Bacteria are tiny, one-celled organisms which decompose organic matter, mineralize and immobilize nutrients, suppress diseases, fix nitrogen, and solubilize phosphorus. In agriculture soils a desirable range is 100 million to 1 billion bacterial organisms in one gram of dry soil.
Fungi are microscopic cells that grow as long threads or strands called hyphae. These single-celled strands push their way between rocks, soil particles, and roots. When these strands fuse together they look like fungal roots called mycelia. Fungi decompose organic matter and crop residues, solubilize nutrients from parent rock, and physically bind soil particles into aggregates which improves soil structure. In agriculture soils a desirable range is 150 to 300 meters of beneficial fungi in one gram of dry soil.
Nematodes are non-segmented microscopic worms. The majority of free-living soil nematodes function as beneficial organisms in the soil foodweb: bacterial-feeding, fungal-feeding, and predatory nematodes. A fourth type, plant-parasitic nematodes, are widely recognized in agriculture because they infest roots and injure crops, but when their numbers reach injurious levels it is an indication of a soil foodweb imbalance. Beneficial nematodes are the ‘balancers’ and play a critical role in nutrient cycling by consuming bacteria, fungi, and other nematodes. In turn, they serve as prey for higher-level predators. If fungal biomass is low certain types of nematodes will “switch” and consume plant roots, thus they also serve as biological indicators of soil health. In agricultural soils a desirable range is 10 to 30 bacterial- and fungal-feeding nematodes and a few predatory nematodes per gram of dry soil.
Protozoa, Fungi and the rest of the gang)
Flagellates and Amoebae are single-celled animals that feed primarily on bacteria, but also eat other protozoa, nematodes, organic matter, and sometimes fungi. Ciliates are the largest of the protozoa and they feed primarily on anaerobic bacteria. Protozoa facilitate nutrient cycling by grazing on bacteria and releasing excess nitrogen into the soil environment. This process of bacterial grazing stimulates further growth of bacterial populations. In agriculture soils a desirable range is several thousand flagellates and amoebae and one hundred to several hundred ciliates in one gram of dry soil. Earthworms are said to prefer protozoa.