Bacteria from soil can be divided into two broad categories: The indigenous species that are true native residents, and the invaders. The indigenous types may have resistant stages and endure for long periods without being metabolically active, but at the same time these natives multiply and participate in the biochemical functions of the community. In contrast, the invader species do not importantly participate in community activities. These species enter the soil system with rain, disease tissues, animal waste or sewage sludge. They may persist for some time in a resting state, and sometimes even grow for a short period of time; but they never contribute measurably to the various ecologically significant transformations or interactions.
Bacteria flourish dramatically when rapidly available nutrients are added to the soil. These actively metabolizing bacteria need nutrients provided from outside sources to sustain this rapid growth. Bacteria respond promptly to soil amendments, become and remain numerous as long as the nutrients are available, and then decline once the food source is depleted.
Environmental conditions affect the density and composition of the bacterial flora. The primary environmental variables that influence soil bacteria include moisture, aeration, temperature, organic matter, pH and inorganic nutrient supply. Highly acidic or alkaline conditions inhibit many common bacteria. The optimum pH range for most species is about 6.4 to 6.8. Generally, the greater the hydrogen ion concentrations, (the higher the acidity) the smaller the size of the bacterial community.
Bacteria and fungi play an essential role in decomposition, and are also crucial to life on earth because of their exclusive ability to perform key biochemical changes:
- NITRIFICATION: Changes ammonia, a product of decomposition, into a form of nitrate used by plants.
- SULFUR OXIDATION: Sulfur is oxidized and made usable to plants through the sulfur cycle, which is similar to the nitrification cycle.
- NITROGEN FIXATION: Introduces atmospheric nitrogen into the food chain.
- MYCORRHIZAL ASSOCIATION: Fungal penetration of plant roots increases nutrient uptake.
Microbial activity is important in producing soil structure changes and stability in four ways:
- Filaments of microbial tissue form a network throughout the soils mineral particles.
- Certain soil organisms produce polysaccharide (complex sugars) that have a mucilaginous nature and cement mineral particles together. This results in up to 40% improved erosion stability in some soils.
- Other nonpolysaccharide microbes produce binders for reducing erosion and leaching and include lignin like and humic substances.
- Deposition of organic matter that strengthens clay particle bonding and assists in aggregation of soil particles into water-soluble units.
Benefits of this soil structuring also include plant root growth, adequate aeration and improved nutrient holding ability. Microbial transformation of elements such as nitrogen, carbon, phosphorus, sulfur, iron, potassium, manganese, selenium, zinc and copper occur in the soil system on a regular basis. Bacteria and fungi also control diseases of plants and soils in various ways:
- SUPPRESSION (production of antibiotics)
- COMPETITION (striving for the same object)
- ANTAGONISM (active opposition)
- PREDATOR (plunders or kills)