit weight of roots. Such uptake implies a disruption in root metabolism and reducing the effective root surface area available for ion uptake. Many such evidences indicate that dysfunction in nutrient absorption by roots under flooding conditions is largely caused by lack of O2 and attendant deleterious metabolic effects. The accumulation of dioxygen in the earth’s atmosphere allowed for evolution of aerobic organisms that uses O2 as the terminal electron acceptor, thus providing a higher yield of energy compared with fermentation and anaerobic respiration. For example, in aerobic metabolism, the complete breakdown of one molecule of glucose yields a total of 38 molecules of adenosine triphosphate (ATP), whereas the anaerobic breakdown of this same glucose molecule to ethanol and CO2 yields only 8 molecules of ATP (Scandalios, 1993). Oxygen stress may inhibit guttation either by increasing the root resistance to water movement or by inhibition of ion transport to the xylem and arrest of vegetation and reproductive growth.
Flooding X Salinity interactions on H2S gas evolution: Bandyopadhyay et al. (1995) measured H2S production in coastal saline and flooded rice field in Sundarbans, West Bengal, India. They observed that sulphates constitute about 40-50 percent of salts in these soils undergoing reduction reactions resulting in transformation of sulphate salts into sulphides including H2S gas which is toxic to plants. It was estimated that during peak periods about 390 to 1180 cm3 of H2S gas per m2 per 24 hours (day time) escaped to the atmosphere from waterlogged rice field, extent of which depended on the soil salinity (ECe 5-9 dSm-1) values and dose of green manure (Sesbania aculeata) applied (0-5 Mg ha-1) in the experiment. Higher soil salinity and application of green manure increased the production of H2S. The concentration of P and micronutrients in both shoot and root of rice decreased considerably at higher concentration of H2S in irrigation water. The insoluble sulphides precipitated on root surfaces (Table 2) and possibly interfered with the growth and nutrient uptake by plants. The acid soluble metallic sulphide (FeS equivalent) content of soil increased from 221 mg per kg soil to 381 mg per kg soil as the H2S content of irrigation water increased from 0 to 80 ppm. The primary cationic constituent of the precipitate on root was Fe, the solubility of which increased considerably after submergence of soil accompanied by a sharp fall in the redox potential. It was inferred that besides the direct toxic effect of H2S gas the indirect effect of sulphate reduction in interfering with the nutrient uptake by plants was also responsible for decrease in yield of rice.
Table 2. Nutrient content in the extract of root coating (10 g fresh root per 20 ml 6N HCl) and the insoluble metallic sulphides (FeS equivalent) in soil at flowering stage of rice in pot culture experiment
H2S in irr. water
(ppm) Soil salinity, ECe 7.4 dSm-1 Soil salinity, ECe 3.8 dSm-1
Insol.
sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm) Insol. sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm)
Fe
Mn
Zn
Cu Fe
Mn
Zn
Cu
0 221 2913 63 68 13 204 2217 52 71 11
20 253 8492 121 142 22 244 7992 98 125 20
40 280 11948 144 181 24 265 10731 127 174 20
60 310 15316 175 218 26 309 14986 160 215 25
80 381 17723 225 268 32 355 17265 196 257 30
LSD
(p=0.05) 26 150 4 5 3 21 161 4 6 2
Adapted from Bandyopadhyay et al., 1995
Biological factors
It is not only the physico-chemical property of soil that is adversely affected by salinity and alkalinity, the soil biology is also highly affected by presence of excess salts in soil, and this, in turn, would affect nutrient availability.
Saline soils
In general, the biological activity of soil microorganisms is low in salt-affected soils due to high salt content. Carbon dioxide evolution and microbial count in soil decrease at higher concentration of salts. There is decrease in free living diazotroph and nitrogenase activity with increasing soil salinity. The inhibition effect also varied with kind of salt being more with Ca and Na type of salts than with K and Mg type of soils among the cations. Among the anions the adverse effect was more with CO3 and Cl salts than with SO4 type of salts. Initial total microbial count of heavy clay saline soil has been reported to have increased (Mahmoud et al., 1972) after reclamation (Table 3). Enzymatic activity in soil also decreases due to increase in soil salinity. The low biological activity of saline soil is largely due to the osmotic and ionic stress induced by salts. In saline soils organic amendments may improve the biological activity up to the salinity level of ECe 26 dSm-1. Thus, there is an urgent need of applying organic matter to saline and sodic soils for their efficient management. At higher salinity, the osmotic and specific ion effects predominate over organic matter in influencing the biological activities in soil. Thus, addition of organic matter alone may not be enough to improve the biological activity. It has been observed that desalinization immediately increases the biological activity, although not completely restored.
