The soil population must be considered from the point of view of a biological complex it is not sufficient to separate it into different constituent groups.
—S.A. WAKSMAN, 1923
A diverse biological community in soils is essential to maintaining a healthy environment for plant roots. There may be over 100,000 different types of organisms living in soils. Most are providing numerous functions that assist plants, such as making nutrients more available, producing growth-stimulating chemicals, and helping form soil aggregates. In a teaspoon of agricultural soils it is estimated that there are from 100 million to 1 billion bacteria, several yards of fungi, and several thousand protozoa. It may hold 10 to 20 bacterial-feeding nematodes and a few fungal-feeding and plant parasitic nematodes. Arthropods can number up to 100 per square foot, and earthworms from 5 to 30 per square foot.
Of all the organisms in soils, only a small number of bacteria, fungi, insects, and nematodes might harm plants in any given year. Diverse populations of soil organisms maintain a system of checks and balances that can keep disease organisms or parasites from becoming major plant problems. Some fungi kill nematodes, and others kill insects. Still others produce antibiotics that kill bacteria. Protozoa feed on bacteria and may attack fungi. Some bacteria kill harmful insects. Many protozoa, springtails, and mites feed on disease-causing fungi and bacteria.
Beneficial organisms, such as the fungus Trichoderma and the bacteria Pseudomonas fluorescens, colonize plant roots and protect them from attack by harmful organisms. Some of these organisms, isolated from soils, are now sold commercially as biological control agents. The effects of bacteria and fungi that suppress plant disease organisms are thought to arise from competition for nutrients, production of antagonistic substances, and/ or direct parasitism. In addition, a number of beneficial soil organisms induce the immune systems of plants to defend the plants (systemic acquired resistance; see discussion in chapter 8). Also, roots of agronomic crops usually have their own characteristic microbial communities with numerous interactions.
Soil management can have dramatic effects on soil biological composition (see figure 4.4 for management effects on organisms). For example, the less a soil is disturbed by tillage, the greater the importance of fungi relative to bacteria. Thus, promotion of cropping practices that encourage abundance and diversity of soil organisms encourages a healthy soil. Crop rotations of plants from different families are recommended to keep microbial diversity at its maximum and to break up any potential damaging pest cycles. Additional practices that promote the diversity and activity of soil organisms include low amounts of soil disturbance, use of cover crops, maintaining pH close to neutral, and routine use of organic sources of slow-release fertility.