Soil Biology and Microbiology

Soil organisms are fauna and flora that spend all or part of their life in the soil. They play a vital role in the maintenance of soil fertility through processes such as the accumulation of soil organic matter, soil aggregation, and the mineralization of organic matter which releases nutrients available to higher plants. Moreover, many antibiotics are produced from microorganisms isolated from soils. Soil fauna include macrofauna (> 2 mm in width, such as mice, earthworms, termites, and millipedes), through mesofauna (0.2-2 mm, such as collembola and mites), to microfauna (<0.2 mm, such as nematodes and protozoa). Soil flora include macroflora (such as the roots of higher plants), and microflora (such as algae, fungi, actinomycetes, and bacteria). The activities of soil fauna and flora are intimately related in what ecologists call a food chain or, more accurately, a food web. Higher plants play the role of primary producers by using water and energy from the sun, and carbon from atmospheric carbon dioxide to make organic molecules and living tissues. Soil organisms that eat live plants, such as mice or termites, are called herbivores. Most soil organisms, however, use the debris of dead tissues left by plants and animals (detritus) as their source of food, and are called detritivores. Soil organisms that consume live animals, such as centipedes, mites, spiders, or nematodes, are predators and are called carnivores. Some organisms that live off, but do not consume, other organisms are called parasites. Mycrophytic feeders are organisms that use microflora as their source of food, and include certain collembola, mites, termites, nematodes, and protozoa. The actions of soil fauna in the food web are both physical and chemical, while those of the microflora are mostly biochemical. The actions of mesofauna and macrofauna enhance the activities of the microflora in several ways. First, the chewing action fragments the litter to expose the more easily decomposed cell contents for microbial digestion. Second, the fragmented plant tissues are thoroughly mixed with microorganisms in the animal gut, where conditions are ideal for microbial action. Third, the mobile animals carry microorganisms with them and help them to disperse and find new food sources.