They resemble molds in that they are filamentous and often profusely branched. They are smaller and similar to bacteria in being unicellular. They are prokaryotes and are classified with bacteria in the kingdom Monera. Soil Actinomycetes are of great importance in the decomposition of soil organic matter and certain ones can fix atmospheric nitrogen into ammonium which is available to higher plants. They are important in the later stages of decay as they can reduce to simpler forms even the more resistant compounds, such as cellulose, chitin, and phospholipids.
Many species produce antibiotic compounds which kill other microorganisms.
Soil Bacteria are extremely diverse, single celled prokaryotic organisms with typically some 20,000 different species in one gram of soil. Their capacity for rapid reproduction allows bacteria to adjust their activities quickly in response to changes in their environment. Soil bacteria are either autotrophic or heterotrophic. The autotrophs obtain their energy from sunlight or the oxidation of inorganic substances such as ammonium, sulfur, and iron and most of their carbon from carbon dioxide. They are vital in controlling nutrient availability to higher plants.