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Soil Biology

Biological Aspects of the Soil System

The cycles that permit nutrients to flow from soil to plant are all interdependent, and proceed only with the help of the soil community.  Soil microorganisms are the key link between mineral resources and plant growth.  Soils contain five major groups of microorganisms: Bacteria, Actinomycetes, Fungi, Algae and Protozoa.  The Bacteria are most prominent because of the many different populations in a given soil, and are the most abundant group, usually more numerous than the other four combined.  Although the other groups carry out many changes, similar to those of Bacteria, the Bacteria stand out because of their capacity for rapid growth and fast decomposition many natural materials.

Bacteria from soil can be placed in two broad divisions: The indigenous species that are true native residents, and the invaders.  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 invader species do not participate in any important way in community activities.  They 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 short periods; but never do they contribute measurably to the various ecologically significant transformations or interactions.

Bacteria flourish dramatically when readily available nutrients are added to the soil.  These actively metabolizing Bacteria need nutrients provided from outside sources for their rapid growth, but the supply is quickly exhausted.  Bacteria respond promptly to soil amendments, become and remain numerous as long as the nutrients are available, then decline once their food source is depleted.

Environmental conditions effect the density and composition of 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, but they are also crucial to life on Earth because of their exclusive ability to perform key biochemical changes:

1)     Nitrification:  Changes ammonia, a product of decomposition, into a form of nitrate used by plants.

2)     Sulfur oxidation: Sulfur is oxidized and made usable to plants through the Sulfur cycle, which is similar to the nitrification cycle.

3)     Nitrogen fixation: Introduces atmospheric nitrogen into the food chain.

4)     Mycorrhizal association: Fungal penetration of plant roots increases nutrient uptake.

Microbial activity is important in producing soil structure changes and stability in four ways:

1)     Filaments of microbial tissue form a network throughout soil mineral particles.

2)     Certain soil organisms produce polysaccharides, (complex sugars), that have a mucilaginous nature and cement mineral particles together.  This results in up to 40% improved erosion stability in some soils.

3)     Other non-polysaccharide microbial produced binders for reducing erosion and leaching include lignin - like and humic substances.

4)     Deposition of organic matter that strengthens clay particle bonding and assists in aggregation soil particles into water-soluble units.

The benefits of this soil structuring also include improved 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 control diseases of plants and soils in various ways:

1)     Suppression (Production of antibiotics)

2)     Competition (Striving for the same object)

3)     Antagonism (Active opposition)

4)     Predator (Plunders or kills)


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