The soil ecosystem is tremendously varied – more so than many above- ground plant and animal food webs.
Each species has slightly different requirements. Aerobic microbes require oxygen. Anaerobes require the absence of oxygen. Some prefer either a high or a low pH, or high or low moisture. Many organisms can digest simple sugars, while only a few species have the enzymes to digest lignin, a major component of woody tissue.

Microbes differ greatly in how they get energy. Most soil organisms are heterotrophs that get their energy and carbon from breaking down organic components. In contrast, the autotrophs use inorganic carbon (carbon dioxide). There are two groups of autotrophs. Phototrophs, such as plants and a few soil organisms, get their energy from light. Chemotrophs are small, but important, group of soil bacteria that get their energy from oxidizing inorganic compounds including ammonium, nitrite, and sulphur compounds.

Healthy soil is a jungle of rapacious organisms devouring everything in sight (including each other), processing their prey or food through their innards, and then excreting it. The value of these creatures to growers lies in:

• Cycling nutrients.
• Enhancing soil structure, which improves water and air movement.
• Controlling disease and enhancing plant growth.

One of the important functions of the soil biological community is managing nutrients. Soil organisms continually transform nutrients among many organic and inorganic forms. Plants primarily need simple inorganic forms of each nutrient. Soil organisms create many of these plant-available nutrients and help store nutrients in the soil as organic compounds.

Most crops grow best in crumbly soil that roots can easily grow through and that allows in water and air. Soil organisms play an important role in the formation of a good soil structure.

Soil organisms have many methods for controlling disease-causing organisms. Protozoa, nematodes, insects and other predatory organisms help control the population levels of their prey and prevent any single species from becoming dominant. Some bacteria and fungi generate compounds that are toxic to other organisms and some organisms compete with harmful organisms for food or a location on a root.

The lives of plants and soil organisms are closely intertwined. Some plant and microbe species have developed symbioses, or mutually beneficial relationships. Rhizobium and other bacteria can invade roots and get sugars from the plant, and in return, they fix atmospheric nitrogen into a form that plants can use.

Another group of friendly root-invaders are the mycorrhizal fungi. Mycorrhizal improve phosphorus nutrition by producing acids that convert phosphorus into plant-available forms and transport phosphorus back to the roots.

Not all plant/microbe interactions are invasions. The rhizosphere (the narrow region surrounding each root) is rich in biological activity as bacteria and other microbes feed on the carbon compounds exuded by roots. Plants may exude compounds that attract certain species to the rhizosphere that protect the roots from disease-causing species.

Supply organic matter – most soil organisms rely on organic matter for food.
Maximise crop residue – crop residue is a convenient and valuable source of organic matter.
Apply compost or manure – compost inoculates the soil with a wide variety of organisms and provides them with a high quality food source.
Plant cover crops – cover crops extend the growing season and increase the amount of roots and above-ground growth that becomes part of soil.
Rotate crops – crop rotation puts a different food source into the soil each year. This encourages a wider variety of organisms and prevents the build-up of a single pest species.
Reduce tillage – tillage enhances bacteria growth in the short-term by aerating the soil and by thoroughly mixing the organic matter. A single tillage event is generally inconsequential to micro-organisms, but repeated tillage eventually reduces the amount of soil organic matter that fuels the soil food web.
Minimize compaction – compaction reduces the space available for larger organisms to move through the soil.
Minimize fallow periods – during long fallow periods, most arthropods will emigrate or die of starvation. Mycorrhizal fungi also ‘starve’ during a fallow period.
Minimize the use of pesticides – all pesticides will impact some non-target organisms.

Reduced input costs. Less fertilizer may be needed if nutrient cycling becomes more efficient and less fertilizer is lost from the rooting zone. Fewer pesticides are needed where a diverse set of pest-control organisms are active. As soil structure improves, tillage becomes easier and potentially less costly.

Pollution prevention. Soil organisms filter and detoxify chemicals and absorb the excess nutrients that would otherwise become pollutants when they reach groundwater or surface water.

Improved yield and crop quality. Soil organisms are key to forming good soil structure or tilth. Good tilth promotes better root development and water storage. Many micro-organisms enhance crop growth or reduce the activity of disease organisms that can degrade the quality of food or feed.

The above information is provided with the permission of University of Minnesota Extension Service. Reference to publication No. 07403 Soil Biology and Soil Management, Soil Management Series, A. Lewandowski. Copyright 2002. All rights reserved. Refer www.extension.umn.edu.