One of the most efficient ways of increasing the N status and sustaining crop production in the salt-affected soils is to exploit the benefit of biological N fixation, specially by cultivation of annual and perennial legumes in crop rotation, application of azolla and non-symbiotic N fixing bacteria, etc. Rhizobium spp. show considerable tolerance to high pH and can thrive well in sodic soils but these are considerably sensitive to salinity. Though Rhizobia could thrive and multiply in sodic soil with pH as high as 10.0 the effective contribution of the bacteria towards N-fixation reduces because of sensitivity of the host plants to salinity of soil. Nodulation may be reduced at soil salinity (ECe) as low as 5.0 dSm-1 in barseem (Trifolium alexandrnum), while nodulation was delayed and ineffective nodules were produced in the crop at ECe 15.0 dSm-1 (Bhardwaj, 1975). Legume crops vary in their nodulation performance on different soils (Table 4). Nodulation and N-fixation increase considerably after reclamation of the soils. No nodule may be formed on root when total soluble salt content of soil is about 5.16%. Application of phosphate fertilizer shows improvement in nodulation, and the phosphate x salinity interaction is found significant (Bajpai and Gupta, 1979). At moderate salinity level of ECe 5.0 dSm-1, application of 100 kg P2O5 ha-1 gave symbiotic performance similar to the application of 50 kg P2O5 ha-1 in normal soil. The performance of some forage legumes with Rhizobium spp. (Barseem with: Trifolium alexandrinum, Shaftal with: T. resupinatum, Lucerne with: Medicago sativa, and Senji with: Melilotus parviflora) tested under different soil salinity conditions shows that the mean nodule number, weight and yield of legumes reduced with increase in soil salinity (Table 5). There exists a considerable variation among the Rhizobium spp. in their tolerance to salinity. For example, there may be complete inhibition of barseem Rhizobia as has been obsreved by Pillai and Sen (1966) at 0.5- 0.7% NaCl in culture media, whereas strains from Dolichos showed highest growth at 0.7-1.2 % (Pillai and Sen, 1973). However, addition of organic material (green manure, FYM, etc.) or gypsum to soil may greatly improve the proliferation of Rhizobia.
Effect of salinity on the microbial and biochemical parameters of the salt-affected soils in Sundarbans (India) was studied at nine different sites. The study revealed that the average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating the adverse effect of soil salinity (Tripathi et al., 2006). It was suggested that integrated nutrient management should be very effective for increasing its use efficiency for higher and sustainable yield of crops (Bandyopadhyay et al., 2006, Tripathi et al., 2007). Bandyopadhyay and Rao (2001) were of the opinion to introduce systems approach involving organic, inorganic and biofertlizers compatible with the farmers’ practice. In coastal soil at Tamil Nadu (India), application of agro-industrial wastes significantly improved soil organic carbon, pH, EC and soil bacteria, fungus and actinomycetes population and enhanced the soil fertility status (macro and micro nutrients) and improved the crop productivity of finger millet. Application of pressmud @12.5 t ha-1 recorded better growth and yield of finger millet followed by composted coirpith @ 12.5 t ha-1 (Rangaraj et al., 2007).
Sodic soils
Sodic soils show low urease activity, but high pH does not affect the ammonifying or ureolytic bacteria as much as it affects the nitrifiers which are highly sensitive to both salinity and alkalinity. Production of ammonia from urea may not be affected although nitrification may be severely affected by NaCl, Na2SO4 or CaCl2. However, urease activity is also reduced at higher pH but enhanced with organic carbon content of soil. Among the nitrifiers Nitrobacter is more sensitive than Nitrosomonas. Both salinity and alkalinity do not show marked influence on the growth of Actinomycetes, but fungi and Azotobacter are very sensitive. Blue green algae are tolerant to high pH. Sulphur oxidizing bacteria, Thiobacillus thiooxidans and T. novellas, and iron-sulphur oxidizing bacteria, T. ferrooxidans, have been isolated from sodic soils (Rupela and Tauro, 1973), although their activities are poor. High concentration of sodium chloride inhibits nitrogen fixation by Azotobacter, and the strains isolated from saline soil are less inhibited than others. Negative correlation exists between the Diazotrophic bacteria and salinity (Hassouna et al., 1995). On the other hand, Azospirillum, a non-symbiotic root associative nitrogen fixing bacteria may be recommended as a bio-fertilizer up to total soluble salt content of 2500 ppm in soil. Growth of non-symbiotic bacteria and their N fixation may be improved by application of chemical amendments and phosphatic fertilizers to sodic soils.
