Bacteria are essential for many of the processes which occur in soils including ecosystem processes such as nitrogen fixing, nutrient turnover, decomposition and soil formation (Ehrlich, 1996). They can be found everywhere except in the very driest of environments.
In the main, bacteria are single celled but may grow as chains or clump together. In soil you are unlikely to find cells on their own but are grouped together unless disturbed by water movement or predation. They grow as a variety of shapes and sizes: rods (e.g. bacilli, Pseudomonas), cocci (e.g. Streptococci), single cells growing in chains held together by a sheath to form a filament (e.g. Sphaerotilus) or stalked (e.g. Caulobacter) see diagram below). Some are attached to particles whilst others are free to actively swim in the water film surrounding soil particles using either cilia (hairs surrounding the cell) or flagella (long whip-like hair). In nutrient poor environments bacteria tend to be much smaller in size than those grown in laboratory cultures. In non-soil environments such as in water and insect guts, comma shaped vibrio species and corkscrew-like spirochaetes may also be found. Other species such as Wolbachia live as intracellular parasites of insects whilst others (Bdelovibrio) are parasites of other bacteria or as endophytes of plants. Many soil species form survival structures (spores) which help the bacteria survive adverse conditions such as low oxygen or low nutrient levels or extremes of temperature. Extreme environments, such as those at high or very low pH, temperature, salinity and oxygen can harbour a unique bacterial flora found no where else.
In one gram of soil it is thought there are about 1010-1011 bacteria and some 6000 - 50,000 species of bacteria (Curtis et al., 2002). In a tilled fertile soil this represents a biomass of about 400-5,000 kg/ha. Estimates of the number of bacteria and the number of species in the environment are constantly being revised, particularly with studies on DNA.
As a group, bacteria possess a wide range of capabilities. They can be differentiated on the basis of their nutritional and energy source. Autotrophs are vital in controlling nutrient availability to higher plants. Most bacteria are however heterotrophs utilising ready made sources of carbon and energy, and along with actinomycetes and fungi account for the general breakdown of organic matter in soils. Bacteria are able to respond rapidly to changes in their environment and tend to be active in nutrient rich environments where there is high turnover in nutrients but which are poor in organic matter.
Nearly all the nutrients found in soil are concentrated around plant roots or from any dead and decaying organic matter material added to the soil. These form a nutrient “hot spots”. This is the “rhizosphere” around a plant root. The actual amount of plant root can be very considerable. In one study it was estimated that wheat (Triticum aestivum) produced 71,000 m total root length when all the root hairs etc were accounted for. In this zone, the plants produce a large amount of nutrient which is continuously exuded through their roots and into the surrounding soil (root exudates).
Bacteria tend not to be found free living in the space between soil particles where they would be susceptible to predation by protozoa. Instead, they are often closely associated with soil particles, inhabiting the soil pores which make up soils.
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