BACTERIA

Bacteria (singular - bacterium) are simple, prokaryotic, microscopic organisms that lack chlorophyll. They can be spherical, rod-shaped, comma-shaped, corkscrewshaped, spiral or filamentous. Most of the bacteria range in size from 0.5 to 5.0 pm, and have almost an unlimited capacity to multiply, which happens by simple fission. Bacteria that use atmospheric oxygen to function are classified as aerobic bacteria and those that get their energy by reducing simple compounds like sugars, nitrates and sulphates, are called anaerobic.
Facultative bacteria mostly utilize atmospheric oxygen but they can also respire anaerobically under conditions of low oxygen supply, as in waterlogged or poorly drained soils.
Depending on their source of nutrition and energy, soil bacteria are classified as follows: (i) Autotrophic bacteria obtain energy from sunlight (photoautotrophs) or from oxidation of minerals such as ammonium, sulphur, iron and carbon primarily from carbon dioxide (chemoautotrophs). Bacteria that oxidize ammonium ions to nitrite (Nitrosomnas spp.) and nitrite to nitrate (Nitrobucter spp.) are examples of autotrophs. (ii) Heterotrophic bacteria obtain their energy and carbon directly from the soil organic matter. For example, nitrogen-fixers such as symbiotic Rhiwbium spp., nonsymbiotic htobacter spp. and Clostridium spp. are heterotrophic. Decomposing and ammonifying bacteria also belong to this group.
Soil bacteria generally thrive best when they have (a) adequate oxygen in a gaseous or combined form, (b) temperatures ranging between 20 and 40°C, (c) Soil moisture above 60% RH, and (d) adequate organic matter and a large amount of exchangeable calcium ion (Ca²⁺). Though a pH of 6 to 8 is suitable for most soil bacteria, a few species can function at a pH values as low as3.
Bacteria are largely responsible for the decay and decomposition of organic matter as well as for recycling elements such as carbon, oxygen, nitrogen and sulphur. Some bacteria, including cyanobacteria (blue-green algae), get their food through photosynthesis. Some bacteria are saprotrophs while others are diseasecausing parasites.
Bacteria are divided into two subgroups: Archebacteria comprising methanogens and species capable of tolerating extremely high temperatures (thermophilic) or salty environments, and eubacteria that include the remaining vast majority of bacterial species.
In general, the presence of bacteria in soil is a sign of good agricultural soil.