Blue Green Algae (BGA) can be used to reclaim the sodic soils due to their ability to secrete organic acids and immobilize Na in the biomass. Though some contradictory reports are available it is true that blue green algae increase the organic matter and N content of saline and sodic soils. Azolla, the water fern, having symbiotic association with BGA, can be grown as a pioneer plant to improve the health, N status, and reclamation of salt-affected soils. By cultivating azolla for a few successive years, the salt content of saline soils may be decreased considerably. The desalinization rate is 1.8 times higher than leaching through water, or 2.1 times than desalinization through Sesbania incorporation (Shang et al.,1987).
Phosphate solubulizing bacteria do not show much effect on the available P status of sodic soil, whereas VAM (vesicular-arbuscular micorrhizae) can grow with their extensive hyphal network up to a salinity level of 12 dSm-1. It may also help the plants in scavenging phosphorus, nitrogen, sulphur and micronutrients in salt-affected soils. The data in Table 6 show that inoculation with VAM resulted in significant increase in VAM colonization of wheat roots and VAM spore count in sodic soil (pH 8.8) over uninoculated controls. The increase in Olsen’s P, as a result P application, inhibits VAM colonization and spores. A positive relation (r=0.74) exists between the VAM colonization and soil available P which indicates mobilization of P by VAM. The inoculation with Glomus mossae and native VAM results in increase in wheat yields. These was no significant increase in P content of both straw and grain as a result of inoculation with VAM in treatments without P application. The inoculation with VAM results in significant increase in Zn content of grain and a positive relation has been reported between the Zn content and VAM in roots.
Table 6. Effect of inoculation with VAM cultures and P application on wheat yield,
available P, VAM spores and % VAM colonization of roots
P level VAM Culture Available P Wheat yield (g pot-1) VAM spores %VAM P Zn
(mg kg-1) Straw Grain (10 g-1) Colonisation - (kg ha-1)-
0 None 4.8 19.3 13.6 17.0 9.7 2.2 53.3
0 Glomus mossae
6.3
24.4
17.6
22.4
76.1
2.1
48.5
0 Native VAM 6.0 24.6 13.6 19.9 59.2 2.2 51.8
40 None 21.5 24.6 17.5 18.7 2.0 3.0 21.5
40 Glomus mossae
19.4
25.6
15.3
10.7
44.5
3.5
36.6
40 Native VAM
17.1 24.3 17.3 15.5 18.4 3.4 36.8
CD (P=0.05) P 1.2 0.7 1.0 NS 5.7 0.2 4.5
VAM NS 0.9 NS NS 7.0 NS 5.5
P X VAM 2.2 1.3 1.8 NS 9.9 NS NS
Source: Sharma and Swarup (1996)
Use of green manure crops like Sesbania spp. (S. aculeata) has been proved very suitable for saline sodic soils (Keating and Fisher, 1985, Rao, 1986, Evans and Rotar, 1986). In favourable climate with proper management, these crops accumulate well over 100 kg N ha-1, mostly through biological N fixation, in 50-55 days, thereby increasing the yield of the following rice crop significantly (Singh et al., 1991). Likewise, application of green leaves of Glyricidia maculata @ 10 tonne ha-1 to the puddled soil before rice transplanting has been found comparable in terms of yield with inorganic fertilizer application of N, P and K at 100, 50 and 50 kg ha-1, respectively (Chavan and Dongale, 1994). Buresh and De Datta (1991), however, were of the view that leguminous green manure and its residue normally are able to meet only partially the N requirement for the following high yielding rice variety. They focused attention on the high loss of N under the anaerobic-aerobic soil cycles typical of legume-lowland rice sequence, as well as under higher production of methane and nitrous oxide from the lowland rice field. However, in view of rising energy cost and limited input availability, recycling of organic wastes and use of renewable sources of biofertilizers, viz. rhizobium cultures for pulse or legume, and blue-green algae for waterlogged rice field may play a significant role in terms of integrated nutrient management for rice in coastal saline soils (Kundu and Pillai, 1992).
