A negative aspect of bacteria is their anaerobic decomposition, where they are responsible for toxic gas production and denitrification. In light of this it is important to keep aerobic conditions up in soils, and as we shall see in a later chapter, composting should be carried out outside of the soil as it is more easily controlled. Many anaerobic bacteria are very useful and needed in soils. Most Fungi are mycelia forming microorganisms. 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.