The regulation of the soil biota is done through predation and parasitism. Protozoa actively fed or graze on bacteria thereby controlling numbers. Viruses also limit microbial numbers by lytic activity and predatory fungi (over 150 species) trap nematodes.  These activities are in turn controlled by the soil physical environment, whether it’s the spatial separation of predator-to-prey or in the interaction with soil minerals such as clays which may bind cells to soil particles. These are affected by soil physico-chemical properties such as pH and water retention.

Many microorganisms are antagonistic to each other and to larger species. This can be either through production of a chemical or enzyme, or through removal of an essential growth nutrient such as iron. This has been used in agriculture to biological control pests and diseases for example in the production of ethylene gas from the bacterium Pseudomonas  to control Striga (Berner et al., 1999) or in the control of the take all fungus Gaemannomyces gramminis.

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