Soil quality
The importance of improved soil quality in the coastal plains through higher SOC level of the soils was highlighted by Mandal et al. (2008). Very recently, IRRI characterized lowland rice soils (excluding deepwater rice) in Asia in respect of soil quality (Haefele and Hijmans, 2009), and the study included large areas under coastal plains (Fig. 2). They grouped soil qualities into four categories. These were: Good, Poor, Very Poor and Problem soils. ‘Good’ and ‘Poor soils’ represent those with different degrees of weathering but without major constraints; ‘Very Poor’ represents soils with multiple chemical constraints (acidity, deficiency of phosphorous, or toxicities of iron and aluminum); while ‘Problem soils’ represent those with the most frequently cited soil problems, including acid sulphate, peat, saline, and alkaline soils, which partly cause low fertility, and partly soil chemical toxicity.
Nutritional disorder in plants
Nitrogen: Under salt-stress conditions, the uptake of N by crop plants is generally affected (Alam, 1999). A substantial number of laboratory and greenhouse studies have shown that salinity reduces N accumulation in plants. It has been reported that an increase in Cl- uptake and accumulation is accompanied by a decrease in shoot nitrate concentration. In their experiment, Aslam et al. (1984) have reported that Cl- inhibited NO3- uptake more than SO42- when these anions were present on an equal osmolarity basis. In contrast to the effect of Cl- on NO3- uptake, reported data indicated that increased NO3- in the substrate decreased Cl- uptake and accumulation. The possible decrease in N uptake by increasing salinity has been partly attributed to a probable substitution of Cl- for NO3-. Both the chloride salts of Na and K inhibited the nitrate uptake similarly, suggesting that the process was more sensitive to anionic salinity than to cationic salinity (Aslam et al.,1984). Although, Cl- salts were primarily responsible for reduced NO3- uptake by plants, NO3- reduction in plants was not affected by salinity in studies with barley (Aslam et al.,1984).
Salinity also stimulated nitrate reductase activity in peanut plants but decreased the nitrate reductase activity in tomato and cucumber (Cucumis sativus L.) plants. Reduction in NRA may be due to inhibition of NO3-uptake by Cl- in plant species (Abdul-Kadir and Paulsen, 1982) .
Potassium, Sodium, Calcium & Magnesium: The higher K/Na ratio in shoots of barley cultivars compared with that in root medium solution indicated selective uptake of K, which seems to be among the processes involved in tolerance of cultivars to salinity stress (Alam, 1999, Niazi et al., 1992). Addition of K suppressed the uptake of other cations by rice and tomato plants in the order of Na>Mg>Ca. The depression of Na uptake by K could be due to the antagonism between the two cations. It is widely recognized that a high Na concentration inhibits K uptake by plants. On the other hand, Na appeared to stimulate the K uptake by plants.
Salinity stress has significant inhibitory effects on the concentrations of K, Ca, and Mg as well as stimulatory effects of these nutrient elements on different crop plants (Alam,1999). With the increasing concentration of NaCl salts, K concentration decreased in the leaves, stems and roots, and was accompanied by a substantial increase of Na in the organs.
In lowland rice plants adapt to saline conditions and avoid dehydration by reducing the osmotic potential of plant cells. Antagonistic effects on nutrient uptake may occur, causing deficiencies, particularly of K and Ca under conditions of excessive Na content. For example, Na is antagonistic to K uptake in sodic soils with moderate to high available K, resulting in high Na:K ratio in the rice plant and reduced K transport rates. Sodium-induced inhibition of Ca uptake and transport limits shoot growth. Increasing salinity inhibits nitrate reductase activity, decreases chlorophyll content and photosynthetic rate, and increases the respiration rate and N content in the plant. Plant K and Ca contents decrease but the concentrations of NO3-N, Na, S, and Cl in shoot tissue increase.
The factors related to nutrient disorder that affect different rice varieties are:
• Differences in nutrient uptake under Na stress: Tolerant cultivars have a narrower Na:K ratio (higher K uptake) and greater leaf Ca2+ content than susceptible cultivars.
• Efficient exclusion of Na+ and Cl-: Salt-tolerant rice varieties have a reduced Na+ and Cl- uptake compared with less tolerant cultivars.
Calcium plays a vital nutritional and physiological role in plant metabolism. Calcium, which like K also is an essential mineral nutrient, helps in maintaining membrane integrity, is important in senescence processes, and is known to counteract the harmful effects of Na on crops (Lahaye and Epstein, 1971). Plant growth is dependent on Ca2+, and both cell division and cell elongation processes are affected by the Ca2+ ion concentration.
