Soil supports the growth of a variety of unstressed plants, animals, and soil microorganisms, usually by providing a diverse physical, chemical, and biological habitat.
The ability of soil to support plant and animal life can be assessed by measuring the following indicators:
Biological Diversity Indicators including habitat diversity and diversity indices for organisms such as bacteria, macro and microarthropods, nematodes, and plants.
What do plants, animals, and microbes need from soil?
Microbes need soil for:
- Food. Most microbes need regular inputs of organic matter (e.g. plant residue) into the soil.
- Space. Larger soil organisms such as nematodes and insects need enough space to move through soil.
- Air. Most soil organisms require air, though some require a lack of oxygen. They live in low-oxygen micro-sites such as within soil aggregates. Generally, soil biological activity is enhanced by an increase in soil aeration.
Plants need soil for:
- Support of the microbiological activity necessary for plant growth.
- Support for, and minimum resistance to, root penetration.
- Intake and retention of water in soil, while maintaining adequate aeration.
- Exchange of soil air with the atmosphere.
- Resistance to erosion.
- Mineral and organic sources of nutrients.
- In addition, farmers need adequate traction for farm implements to grow crops.
Animals and people need soil for:
- Healthy plant growth.
- Availability of nutrients essential for animal health. These are absorbed by plants, but are not necessarily essential for plant health.
All organisms need:
- Low levels of toxic compounds.
- Filtering of water and air.
At a landscape scale, a variety of soil environments are needed to support a variety of plants, animals, and microorganisms. (Lists adapted from Yoder, 1937, and Cihacek, 1996.
Diversity of soil and soil organisms
Each animal, plant, and microbe species requires a slightly different habitat. Thus, a wide variety of habitats are required to support the tremendous biodiversity on earth. At the microbial level, diversity is beneficial for several reasons. Many different organisms are required in the multi-step process of decomposition and nutrient cycling. A complex set of soil organisms can compete with disease-causing organisms, and prevent a problem-causing species from becoming dominant. Many types of organisms are involved in creating and maintaining the soil structure that is important to water dynamics in soil. Many antibiotics and other drugs and compounds used by humans come from soil organisms. Most soil organisms cannot grow outside of soil, so it is necessary to preserve healthy and diverse soil ecosystems if we want to preserve beneficial microorganisms. Estimated numbers of soil species include 30,000 bacteria; 1,500,000 fungi; 60,000 algae; 10,000 protozoa; 500,000 nematodes; and 3,000 earthworms (Pankhurst, 1997).
Cihacek, L.J., W.L. Anderson and P.W. Barak. 1996. Linkages between soil quality and plant, animal, and human health. In: Methods for Assessing Soil Quality, SSSA Special Publication 49.
Pankhurst,C.E. 1997. Biodiversity of soil organisms as an indicator of soil health. In: Biological Indicators of Soil Health. CAB International.
Yoder, R.E. 1937. The significance of soil structure in relation to the tilth problem. Soil Sci. Soc. Am. Proc. 2:21-33.