Input supplied to Prof. Biswapati Mandal,BCKV on Soil Health Assessment methods

Soil quality assessment methods

Three (3) types of soil quality assessment approaches are in vogue.

1.      Modeling based approach

2.      Indicator based approach

3.      Integrated approach

Modeling based approach

The models provide a tool for predicting the change in outcome caused by the changes in input parameters. By using land-evaluation models, it is possible to predict the rates and direction of many soil-quality changes.

The two principal land-evaluation modeling approaches are:

(i)                 Empirical-based modeling, and

(ii)              Process-based modeling.

The basic idea of empirical modeling for land evaluation is that observed relations are quantified and these once analyzed (i.e., in a limited number of locations) are applicable for predicting future situations. However, this will not work unless there are sufficient data on which to base the inferences, so the methodology is not appropriate for new land uses or areas from which sufficient samples have not been taken.

The process-based models for land evaluation have been basically developed to simulate the growth of crops, along with associated phenomena that influence crop growth such as water and solute movement in soil. These simulation models are deterministic and based on an understanding of the actual mechanisms, but used to include a large empirical component in their descriptions of subsystems. The so-called Wageningen models (e.g., WOFOST and CGMS) are based on soil processes and plant physiology to predict yields under several production levels.

 But, since soil formation and soil processes are function of climate, modeling based approach will not hold universally fitting, i.e., a model developed for a particular region will not hold true for a land situation under altogether different macro-climatic or, even, micro-climatic conditions.

Indicator based approach

Indicator based soil quality assessment is much more widely acceptable one than modeling based approach. Indicators can be physical, chemical, and biological properties, processes, or characteristics of soils. They can also be morphological or visual features of plants. Selection of suitable indicators is the cornerstone of this type of evaluation.

The selection of indicators should be based on,

(i)                 Land use,

(ii)              Relationship between an indicator and the soil function being assessed,

(iii)            Ease and reliability of the measurement,

(iv)            Variation between sampling times and variation across the sampling area,

(v)               Sensitivity of the measurement to changes in soil management,

(vi)            Compatibility with routine sampling and monitoring,

(vii)          Skills required for use and interpretation.

(viii)       Cost of analysis

The list of different indicators to choose from as suggested by USDA is as below,

Physical

·         Aggregate Stability 

·         Available Water Capacity 

·         Bulk Density 

·         Infiltration 

·         Slaking

·         Soil Crusts

·         Soil Structure and Macropores

Chemical Properties

·         Reactive Carbon 

·         Soil Electrical Conductivity 

·         Soil Nitrate 

·         Soil pH 

Biological Properties

·         Earthworms 

·         Particulate Organic Matter 

·         Potentially Mineralizable Nitrogen 

·         Soil Enzymes 

·         Soil Respiration 

·         Total Organic Carbon 

The weaknesses of this approach are,

1.      Working with a whole lot of indicators is arduous

2.      Extraction of a Minimum Data Set (MDS), since done by statistical procedures, is subjected heavily to sampling and analytical accuracy.

3.      This does not take into account the interrelation or interaction between two or more interrelated indicators.

4.      It is not farmers’ centric per se, i.e., farmers perspectives regarding quality of their soil is given less scope. Only ease of estimation of an indicator is considered from farmers’ viewpoint.

   

Integrated approach

Considering the onerous task of development of relationships between all the soil-quality indicators and the numerous soil functions, a stepwise agroecological approach for soil-quality evaluation and monitoring can be much effective as well as accurate as proposed by De la Rosa (2005).

This is done in two steps,

Step 1:

Land evaluation is an appropriate procedure for analyzing inherent soil quality from the point of view of long-term agroecological changes. Within this complex context, land-evaluation models may serve as a first step to develop a soil quality assessment procedure. The first step will result in defining agroecological zones, land suitability, and vulnerability classes.

Step 2:

A short-term evaluation and monitoring procedure would be basically considered for the soil biological quality in each agroecological zone defined in the first step. By measuring appropriate indicators, changes in soil dynamic quality can be assessed.

Because soil biological parameters are most variable and sensitive to management practices, a monitoring system (observed change over time) would provide information on the effectiveness of the selected farming system, land-use practices, technologies, and policies. Also, enzyme activities have been found to be very responsive to different agricultural management practices such as no-tillage. 

Because of the complex nature of the soil and its high spatial and temporal variability, it is appropriate to develop soil-quality assessment based on biological indicators after the traditional land evaluation using basically physicochemical parameters. This agroecological approach should focus on dynamic soil aspects (biological factors) but with awareness of inherent soil aspects (physical and chemical factors).

Graphical representation of a stepwise agroecological approach for soil-quality assessment

(Soil Quality and Methods for its Assessment: Diego De la Rosa and Ramon Sobral,  https://www.google.co.in/?gws_rd=ssl#q=Soil+Quality+and+Methods+for+its+Assessment:+Diego+De+la+Rosa+and+Ramon+Sobral)

General remarks

The issue I wish to drive in additionally emphasizing gaps in our understanding for assessment of soil health is that we possibly never take into consideration the ‘interaction’ between different indicators. To elaborate this issue I stress upon the fact that we make measurement of the static indicators. Especially in respect of soil physical and biological/ enzymatic aspects, the interactions among the indicators, though may play vital role in crop productivity function, remain completely overlooked. This, to my understanding, needs focus for the future workers through detailed discussion among the relevant scientists to unravel major areas of such interactions in the first place. This will possibly further entail significant overhauling of the measurement and assessment techniques currently in vogue.