Soil Biological Fertility

What is soil biological fertility?

Soil fertility is the combined effects of three major interacting components. These are the chemical, physical and biological characteristics of the soil. The physical and chemical characteristics of soil are far better understood than that of the biological component. It is still difficult to define the desired biological status of soil because of its dynamic nature and changes occur in much shorter time periods than physical and chemical changes in the soil.

Some aspects of soil biology may be able to provide early warnings of land degradation, such as determining the amount of carbon in microbial pools as opposed to the total amount of carbon in the soil.

The biology of soil is complex and we need a better understanding of the mediatory effect that biological components have on chemical and physical fertility. We have yet to determine desirable levels of activity, numbers and diversity of soil organisms to maintain a fertile and productive soil. Since these will change depending on the different type of soils, single optimal values will not be useful.

What are the ten key principles of soil biological fertility?

1. Soil organisms are most abundant in the surface layers of soil
2. Soil organic matter is necessary for nutrient cycling and soil aggregation
3. Maximum soil biological diversity depends on the diversity of organic matter and habitats
4. Nitrogen fixing bacteria form specific associations with legumes under specific conditions
5. Nitrogen is released during organic matter breakdown, either into soil or into the soil microbial biomass,
6. Arbuscular mycorrhizal fungi can increase phosphate uptake into plants in P-deficient soils
7. Soil amendments can alter the physical and chemical environment of soil organisms
8. Some crop rotations and tillage practices decrease the suitability of soil for plant pathogens
9. Production systems based on soil biological fertility can be profitable
10. Soil biological processes develop slowly, and the time required will differ for different soils, environments and land management practices.

What are the corresponding land management guidelines relating to these ten key principles of soil biological fertility?

1. Soil erosion should be controlled to minimise loss of soil organisms
2. Plant organic matter should be retained to maximise nutrient cycling and soil aggregation processes
3. Some disturbance of soil is necessary to maximise soil biological diversity
4. Nitrogen fixing bacteria should be selected that match the host, soil characteristics (such as pH) and environmental conditions
5. Inputs of nitrogen fertiliser should be calculated to complement nitrogen cycling from organic matter
6. Inputs of phosphorus fertiliser should be calculated to complement and enhance the activities of arbuscular mycorrhizal fungi
7. Any substance added to soil should be assessed in terms of its effects on soil biological processes and soil biological diversity
8. Crop rotations and tillage practices should be selected to avoid development of soil conditions that enhance the growth and survival of plant pathogens
9. The capacity of a management practices to produce a commercial product should be considered in parallel with its capacity to maintain and/or increase soil biological fertility
10. Sufficient time should be allowed for establishment or restoration of a level of soil biological fertility appropriate for particular soils and land management.