The presence of Ca2+ as the dominant cation in agricultural soils generally ensures that the absolute Ca2+ level is not a primary growth-limiting factor. As salinity increases, the requirement of plants for Ca2+ increases. The uptake of Ca2+ from the soil solution may decrease because of ion interactions, precipitation, and increases in ionic strength that reduce the activity of Ca2+. In citrus, Ca was found to be effective in reducing the transport of both Na and Cl from the roots to leaves. Maintaining an adequate supply of Ca2+ in the soil solution is an important factor in controlling the severity of specific ion toxicities.
The magnesium content of the leaves of saline-treated bean plants increased, whereas it decreased in the root. Hodson et al. (1982) found potentially toxic concentrations of Mg in salt-marsh soil solution samples and demonstrated that a salt-marsh clone, Agrostis stolonifera, was considerably more tolerant to Mg2+ than was an inland clone. Magnesium concentration of avocado leaves was decreased with an increase in the exchangeable Na in the soil. In rice, Mg transport to the tops was suppressed by Na compared with Mg uptake (Song and Fujiwara, 1996). The Mg content in the roots revealed the competition between Mg and Na uptake and transport to the tops (Alam, 1999, Alfocea et al., 1993).
Phosphorus: Phosphorus, which has a crucial role in the energy metabolism of cells, is involved in a number of anabolic and catabolic pathways (Alam, 1999). A recent study indicates that salinity may increase the P requirement of certain plants. Awad et al. (1990) found that when NaCl increased in the substrate from 10 to 50 or 100 mM, the P content in the tomato leaf increased from 58 to 70 and 97 mmol kg-1 dry weight. The influence of salinity on P accumulation in crop plants is variable and depends on the plant and experimental conditions. In many cases, salinity decreased the P concentration in plant tissue (Sharpley et al., 1992). It is unlikely that Cl and H2PO4 ions are competitive in terms of plant uptake. However, it has also been observed that Cl may have a suppressing effect on P uptake in tomato shoots (Papadopoulos and Rendig, 1983). The presence of Cl as well as SO4 reduced P uptake in barley and sunflower plants. In other cases, a reduction in plant P concentration by salinity may result from the reduced activity of P in the soil solution due to the high ionic strength of the growth media (Awad et al.,1990). Phosphate solubility and its availability are reduced in saline soils not only because of ionic strength effects that reduce the activity of phosphate but also because the P concentration in soil solution is tightly controlled by sorption processes and by the low solubility of Ca-P minerals. It is, therefore, understandable that P concentrations in field-grown agronomic crops decreased as salinity increased in the media. When plants are P-deficient, they may be more sensitive to salinity (Sharpley et al., 1992).
Gibson (1988) observed that adequate phosphorus nutrition was essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed wheat.
Micronutrients: The concentrations of micronutrients in the soil solutions, with the exception of Cl, seem to be low and depend on the physical and chemical characteristics of the soil bodies. The availability of most micronutrients depends on the pH of the soil solution as well as the nature of binding sites on organic and inorganic particle surfaces. In saline soils, the solubility of micronutrients such as Fe, Mn, Zn and Cl is particularly low and plants grown in these soils often experience deficiencies in these elements (Alam, 1999, Page et al., 1990). Nevertheless, the micronutrient concentration in plant shoots may increase, decrease, or have no effect depending on the type of plant tissue, salt tolerance of plant species, salinity, micronutrient concentration, environmental conditions, and/ or abrupt changes in the permeability of the crop cell membranes.
Both Fe and Mn contents were reported to increase in all parts of the salt-treated peanut plants (Chavan and Karadge, 1980). The increase in Fe contents was more prominent than that of Mn. Salinity increased the Fe concentration in the shoots of pea and rice and decreased its concentration in the shoots of barley and corn (Hassan et al., 1970).
A range of soil conditions have been associated with binding of Zn in less plant available forms: high pH (7.0), prolonged submergence and low redox potential common in coastal soils, high organic matter and bicarbonate content, high Mg:Ca ratio, and high available P. Perennial wetness and low redox potential are the major causes of Zn deficiency in peat and coastal saline soils (Neue and Lantin, 1994; Quijano-Guerta et al., 2002). One remarkable feature of Zn-deficiency tolerance in rice is that the trait seems to be associated with tolerance of other abiotic stresses. Rice germplasm identified with tolerance of Zn deficiency from large-scale screening at IRRI often showed cross-tolerance of salinity, P deficiency, and peat soils (Quijano-Guerta et al., 2002). The reasons behind this cross-tolerance are still unknown and could be attributed to better Zn acquisition when Zn is the most limiting factor.
