Soil Quality Evaluation: An increasingly felt issue in agriculture

Although soil has evolved through millions of years of weathering and is sustaining various forms of terrestrial life, its management has become an essential component of strategy for sustenance of ever increasing population. Assessment of soil quality is prerequisite for devising any intervention(s) in the existing management practices so as to achieve the desired goal. It focuses on the dynamic or management-affected, properties of soil, which are assessed in the context of its inherent capability. Soil quality cannot be measured directly because it is a broad and purpose-oriented concept. Instead, we analyze a variety of proxy measurements (called indicators) that together provide clues about how the soil is functioning as viewed from soil-use perspectives, which are subsequently integrated to arrive at certain numerical value that is indicative of overall quality of soil. Although importance of soil quality evaluation has not been realized in the era of high input dependent agriculture, rising cost of fertilizers/manures, decreasing availability of irrigation water and increasing impact of industrial activity on land are compelling us to devise policies based on soil quality.

Since the origin of this concept in early '90, several workers both in India and abroad have made efforts for identifying indicators for assessing the quality of soil within certain boundary conditions using several techniques, which can be grouped into two broad categories. In the first category, statistical techniques are employed for identification of indicators influencing goal variable (e.g. crop yield, produce quality etc.). As selection of indicators is based on the goal variables, experiments are required to be conducted under controlled condition for eliminating/minimizing influence of other factors not related to soil (like microclimate, disease/pest infestation & control, seed quality, irrigation water amount and quality etc.). Under this background, identification of indicators of soil quality on the basis of crop yield data generated from farmers' fields may not give a true picture. Information generated under an on-going NAIP project has also revealed that many of the important soil parameters known to have strong influence on crop growth did not figure-in as identified indicators and as a result, quite a lower degree (28%) of the variability in yield could be explained by the indicators of soil quality. The result also showed that soil quality index (SQI) values derived from the identified indicators for any agro-ecological sub-region (AESR) were not comparable with SQI values of other AESRs of the country.

Other group of workers/organizations has proposed indicators of soil quality based on the knowledge pool generated through detailed investigations on soil properties and its functions. Soil quality index computation based on this approach appears to be very simple and can be easily adopted by soil testing laboratories. Although this approach appears more appropriate, difficulty arises while deciding upon (1) contribution of each of the indicators towards goal variable, (2) whether a similar set of indicators is applicable to all soil types, cropping types (like annual, perennial or horticultural) and management conditions or otherwise. Detailed investigations as well as thorough discussions are required among peer groups on these issues.