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Soil health - Looking beyond soil fertility

Introduction

Soil health is a fairly new concept that deals with the integration and optimisation of the chemical, physical and biological processes of soil that are important for sustained productivity and environmental quality (Figure 1). Soil health is therefore defined as the continued capacity of soil to function as a vital living system, within ecosystem and land-use boundaries, to sustain biological productivity, maintain or enhance the quality of air and water and promote plant, animal and human health (Karlen et al., 1997).

Components of soil health
Figure 1: Components of soil health

In the past the understanding of the importance of soil chemical and physical properties have been well accepted in the agricultural community. However, it has not been until more recently that the importance of understanding and managing the soil’s biological properties was realised and many producers are now making significant progress on assessing and managing soil biological functioning in agricultural production systems.

Soil health cannot be measured directly because it is a broad, integrative, context-dependent concept. Since soil health is determined by several physical, chemical and biological indicators, as well as the strong interaction among these components (i.e. biophysical, biochemical and physicochemical), several soil health indicators can be used to evaluate soil health status.

What are soil health indicators?

Soil health indicators are key parameters that are specific to the three distinct soil components (physical, chemical and biological) that together provide clues about how the soil is functioning when viewed from one or more soil-use perspectives. These indicators should be easily measured, reproducible and sensitive enough to detect changes in soil function as a result of anthropogenic degradation. Soil health indicators may be quantitative (e.g. the infiltration rate is 25 mm/hr) or qualitative/descriptive (e.g. drainage is fast). Figure 2 shows a graphic representation of the five essential soil functions and associated soil health indicators.

Graphic representation of the five essential soil functions and associated soil health indicators
Figure 2: Graphic representation of the five essential soil functions and associated soil health indicators

Why assess soil health?

Soils vary naturally in their capacity to function; therefore, soil health is specific to each soil type. Soil health is primarily evaluated to measure the effects of management practices on soil function with the objective to improve or maintain soil health. The two main reasons for measuring soil health can therefore be described as follows:

a. Assessment as a monitoring tool.
When soil health is assessed over time, it can provide an indication about the sustainability of current management practices. Soil indicators that decline over time send a signal that some aspect of the management practises should be reconsidered (Figure 3).

Figure 3: Possible temporal trends in dynamic soil health assessments
Figure 3: Possible temporal trends in dynamic soil health assessments

b. Assessment as an adaptive management tool.
Soil health assessments can be used to compare the effects of different management practices on similar soils or fields (Figure 4). For example, it can be used to examine the effects of different tillage practices (such as conventional tillage versus conservation tillage) before actually committing full resources to that change.

Figure 4: Soil health assessments to compare the effects of different management practices on similar soils or fields
Figure 4: Soil health assessments to compare the effects of different management practices on similar soils or fields

Conclusion

The properties observed in a healthy soil will vary from place to place, depending on soil type, management history, the agricultural production system and climate. It is therefore important to carry out soil health assessments over time and at the same site. Results obtained from the first assessment provide a baseline from which to evaluate future assessments and it will provide information about the trend or direction of certain soil properties. Maintaining and enhancing these soil properties above the threshold or critical levels are essential to sustaining/improving soil health.

By Dr. Louis Ehlers – Manager: Agronomic Services