This topic has 3 questions:

  1. What do bacteria look like?
  2. What do bacteria do in soil?
  3. Where do bacteria live in soil?


Question 1. What do bacteria look like?

Most bacteria in soil are about one micron in length or diameter (there are a thousand microns in a millimetre). Some are slightly larger than this, up to several microns, and in rare cases even larger. Their size varies with their environment. Bacteria in environments that have high levels of nutrients may be larger than those in nutrient poor conditions.

The majority of bacteria in soil usually occur as single cells. Bacteria sometimes join together in chains or clusters. They mainly have one of two shapes - spheres (called cocci) and rods (called bacilli). Other bacteria have more varied shapes including spirals and long thin hyphae (although these are less common).

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Question 2. What do bacteria do in soil?

Bacteria are able to perform a wide range of beneficial processes in soil including decomposing organic matter, releasing nutrients from organic matter for plant uptake, forming soil aggregates, degrading toxic substances, fixing atmospheric nitrogen and preventing plant disease.

In general, bacteria are the organisms in soil that are mainly responsible for transforming inorganic constituents from one chemical form to another. Their system of external digestion means that some of the metabolites released by the use of extracellular enzymes may be used by other organisms, such as plants. The bacteria gain nutrients and energy from these processes and provide other organisms with suitable forms of chemicals they require for their own processes. For example, in the conversions of nitrate to nitrite, sulphate to sulphide and ammonium to nitrite.

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Question 3. Where do bacteria live in soil?

Bacteria are aquatic organisms that live in the water-filled pore spaces within and between soil aggregates. As such, their activities are directly dependent on relatively high soil water contents.

Bacteria are normally found on the surfaces of mineral or organic particles or congregate around particles of decaying plant and animal debris. Most are unable to move and hence, their dispersion is dependent on water movement, root growth or the activity of soil and other organisms.

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