Nematodes are often predators in the soil, feeding on other microorganisms. Many of them have a detrimental effect on plant growth. Microscopic protozoa graze on bacteria, moving within thin films of soil water. When water or food are limiting they encyst, waiting until conditions improve. Plants have major effects in the soil environment by the cycling of water and nutrients, production of organic matter, formation of pores by root growth, secretion of root exudates which promote soil aggregation, and production of organic acids which influence nutrient availability.
Bacteria have perhaps the greatest influence in the soil environment of all organisms.
Species are either autotrophic or heterotrophic. They perform many functions in the ecosystem, most importantly in enzymatic transformations. They are able to oxidize or reduce many chemical elements in the soil. For example, nitrogen is tansformed from ammonium to nitrite and nitrate by two different species. Fixation of nitrogen from atmospheric N 2 into organic forms is also performed by bacteria, either in symbiotic associations with plants or independently.