Evaluation Process of Soil Quality: 3 Steps

This article throws light upon the three main steps involved in the evaluation process of soil quality. The steps are: 1. Soil Health Indicators 2. Minimum Data Set: Concept and Application 3. On-Farm Soil Health Assessment.

Step # 1. Soil Health Indicators:

The quality of soil is rather dynamic and can affect the sustainability and productivity of land use (cite). It is the end product of soil degradative or conserving processes and is controlled by chemical, physical, and biological components of a soil and their interactions.

Indicators, however, will very according to the location, and the level of sophistication at which measurements are likely to be made. Therefore, it is not possible to develop a single short list which is suitable for all purposes (Table 28.5).

A. Biological:

Identification of biological indicators of soil quality is reported as critically important by several authors, because soil quality is strongly influenced by microbiological mediated processes (nutrient cycling, nutrient capacity, aggregate stability). Of particular importance is to identify those components that rapidly respond to changes in soil quality.

Biological indicators of soil quality that are commonly measured include soil organic matter, respiration, microbial biomass (total bacteria and fungi,) and mineralizable nitrogen. Soil organic matter plays a key role in soil function, determining soil quality, water holding capacity and susceptibility of soil to degradation.

In addition, soil organic matter may serve as a source or sink to atmospheric CO₂ and an increase in the soil C content is indicated by a higher microbial biomass and elevated respiration. It is also the principal reserve of nutrients such as N in the soil and some tropical soils may contain large quantities of mineral N in the top 2 m depth.

B. Chemical:

In order to achieve high crop yields small-holder farmers have to provide soil nutrients in large quantities. Therefore it is possible to alter the pool of available nutrients by adding inorganic fertilizers, incorporation of cover crops, and using other organic materials in form of manures and composts.

Results of chemical tests are soil quality indicators which provide information on the capacity of soil to supply mineral nutrients, which is dependent on the soil pH. Soil pH is an estimate of the activity of hydrogen ions in the soil solution. It is also an indicator of plant available nutrients. High activity is not desirable and the soil may require liming with base cations Ca or Mg in order to bring the solution back to neutral.

C. Physical:

Soil physical properties are estimate from the soil’s texture, bulk density (a measure of compaction), porosity, water-holding capacity. The presence or absence of hard pans usually presents barriers to rooting depth. These properties are all improved through additions of organic matter to soils.

Therefore, the suitability of soil for sustaining plant growth and biological activity is a function of its physical properties (porosity, water holding capacity, structure, and tilth). There are several criteria to consider when selecting soil health and soil quality indicators.

In general, appropriate indicators should be:

i. Easy to asses

ii. Able to measure changes in soil function both at plot and landscape scales

iii. Assessed in time to make management decisions

iv. Accessible to many farmers

v. Sensitive to variations in agro-ecological zone

vi. Representative of physical, biological or chemical properties of soil

vii. Assessed by both qualitative and/or quantitative approaches.

Summary of Soil Health Indicators used to Asses Soil Function:

Indicator:

i. Soil organic matter (SOM)

ii. Physical: soil aggregate stability, infiltration and bulk density

iii. Chemical: pH, extractable soil nutrients, N-P-K and base cations Ca Mg and K

iv. Biological: microbial biomass C and N; potentially mineralizable N

Soil Function:

i. Soil structure, stability, nutrient retention; soil erosion

ii. Retention and mobility of water and nutrients; habitat for macro and micro fauna

iii. Soil biological and chemical activity thresholds; plant available nutrients and potential for N and P as well as loss of Ca, Mg and K

iv. Microbial catalytic potential and repository for C and N; productivity and N supplying potential

It is postulated that basic soil quality indicators should reflect criteria which are relevant to existing soil data bases. Based on these propositions a list of basic soil properties that should be indicative of soil quality was established (Table 28.5).

Step # 2. Minimum Data Set: Concept and Application:

A minimum data set (MDS) was proposed to measure soil quality and its changes due to management practices through selection of key indicators such as soil texture, organic matter, pH, nutrient status, bulk density, electrical conductivity and rooting depth.

Collecting a minimum data set helps to identify locally relevant soil indicators and to evaluate the link between selected indicators and significant soil and plant properties.

It is a minimum set of indicators required to obtain a comprehensive understanding of the soil attributes evaluated. (Fig. 28.7). More importantly they serve as a useful tool for screening the condition, quality, and health of soil. For small-holder farmers these tools need to be simple measures of soil health and soil quality such as consistency, colour and workability.

Due to these complexities, there is need to develop diagnostic measures and indicators of soil health and soil quality changes in order to derive classifications from the minimum data set (Table 28.5) that can better assist farmers and inform research and extension to target solutions at farm level.

Step # 3. On-Farm Soil Health Assessment:

On-farm assessment of soil quality and health is recommended to assist farmers evaluate the effects of their management decisions on soil productivity. Also, this approach permits interaction between researchers, extension and polity personnel when providing interpretation to link on-farm knowledge to soil health information.

The main challenge is to develop soil quality and soil health standards to assess changes which are practical and useful to farmers. For instance, linking soil health measurements with farmer perceptions of soil quality can bridge the gap in interpretation of complex data sets (Table 28.6).

1. Scorecards:

The use of scorecards for on-farm soil quality assessment is emphasized where qualitative observations of soil health are scored to obtain an overall measure of soil quality. These cards may be developed to evaluate soil health through farmer observations of soil physical, chemical and biological properties.

These soil characteristics are classified in terms of descriptive indicators which are interpreted on a graded scale. Examples such as observations on earthworm numbers can yield a general index of biological activity in the soil.

2. Soil Quality Test Kits:

Assessment tools such as soil quality test kits focus on farmer- based evaluations and education regarding various soil and smallholder management practices. Further, they aim to produce an educational tool to increase public awareness of the importance of soil quality.

Such kits, which are commercially available, provide for measurements of soil respiration, infiltration, bulk density, water content, electric conductivity (EC), pH, soil nitrates, aggregate stability, slake test and earthworm numbers.

3. Soil Quality Indices:

Finally, various soil quality indexing approaches are available and can to apply to derive a range of critical test values within which soil quality and soil health assessments can be defines.