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Beneficial Soil Organisms
Part 2: Bacteria

Paulette Mouchet

Originally published in the Rose Garden newsletter, September, 2003. Updated November, 2006.

If you listen to the news, bacteria are invading and will wipe out life as we know it. Stores are filled with products to rid these pests from the household including antibacterial soaps, antibacterial wipes, antibacterial sprays, and the latest—antibacterial garden hoses. Contrary to news reports and Lysol company advertising, using these products are more hazardous to your health and the environment than using nothing at all. One reason is they contribute to the problem of resistant bacteria. Another reason is they are indiscriminant. They wipe out all bacteria, including the good guys. Yes, there are good guys.

Take for example, nitrogen-fixing bacteria. They form symbiotic relationships with the roots of legumes like alfalfa, beans, and clover. The plant gives the bacteria simple carbon compounds and in return, the bacteria convert nitrogen from the air into a form the plant can use. Then, when the host plant dies and decomposes, it adds nitrogen to the soil. Planting a cover crop of clover takes advantage of this to rebuild depleted soil. When the clover is tilled under, nitrogen in the clover plant is added to the soil that can be used by the next plant crop.

Bacteria are tiny, one-celled organisms that live almost everywhere. Scientists have found bacteria in thermal hot springs in Yellowstone National Park and living in pockets of water embedded 6 feet under solid ice in Antarctica. In the soil, they provide a great number of services. They:

Soil Bacteria
Soil Bacteria

There are four main groups of bacteria:

1) Decomposers. Their most important function is to degrade fresh plant litter and convert it into bacterial biomass. These biomass nutrients are readily available to plants and other soil inhabitants, and the transfer of nutrients is not affected by soil pH. Decomposers are also responsible for degrading pesticides and pollutants.

2) Mutalists form partnerships with plants. Nitrogen-fixing bacteria are an excellent example.

3) Pathogens are the bad guys. An example is agrobacterium that causes bacterial crown gall in roses. Research has shown that a diverse population of beneficial bacteria will out-compete and/or consume pathogens.

4) Lithotrophs. An interesting group of bacteria that don't use carbon as their energy source. Instead, they use hydrogen, nitrogen, sulfur, or iron. Nitrifying bacteria that convert ammonium into nitrate are lithotrophs. Lithotrophs are major players in degrading pollutants.

Beneficial bacteria improve soil health in many ways and that translates to vigorous plants that can resist disease. As organic gardeners, we want to encourage a variety of beneficial soil bacteria. Fish meal, fish hydrolysate, kelp meal, and alfalfa meal—which are common ingredients in an organic fertilizing program—are excellent food sources for beneficial bacteria.

Read Part 3: Protozoa

Photo by Michael T. Holmes, Oregon State University, from the U.S. Department of Agriculture Soil Biology Primer.


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