Microbial Differentiation
Algae have chlorophyll to obtain energy from the sun. There are several main types of algae the green, yellow-green, brown, red, and others. The diatoms, a special form of algae, have a silica outer shell.
Algae obviously live close to the surface to have enough light for photosynthesis. The soil environment has to have adequate moisture for their proper growth; because the surface of the soil is easily dried out, a steady supply of moisture must exist (this presents a problem in many regions).
Algae are found in all soils and bodies of water, and are very tolerant to their environment. Oceanic algae, for example, tolerate water salt saturation from 0% to almost 100%. Algae found under the surface layer of soils are either dormant (latent) or forms which have become through time facultative heterotrophs.
Except in special cases, the contribution of algae to soil is considered minimal in our climatic zones (temperate zone) compared to other microorganisms. In the future there may be algae ponds to harvest organic matter and nitrogen for field use. An incredible amount of nitrogen, up to 750kg per hectare (670 lb/acre), can be fixed by the symbiosis of azolla (water fern) and anabeana azollae (a cyanobacter) in water in one year.
Bacteria are about 0.5 - 2µm in size, are very numerous in soil, and may be up to 7 Mg per hectare (6300 lb/acre). They are highly important in nitrogen fixation and as all-around support for fungi and actinomycetes due to their production of many organic acids from carbohydrates.
The cyanobacteria, sometimes referred to as blue-green algae, are a somewhat special kind of microorganism; they fix nitrogen from the air unlike other algae. Cyanobacteria are more like bacteria than algae (based on cellular differences). Examples of other bacteria utilizing light as their primary energy source (phototrophic bacteria) are chromatiaceae, and chlorobiaceae.
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. Their biomass may reach about 8 Mg per hectare (7200 lb/acre). Unlike bacteria that ordinarily need easily decomposable organic substances, fungi also decompose complex hydrocarbon type molecules (lignin, tannin). They are relatively more advanced than bacteria.
Fungi are important for humus formation and the aggregate structure of the soil. Mycelium binds soil particles and forms larger aggregates; it has been found that in a single gram of soil the length of the mycelium reaches 50 - 200m per gram (up to 7000 ft/oz). Another important property of fungi is their mycorrhiza relationships with plant roots (see ROOTS).
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. Major genus of actinomycetes found in soils are the streptomycetes; these may make up to 70% of the actinomycetesí biomass. Actinomycetes are very important for composting processes.
The term "nitrogen fixation" means conversion of gaseous nitrogen (N2) to ammonia (NH3). This is not to be confused with the term "nitrification" which means, taking the ammonia and oxidizing it to nitrite and then to nitrate (NO3). No proof of fungi fixing nitrogen has been found, but fungi can nitrify ammonia.
It is important to understand that the soil is a complex mixture of organisms; they are not separated in large groups or layers. Aerobes and anaerobes are mutually active and, indeed, dependent on each othersí products.
Many other organisms, important to soil function, like protozoas, which are over 30,000 species, are single cell animals (unicellular microfauna). Here again a disagreement with algologist as some members of the group Sarcomastigophora are photosynthetic (Euglena sp.).
Also, nematoda and rotifera are important groups contributing to soils.
Earthworms, of course, are of vital importance in the development of productive soil. Their excrement is especially rich in N-P-K and calcium as well as microbes. 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. Healthy soil with its normal population of earthworms can produce from ten to ninety tons of earthworm manure per hectare in a single year. Earthworm activated soil contains three to four times more available nutrients for plant life and can raise crop yields 35% or more. Without microbial life in the soil these small microbiological factories cease to exist.
