1. Soil Bacteria
Bacteria are single-celled organisms that make up the largest group of soil microbes. One teaspoon can contain as many as 1 billion bacteria. They are named largely according to the job they perform in the soil. They are divided into 4 primary groups and most are beneficial.
The decomposers: As the name sounds, they decompose everything from tree limbs, fallen leaves, dead animals, organic matter, insect bodies, and more and recycle the carbon into nutrients.
Mutualistic Bacteria: These are the bacteria that take atmospheric Nitrogen (N) and convert it into nitrogen for plants. There are 4 types that do this. Three types live freely in the soil and the Nitrogen can be used by any plant. One type of mutualistic bacteria must have a host plant, all within the legume family, such as alfalfa, soybeans, and clover. It is a symbiotic relationship where the plant supplies the carbon in the form of sugars for the bacteria and the bacteria captures atmospheric Nitrogen and converts it to Nitrogen for the plant. The process is called "Nitrogen fixation" and is quite a complex procedure.
Denitrifying bacteria do the exact opposite. In flooded fields, compacted soils, or oxygen-deficient soils, denitrifying bacteria take soil Nitrogen and convert it back to atmospheric Nitrogen, thereby decreasing the available nitrogen in soils. It is, therefore, important to aerate compacted soils and ensure good drainage.
Pathogenic Bacteria: These bacteria produce diseases in plants. Blights are caused by these pathogenic bacteria and are spread by insects.
Lithotroph Bacteria: These are similar to Mutualistic bacteria except they work on soil sulphur. Some are arobic bacteria (oxygen-rich soil) and anaerobic bacteria (Low oxygen soil).
In oxygen-rich soils they make sulphur more available to plants. However, in low oxygen soils, anerobic bacteria make sulphur less available to plants.
2. Fungi - Beneficial and Pathogenic
Fungi are an important group that includes mushrooms, yeasts, and molds. However, soil fungi are extremely important in the soil. They include decomposers, mutualistic and pathogenic types of fungi.
Mushrooms are not just the things you buy in the store for salads. They are actually the "fruiting bodies" of microscopic fungi in the soil as well as pathogenic and beneficial fungi growing on and inside living and dead trees, and those growing in other material. The mushrooms release spores so the fungi can spread and continue.
3. Nematodes
Nematodes are tiny worm-like organisms less than 1 mm in length. In the UK they are called eelworms. There are some 40,000 different known types of nematodes of which about half are parasitic. None are harmful to humans. There is almost no place on earth where nematodes do not exist from deep oceans to snow-topped mountains.
Most are beneficial and perform a wide range of tasks. Some types feed upon bacteria and fungi and can keep pathogenic microbes under control. Others feed upon and help control damaging insects, such as grubs, fleas, ticks and other soil-dwelling insects. Still, others are involved in nutrient recycling and breakdown of dead insect bodies and plant life into elements plants need to survive. Some nematodes have been commercially harvested and grown as a type of insecticide for grubs, etc.