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Sustainable Agriculture: Programs

Soil Biology

Except for the growing plants, it may not look like there is much living in soils, but soils are home to a complex ecosystem that is critical for maintaining soil quality. Remember that organisms are one of the five soil forming factors, and these organisms range in size from unicellular bacteria to earthworms, insects, and rodents.

Picture of a collembola (a small insect) in the soil
The smallest soil organisms are some of the most important. Bacteria and fungi have an important role of decomposing organic matter in the soil, creating humus, and causing the formation of good topsoil. If you’ve ever smelled a soil, that pungent “dirt” smell can be attributed to actinomycetes and other micro-organisms decomposing plant and animal tissues. When these organisms die, they contribute to the organic matter of the soil for other critters to decompose. In this decomposition process they release nutrients that were incorporated into plant or animal bodies available for plants to use. They also perform many other important functions like fixing nitrogen in legumes (rhizobia bacteria), helping plants obtain nutrients from a larger volume of soil (mycorrhizal fungi), and helping create stable soil aggregates.


Larger soil organisms, such as earthworms, perform the task of soil disturbance (bioturbation) and moving of materials through much of the soil profile, which stimulates soil formation. These organisms also stimulate activity of micro-organisms through the digestion of organic material. Earthworm channels and other burrows allow for movement of plant roots and water transport by increasing porosity and macropores. Other organisms such as collembolan are the first step in breaking down plant leaves or roots to begin the process of incorporating them into the soil as organic matter.


One important concept is that the greater diversity of organisms found in your soil, the more likely disease organisms will be kept in check by competition or predation of other organisms. Soil biology is fascinating and complex. Additional information can be found in Soil Biology Primer.