Micro-organisms in the soil

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Root nodules of the fava bean or broad bean. Each of these nodules contains billions of Rhizobia bacteria that live in symbiotic relationship to Fabaceae or the legume family (beans, peas).
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* This page is under construction. If you have any ideas to contribute to this page, place a comment below in the Comments section. --JE 01:53, 14 June 2012 (PDT)

Provided there is a carbon source, microorganisms exist in large numbers in the soil. There are more microbes in a teaspoon of soil than there are people on the earth. For example, there are 100 million to a billion bacteria in one gram of soil and 100,000 to a million fungi. Their biomass per m2 is for bacteria 40-500 g and for fungi 100-1500g at 0–6 inches (0–15 cm) depth of soil.[1]

Most of the soil microbes tend to be in a dormant state, the more so in tilled soils. Soils that haven't been tilled for a longer time have significantly greater levels of microbes. What microbes need as food are dead plant residues and plant nutrients.

Soils contain soil organic matter. Soil organic matter is composed of:

  • living microorganisms,
  • dead residues of plant and animal material,
  • and the organic material that is resistant to decomposition and that may be thousands of years old (which we call 'humus.')

Microorganisms need regular supplies of new organic material in order to survive.

Microbes grow faster in warmer temperatures. This is why in the tropics we find little or no humus in the soil. Most of the carbon is tied up in above-ground vegetation. In regions with high temperatures and enough moisture, microbes quickly decompose soil organic matter. In colder regions soils contain much larger amounts of soil organic matter.

In acid soils microbes are less effective in decomposing organic material. This is why liming soils often enhances decomposition and more nutrients for plants become available.

Organic material that has been made finer will more easily be decomposed by microbes because the surface area is bigger for finer particles.

Grassland generally forms much organic material in the soil because 50 percent of the grass roots dies each year. This is why agricultural soils of former prairies with thick layers of humus formed in the past are very productive. These soils also contain more microbes and larger fungal populations.

Chunks of information

  • Root nodules of the fava plant or broad bean. Each of these nodules contains billions of Rhizobia bacteria that live in symbiotic relationship to Fabaceae or the legume family (beans, peas). These bacteria fix nitrogen that is in the air and give it to the plant or remain in the soil for a next crop. They grow in healthy soils and reduce the need for fertilizing the soil.

List

Bacteria

Bacteria exist in large numbers in the soil. Because their size is small they have a rather small biomass compared to other soil microorganisms.

Aerobic and anaerobic bacteria

Plowing farmer in ancient Egypt

Bacteria that need oxygen for their survival are called aerobic bacteria and bacteria that do not require oxygen for survival are called anaerobic bacteria. Anaerobic bacteria may die if kept in an oxygenated environment. They can be found deeper into the soil, inside soil particles and also deep in the ocean.

The differentiation between aerobic and anaerobic bacteria is important for how we treat the soil. Aerobic bacteria live closer to the surface and anaerobic bacteria deeper. In general when we cultivate a soil it is preferable to not disturb the soil layers too much and keep above what's up and keep down what's below. This is why most organic gardeners don't plow or dig a soil but don't cultivate it all (only cover the soil with mulch) or cultivate it with a pin similar to how plowing was done in the ancient past.

The nitrogen cycle

Main article: Nitrogen cycle. See also Some science for the gardener, section Nitrogen cycle.

Mention only the involved bacteria here.

Actinobacteria

Actinobacteria or actinomycetes appear 10 times smaller in number in soils compared to bacteria but because their are larger in size, their total biomass is similar.

Rhizobium

Fungi

Although fungi are smaller in population numbers, they dominate the soil biomass in undisturbed soils. Bacteria, actinomycetes and protozoa can tolerate more soil disturbance and therefore dominate in tilled soils. Fungal populations tend to dominate in untilled soils.

Mycorrhizae

Main article: Mycorrhiza

Penicillium

Algae

Protozoa

Nematodes

Nematodes, like fungi, are more present in untilled than in tilled soils.

See also

References

  1. ↑ [http://ohioline.osu.edu/sag-fact/pdf/0016.pdf Understanding Soil Microbes and Nutrient Recycling - The Ohio State University}

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