Boron is highly toxic to almost all plants. Sensitive plants may be affected at concentrations > 0.50 gm-3 in soil saturation extract (Maas, 1986).
Future research options for improved nutrient management
A. Adequate phosphorus nutrition has been found essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed plants. P translocation from roots to young shoots should increase in the presence of an additional supply of Ca2+. An increased Ca2+ supply to the plant could be more efficient than P fertilization itself in restoring the P supply to young tissues under saline conditions.
B. Elevated Ca2+ levels may protect the plant from NaCl toxicity by reducing the displacement of membrane-associated Ca2+ by reducing Na+ uptake and transport to the shoots or by a combination of these effects. Ca2+ also improves K+ uptake under NaCl salinity, effectively improving on the Na/K value in the tissues. An increase in the Cl- concentration, on the other hand, in the nutrient media may lead to a reduction in the NO3 content of plants, observed in case of tomato. Under saline conditions, a high Ca2+ supply should alleviate the inhibition of NO3 uptake and increase Na/K selectivity.
C. Supplementation of Ca2+ may also improve the growth rate of the plants in the NO3 treatment based on which it may be suggested that NO3- is possibly a better N source than NH4+.
D. In coastal flooded saline soils measures should be taken to reduce volatilization loss, in particular, either through placement of N-source (urea) at sub-surface depth, through application of slow release source, through use of urease inhibitor, or by adjusting the time of application coinciding with the plants’ active growth stage, for higher N-uptake.
E. Under flooded condition, soil organic matter contributes to Fe and Mn availability through the formation of metallo-organic complexes with organic substances. This phenomenon may be attributed to the production of chelating agents from compost that generally keep the micronutrient elements soluble and, consequently, more available to crop plants. Increased Fe and Mn solubility in flooded soils benefits rice, which has a higher requirement for these elements, than other plants. There is concomitant increase in pH, CO32-, and DTPA extractable Fe and Mn on the submergence of a lowland rice plants.
F. Under flooded conditions, the production of organic complexing compounds and reductions of Fe and Mn tend to enhance the solubility of Zn and Cu in the growth media. Contradictory results are also, however, available. When a soil undergoes reduction by flooding, the breakdown of Fe and Mn oxides may provide an increased surface area with a high adsorptive capacity onto which Cu and Zn may be firmly adsorbed in some soils. Thus, long-term flooding of noncalcareous soils though generally tends to increase the availability of Cu and Mo may depress that of Zn. Deficiency may also occur primarily during early growth of the crop due to immobilization of Zn in roots by bicarbonate ions that are produced in alkaline soils soon after submergence with increase in CO2 concentration in soil.
G. Plant roots and their interaction with the different abiotic and biotic soil components represent a key point in the acquisition of water and essential nutrients. However, anthropogenic effects on the environment – including soil and water deterioration and contamination – could alter these relationships. In addition to these, vegetable production presents diverse problems, which could be mitigated by the use of plant-growth promoting microorganisms (PGPMs). On the soil, PGPMs could contribute to solubilize and/ or acquire essential minerals, making scarce nutrients more available to the plant. On the host, they stimulate several physiological changes that could lead to better growth and render the plant more tolerant to abiotic stresses. Amongst PGPMs, Azospirillum is one of the most studied genera. Even though it colonizes different plant species in an ample variety of soils, its favourable effects on vegetable germination, emergence and growth have not been thoroughly studied. The review (Barrasi et al., 2007) describes the beneficial effects PGPM inoculation could have on vegetables growing either under normal or stressful conditions including salinity, with an emphasis on the use of Azospirillum. Focus may be made on the recent advances on Azospirillum-plant interactions and the bacterial mechanisms of plant growth promotion.
H. For sustainable soil health in order to ensure improved plant nutrient status and its use by the plants the importance of improved soil quality in the coastal plains through higher SOC level of the soils, for which C sequestration is one of the important pathways, may be emphasized since lowlying coastal soils may be a useful sink for higher organic carbon pool for the terrestrial system.
I. Based on earlier observations on acid sulphate soils attempt may be made, in the absence of soil liming, to explore the utility of organic wastes which may improve the performance of Al-tolerant crops while permitting the cultivation of more Al-sensitive crops on these soils. Further research should aim at identifying most appropriate substrate types and application rates for specific acid soil conditions and crop tolerance levels.
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