Soil Quality: Meaning, Assessment and Calculation

After reading this article you will learn about:- 1. Meaning of Soil Quality 2. Functions of Soil Quality 3. Assessment 4. Calculation.

Meaning of Soil Quality:

The terms soil health and soil quality are becoming increasingly familiar worldwide. Doran and Parkin (1994) defined soil quality as “the capacity of a soil to function, within ecosystem and land use boundaries, to sustain productivity, maintain environmental quality, and promote plant and animal health”. In general, soil health and soil quality are considered synonymous and can be used interchangeably.

The National Resources Conservation Service (NRCS) defines soil quality or soil health similarly, but add inherent and dynamic soil quality to the definition.

The inherent soil quality is defined as “the aspects of soil quality relating to a soil’s natural composition and properties influenced by the factors and processes of soil formation, in the absence of human impacts”. While, dynamic soil quality “relates to soil properties that change as a result of soil use and management over the human time scale”.

The concept of soil quality emerged in the literature in the early 1990s and the first official application of the term was approved by the Soil Science Society of America Ad Hoc Committee on Soil Quality.

Soil quality was been defined as “the capacity of a reference soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation.”

Subsequently the two terms are used interchangeably although it is important to distinguish that, soil quality is related to soil function whereas soil health presents the soil as a finite non-renewable and dynamic living resource.

Functions of Soil Quality:

Soil quality is used to describe the ability of soil to perform the following functions:

(i) Supporting the growth and diversity of plants and animals by providing a physical, chemical and biological environment for the exchange of water, nutrients, energy and air.

(ii) Regulating the distribution of rain or irrigation water between infiltration and run­off, regulating the flow and storage of water and solutes, including nitrogen, phosphorus, pesticides, and other nutrients and compounds dissolved in the water.

(iii) Storing, moderating the release of and cycling plant nutrients and other elements.

(iv) Acting as a filter to protect water quality, air and other resources, and

(v) Supporting structures and protecting archeological treasures.

The USDA Natural Resources Conservation Service defines soil quality as “The capacity of specific kind of soil to function, within natural or managed ecosystem boundaries to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. Changes in the capacity of soil to function are reflected in soil properties that change in response to management or climate.”

Recently the concept of soil quality has been broadened to include attributes for food safety and quality, human and animal health and environmental quality.

Soil quality index is a function of the following parameters:

Soil quality index (SQI) = f(SP, P, E, H, ER, BD, FQ, MI)

where, SP = Soil properties, P = potential productivity, E = Environment factors, H = Health of animal and human, ER = Erodibility BD = Biological diversity, FQ = Food quality safety, MI = Management inputs.

Maintaining the functions of soil is thus central to the achievement of sustainable development. However, no soil is likely to provide all those above functions, some of which occur in natural ecosystems and some of which are the results of human modifications.

Soils have an inherent quality as related to their physical, chemical and biological properties within the constraints set by climate and ecosystems, but the ultimate determinant of soil quality is the land manager.

Perceptions of what constitutes a good soil vary depending on individual priorities with respect to soil function, intended land use and interest of the observer. The assessment of soil quality can be viewed as a primary indicator of the sustainability of land management.

Assessment of Soil Quality:

Basically, two types of approach are employed for evaluating the sustainability of a management system:

(a) Comparative assessment and

(b) Dynamic assessment.

A comparative assessment is one in which the performance of the management system is evaluated in relation to alternatives at a given time only in contrast, in a dynamic approach, the management system is evaluated in terms of its performance over time.

However, soil is not directly consumed by humans and animals, and it is difficult to relate measurable soil quality indicator properties to specific soil functions or management goals. The assessment of soil quality or health has been likened to a routine medical examination for a human being, when a doctor measures a number of key parameters as basic indicators of overall system function.

Because soils perform many simultaneous functions, however, the objectives of relating indicator properties to specific functions or processes are very difficult.

Over last several years, researchers and farmers alike have tried to establish what are now widely called minimum data set of physical, chemical and biological properties that can be used as quantitative indicators in soil health assessments. Indicator properties that are frequently used are presented in Table 28.4.

Characterisation of soil health using these indicators can be quite time consuming and expensive, and is not feasible as a general practice for everyone. A number of soil health scorecards have also been developed as qualitative tools for characterizing soil health.

Soil organic matter (SOM) content is frequently identified as a primary attribute of soil quality assessment. SOM influences many soil properties including infiltration rate, bulk density, aggregate stability, cation exchange capacity, and biological activity, all of which are related to a number of key functions.

SOM serves as a slow release reservoir for plant macro- nutrients especially nitrogen and also helps in plant micro-nutrient nutrition.

It facilitates the infiltration of water and air into the soil, increases water retention by the soil, and it’s important in maintaining soil tilth. Over time, increases in SOM can lead to a greater and more diverse population of soil micro-organisms and may thus enhance the biological control of pests and plant diseases.

Large quantities of fresh organic matter that are added to the soil, however, may stimulate plant pathogenic organisms and seed and seedlings pests as cabbage maggots and wireworms, which can cause serious losses.

Methods for Calculating Soil Quality:

Among methods, the following two methods are generally used for calculating soil quality indices:

(i) Statistical and

(ii) Conventional methods.

1. Statistical:

Soil quality indicators so determined are to be reduced to a minimum data set (MDS) through a series of uni- and multivariate statistical methods using SPSS 10 software.

Both parametric (Randomised Block Design) as well as non-parametric statistics (Kruskal-Wallis x²) are to be used to identify quality indicators with significant treatment differences. Only variables with significant differences between treatments are to be chosen for the next step in MDS formation.

For each statistically significant variable Principal Component Analysis (PCA) may be performed. Within each principal component, only highly weighted factors i.e. those with absolute values within 10% of the highest weight, are to taken for the MDS.

For the reduction of redundancy and rule out spurious groupings among the highly weighted variables within each principal component, the multi-variate correlation coefficients may be used to determine the strength of relationships among variables.

Well correlated variables are to be considered as redundant and also for the elimination from the data set. Summing up absolute values of well correlated groups, the highest correlation sum is the best representing group. Apart from this, any non-correlated highly weighted variables are also to be considered important and retained in the MDS.

2. MDS Validation:

Multiple regression analysis using final MDS components as independent variables and each management goal attribute as a dependent variable is to be made. These regressions serve to check the MDS representation of management system objectives.

3. Indicator Transformation (Scoring):

After determining variables for MDS, every observation of each MDS indicator needs to be transformed for the inclusion in the soil quality index (SQI). Linear scoring technique may be used. However, soil quality indicators are to be ranked in ascending or descending order depending on whether a higher value a higher value considers “good” or “bad” with respect soil functions.

For more is better indicators, each observation divides by the highest observed value such that the highest observed value receives a score of 1. For less is better indicators, the lowest observed value divides by each observation such that the lowest observed value receives a score of 1.

4. Indicator Integration into Indices:

Two soil quality indices may be used for comparison an additive SQI and a weighted additive SQI. The additive index is the summation of scores from MDS indicators. From the summed scores, the additive soil quality index treatment means and standard deviations are to be calculated. In the weighted additive index after transformation, the MDS variables for each observation are to be weighted based on PCA results.

Each PC explains a certain amount of the variation in the total data set. The percentage is to be standardized to unity, provided the weight for variables chosen under a given PC. Then summing up the weighted MDS variable scores for each observation and calculated the treatment means and standard deviations.

For all the indexing methods, SQI scores for the management treatment are to be compared using a two way ANOVA. Higher index scores are considered to be a mean better soil quality.