They are relatively more advanced than bacteria. Fungi are important for humus formation and the aggregate structure of the soil. Actinomycetes are microorganisms that fit between bacteria and fungi. They can utilize hard to decompose materials as fungi can, and some can fix nitrogen as bacteria do. Their biomass per hectare may reach that of bacteria.
Actinomycetes are very important for composting processes.
No proof of fungi fixing nitrogen has been found, but fungi can nitrify ammonia. Aerobes and anaerobes are mutually active and, indeed, dependent on each others' products. Also, nematoda and rotifera are important groups contributing to soils. Earthworms , of course, are of vital importance in the development of productive soil. Earthworm activity aerates, drains and transports organic matter to the deep layers and in so doing brings microlife to the lower levels of the soil.