Soil Bacteria

 Common Soil Bacteria
Bacteria are a major class of microorganisms that keep soils healthy and productive. Microbes in the soil are directly tied to nutrient recycling especially carbon, nitrogen, phosphorus and sulphur. A teaspoon of productive soil generally contains between 100 million and 1 billion bacteria which can be classified into four main groups.

 Bacteria Shape

Bacteria generally have three major shapes: rod, sphere or spiral. Actinomycetes are still classified as bacteria but are similar to fungi except they are smaller in size. Classifying bacteria by shape is complex because many bacteria have different shapes and different arrangements.  


Aerobic & Anaerobic Bacteria

Soil oxygen levels often determine soil bacteria activity. Most soil bacteria prefer well-oxygenated soils and are called aerobic bacteria and use the oxygen to decompose most carbon compounds.

Most anaerobic bacteria prefer an environment without oxygen. Anaerobic bacteria are generally found in compacted soil, deep inside soil particles (microsites), and hydric soils where oxygen is limiting. Many disease causing bacteria prefer anaerobic soil conditions and are known to outcompete or kill off aerobic bacteria. Many anaerobic bacteria are found in the intestines of animals and are associated with foul odour.

Gram Positive & Negative Bacteria

Bacteria can be characterised as gram negative or gram positive depending on their response to a laboratory staining test with the staining agent attaching to cell walls giving a visual display under the microscope.

Gram positive bacteria are generally larger than gram negative and have thicker cell walls making them more tolerant to drought conditions and water stress.

Method of Reproduction

Another way to classify bacteria is by the way in which they reproduce.  Autotrophic bacteria process carbon dioxide to get their carbon with some also using photosynthesis to produce the sugars they need to survive.  Cyanobacteria and Algae are examples of autotrophic bacteria.  Heterotrophic bacteria on the other hand require a chemical element source of energy, like ammonia etc.  An example of a heterotroph would be Arthrobacter which is involved in the nitrogen nitrification process.



How does soil benefit from bacteria

There are basically four functional soil bacteria groups including decomposers, mutalists, pathogens and lithotrophs. Decomposer bacteria consume simple sugars and simple carbon compounds, while mutualistic bacteria form partnerships with plants including the nitrogen-fixing bacteria (Rhizobia). Bacteria can also become pathogens to plants and lithotrophic bacteria convert nitrogen, sulfur, or other nutrients for energy and are important in nitrogen cycling and pollution degradation. Bacteria have the ability to adapt to many different soil microenvironments (wet vs. dry, well oxygenated vs. low oxygen). They also have the ability to alter the soil environment to benefit certain plant communities as soil conditions change.

Bacteria can multiple quickly in the right conditions where easily digestible simple sugars are available around the rhizoshpere (the zone immediately around a plants roots where bacteria live).  Complex polysaccharides, simple sugars, root castings and dead plant debris are commonly found in the rhizosphere.

Bacteria are important in producing polysaccharides that hold silt, sand and clay particles together to form microaggregates and improve soil structure.  Bacteria by their nature do not move far in substrates and normally migrate by hitch hiking in water flow or by way of earth worms, spiders or other soil inhabiting fauna.

Most bacteria thrive in well oxygenated pH neutral soils where they provide large quantities of nitrogen to plants which is often lacking in soils.  Many bacteria also secrete enzymes to makes phosphorus more soluble and plant available.