Soil Bacteria—Important Partners in Plant Growth

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What bacteria lack in size, they make up in numbers. These tiny, one-celled organisms are found soil to the tune of 100 million to 1 billion per teaspoon of soil.

Three Groups of Soil Bacteria

There are three functional groups of bacteria: decomposers, mutualists, and pathogens.

Most soil bacteria species are decomposers that consume simple carbon compounds.

A number of decomposers can break down pesticides and other contaminants in the soil.

Decomposers are especially important in preventing the loss of nutrients, such as nitrogen, from the rooting zone.

The second group are the mutualists that form partnerships with plants. The most well-known of these are the nitrogen-fixing bacteria that work with legumes.

The third group is the pathogens, the disease-causing bacteria species.

Functions

Bacteria perform important services related to water dynamics. They're also very important in nutrient cycling (converting nutrients that are unusable to plants to ones that the plants can use).

They also help with disease suppression (unless, of course, they're causing a disease). Some bacteria species compete with disease-causing organisms in the soil and on plants surfaces.

A very important function that some species perform is the conversion of nitrogen from the air into a form that leguminous plants can use. The plants supply simple carbon compounds to the bacteria, which use them as their food source.

In return, they convert N₂ from air into a form that plants can use. When leaves or roots from the host plant decompose, soil nitrogen increases in the surrounding area, improving the soil for all plants.

Nitrifying changes ammonium to nitrite, then to nitrate, which is the preferred form of nitrogen for grasses and most row crops. Nitrate is leached more easily from the soil, so some farmers use nitrification inhibitors to reduce the leaching.

Nitrification is suppressed in forest soils, so that most of the nitrogen remains as ammonium.

Denitrifying converts nitrate to nitrogen (N₂) or nitrous oxide (N₂O) gas. Denitrifiers are anaerobic bacteria, meaning they are active where oxygen is absent, such as in water-saturated soils or inside soil aggregates.

Actinomycetes are a large group that grow like fungi, using hyphae to reach into the soil for nutrients. They are responsible for the characteristically "earthy" smell of freshly turned, healthy soil.

Actinomycetes decompose a wide array of hard-to-decompose compounds and are active at high pH levels. (Fungi are more important in degrading these compounds at low pH).

Bacteria Locations in the Garden

Bacteria are found where easy-to-metabolize substrates are present, such as fresh, young plant residue, and the compounds found near living roots. They're especially concentrated in the region next to and in the root.

Some researchers believe that some plants produce certain types of root exudates (chemical compounds released by the roots) to encourage the growth of protective bacteria.

Certain species of soil bacteria have anti-fungal activity that inhibits some plant pathogens. Those species can increase plant growth in several ways. Some produce a compound that inhibits the growth of pathogens or reduces invasion of the plant by a pathogen. Others produce growth factors that directly increase plant growth.

The soil bacteria that promote plant growth occur naturally in soils, but may not be in high enough numbers to have a great effect.

Soon, you may be able to buy seed coated with this anti-fungal to ensure that there are sufficient bacteria to reduce the pathogens around the seed, and, as the plant grows, the root of the crop.

From Soil Bacteria to Microorganisms