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.
