Harper and Brothers Publishers, New York. Soil is not a sterile homogeneous material. Soil in its natural state is a living environment, teeming with biological activity functioning in a complex ecosystem. The types of life found in the soil are many, and the numbers of organisms are enormous. A handful of such soil may also contain several earthworms and many plant roots, insects, mites and spiders, and perhaps an indignant gopher or prairie dog.
Organisms serve a variety of functions in the soil ecosystem.
Recycling of plant nutrients - The decomposition of organic matter transforms plant nutrients into available forms which can be taken up by plants. Some organisms cause or transmit diseases while others, such as Penicillium , produce substances which control disease organisms. Nitrogen fixation - Some organisms take nitrogen from the atmosphere and transform it into organic forms which are then utilized by other organisms in the ecosystem. Such organisms include bacteria in symbiosis with legume plants, blue-green algae, and some actinomycetes. Soil mixing - Organisms that burrow through the soil mix it through their digging.