Soil Microorganisms

groups of microorganisms for which the soil is the natural habitat. They play an important role in the cycle of matter in nature, soil formation, and soil fertility. Soil microorganisms can develop directly in soil, as well as in decomposing plant residues. Some pathogenic microbes and aquatic microorganisms may accidentally enter soil during the decomposition of dead bodies, from the gastrointestinal tract of animals and man, with irrigation water, or by other routes, but they generally die quickly. However, some of them persist a long time (for example, anthrax bacilli and the causative agents of tetanus) and can become a source of infection for man, animals, and plants.

Soil microorganisms constitute a large part of the microorganisms on our planet: 1 g of chernozem contains about 10 billion or more living microorganisms (10 tons per hectare). Soil microorganisms consist of both prokaryotes (bacteria, actino-mycetes, blue-green algae) and eukaryotes (fungi, microscopic algae, protozoans). Owing to the use of such modern methods as electron and capillary microscopy, many new species of soil microorganisms are discovered every year.

Varying greatly in their properties and functions, soil microorganisms include heterotrophs and autotrophs, aerobes and anaerobes. They differ sharply in optimum pH, relation to temperature, osmotic pressure, and source of organic and inorganic matter used. Despite different and sometimes directly opposite requirements, many of them develop in the same soil, which consists of a large number of very different microenvironments. The number of soil microorganisms varies with the season: there are more in spring and fall and fewer in winter and summer. The top layers of soil have more microorganisms than do the lower layers. Microorganisms are particularly abundant in the root zone of plants, or rhizosphere.

Cultivation, the addition of fertilizers, and the creation of a favorable water regime help soil microorganisms become more numerous and active. The most important planetary function of soil microorganisms is to participate in the cycle of matter and in the conversion processes of important biogenic elements—O, C, N, P, S, and Fe. Soil microorganisms are capable of breaking down all natural organic compounds and some organic compounds not found in nature. They perform an important role in freeing the biosphere from pollutants, chiefly by decomposing pesticides and oxidizing carbon monoxide. The properties of different soil groups and the variations in their fertility are largely determined by the nature of the soil microorganisms and by their activity.

Some species of soil microorganisms are used in the microbiological synthesis of antibiotics, vitamins, enzymes and other proteins, amino acids, and gibberellins. Most antibiotics, for example, are produced by growing soil actinomycetes in cultures.