The role of soil microorganisms

The soil is a thriving biological laboratory, and a teaspoonful easily contains a billion microorganisms such as fungi and bacteria. Some cause plant diseases, but most are beneficial to agriculture. Some examples:

• Humus production: Many kinds of soil bacteria and fungi decompose organic matter into crumbly humus that does all those great things for the soil. The compounds produced by decomposition are also beneficial.

• Release of plant nutrients from organic matter: Most of the nitrogen, phosphorus, and sulfur in fresh plant residues is tied up in the unavailable organic form which plants can't use. Soil microbes change these tied-up nutrients into available inorganic (mineral) forms which plants can use.

• Mycorrhizae are a kind of mushroom fungi commonly found in most soils and infest the roots of many plants and trees. They cause no harm but actually enhance the host's uptake of plant nutrients, especially phosphorus (P); they also improve water uptake, lessen the toxicity of salinity or excess aluminum, and stimulate the growth of other beneficial microbes like rhizobia. They may even secrete growth-promoting hormones. In return, the plant provides the fungi with simple sugars for food. It's believed that mycorrhizae play a particularly important role in aiding the P uptake in some crops like sweetpotatoes and cassava (manioc) which seem to tolerate soils with low levels of available P. In the case of sterilized field or greenhouse soils that lack the fungi, considerable savings in phosphorus fertilizer have sometimes been obtained by innoculating them with a mycorrhizae culture, notably in the case of citrus nurseries. Topsoil from a disease-free, actively growing organic garden is likely to contain an especially good population of the fungi, and a few shovelfuls can be transferred to a new plot to encourage development. (However, mycorrhizae do not colonize the roots of beets, spinach, chard, and brassica [crucifer] family plants such as cabbage, broccoli, radish, turnip, and pak choy.)

• Nitrogen fixation by rhizobia: Several kinds of bacteria "fix" (capture) nitrogen from the air and convert it to a form that plants can use. The most important type are rhizobia bacteria (of the genus Rhizobium) that live in small nodules on the roots of legumes. (Legumes are plants that produce their seed in pods such as beans, peas, and peanuts.) The rhizobia have a symbiotic (mutually beneficial) relationship with legumes. The bacteria live off sugars provided by the plant and supply their host with nitrogen. Some legumes such as cowpeas, peanuts, mungbeans, soybeans, and pasture legumes like clovers receive all the N they need from the rhizobia if the right strain is present.

• Other kinds of N fixation:

•• Blue-green algae (cyanobacteria) inhabit flooded rice soils and fix N. Free-living types (i.e. those requiring no host) fix modest amounts of N, and farmers in Egypt, India, and Burma purposely inoculate their rice paddies with these algae.

•• The Azolla plant is a low-growin8, aquatic fern which harbors a type of N-fixing, bluegreen algae (Anahaena azollae) in its leaves. Azolla has been used as a green manure and also intercropped (grown in combination) with flooded rice for centuries in China and Viet Nam and can supply considerable N to the rice plants. (For more information on Azolla, refer to the section on rice in Chapter 10.)

•• Azotobacter are free-living, N-fixing bacteria commonly found in unflooded soils of warm areas.

•• Casuarinas are pine-like trees used for firewood, soil stabilization, and windbreaks in warm climates. Although not a legume, they do fix N, thanks to an association with an Actinomycete bacteria of the genus Frankia.