organisms are creatures that spend all or part of their
lives in the soil. There are several criteria that can be used
to classify soil organisms. For example, soil organisms are
classified according to their size into:
macro-organisms ( > 2 mm in width),
meso-organisms (0.2 to 2 mm in width), and
micro-organisms (< 0.2 mm in width).
on the ecological functions that soil organisms perform they can
be classified as:
herbivores that subsist on living plants,
detritivores that subsist on dead plant debris,
predators that consume animals,
fungivores that eat fungi,
bacterivores that eat bacteria,
parasites that live off, but do not consume, other
classification of soil organisms groups them into:
heterotrophs that rely on organic compounds for
their C and energy needs, and
autotrophs that obtain their C mainly from CO2
and their energy from photosynthesis or oxidation of various
Most Common Groups of Organisms
Higher plants provide a large quantity of organic residue to the
soil through roots and litterfall. Roots consume oxygen, water
and nutrients, while releasing CO2 and exudates
(mixture of organic acids, sugars and other soluble plant
compounds that escape from roots). Roots influence microbial
activities especially in the rizosphere.
Algae belong to soil microflora. They are autotrophic (i.e. have
chlorophyll and are capable of performing photosynthesis) and
eucariotic organisms. Algae are most active and abundant in wet
soils. In dry soil, the water film that surrounds soil particles
becomes too thin for them to move freely. Because they need
light for photosynthesis, algae are most common at or very near
the soil surface. Some algae make symbiotic associations with
fungi forming lichens
that are important in colonizing bare rocks and other
low-organic-matter environments (deserts). The most common
groups of algae present in soil are:
In addition to producing a substantial amount of organic
matter in some fertile soils, certain algae excrete
polysaccharides that have very favorable effects on soil
Fungi belong to soil microflora (heterotrophic, aerobic) and are
eucariotic organisms. As heterotrophs, fungi depend on living or
dead organic materials for both their C and energy. They may be
divided in the following three groups:
– single-celled organisms that live in waterlogged, anaerobic
(3) mushroom fungi.
Fungi play an important role in the decomposition of
various organic substances (cellulose, hemicellulose, lignin),
they may form symbiotic relationships with plants (mycorrhizae)
or act as pathogens, and they are play a major role in humus
formation and aggregate stabilization.
Actinomycetes belong to soil microflora (heterotrophs, aerobic).
They are single-celled, prokaryotic, filamentous and
often profoundly branched organisms. They are of great
importance in the decomposition of soil organic matter and the
mineralization of nutrients, especially in alkaline soils. Many
actinomycetes produce antibiotic compounds that kill other
microorganisms (e.g. streptomycin is produced by growing soil
actinomycetes in pure culture).
belong to the kingdom Monera and can
be either autotrophic or heterotrophic and can
be both aerobic and anaerobic. They are single-celled,
prokaryotic organisms and are the most diverse group of soil
organisms. A gram of soil may contain 20,000 different species.
They have evolved mechanisms to adapt to life in the most
extreme of environments. Due to their diversity they play many
roles in the soil and are involved in all of the organic
transactions. Their most important role is in decomposition of
dead organic matter and mineralization of nutrients such as N
and S to forms available to plants. Another critical process in
which bacteria are prominent is N-fixation.
Cyanobacteria (formerly called blue-green algae) belong to soil
microflora and are either autotrophic or heterotrophic. They
are also prokaryotic and among the most ancient of organisms.
Many genera of cyanobacteria can fix atmospheric N (e.g. Anabaena
sp.)and are especially plentiful in rice paddies and other
wetland soils, where they fix large quantities of N.
animals often account for only a small fraction of the total
breakdown of organic matter, which is mostly due to microbes.
However, they have a strong influence on the rate of organic
matter breakdown, because their chewing and shredding activity
divide organic matter into smaller particles, which increases
the surface area exposed to microbial attack.
Vertebrates (mice, moles, ground squirrels, etc.) are an example of
soil macrofauna. They
mix soil through their burrowing activity and contribute to the
formation of an Ah horizon.
Annelida (segmented worms) are another example of soil macrofauna.
They are decomposers (detritivores). The most important species
of Annelida are earthworms (especially those from genera Lumbicids).
Earthworms are the original tillers of soil and can thus improve
the rooting environment for plants and increase the amount of
water that infiltrates the soil. Earthworms mix soil and organic
matter and leave casts (their wastes) in the soil to become soil
University of California
Sustainable Agriculture Research &
Arthropods are fauna with a jointed exoskeleton and can belong to
the macro or meso group. They are a diverse group of bizarrely
shaped spiders, mites, pseudoscorpions, and insects. Many
arthropods prey on disease-causing pests. Others help to shape
soil structure and thus can improve root development, water
infiltration, drainage, and aeration. Some arthropods are the
front line in decomposition of organic matter. They shred plant
residues, mix them with soil, and stimulate decomposition within
their intestines. They are especially important in forests,
rangelands, no-till cropland, and in other areas where the soil
is minimally disturbed and covered all year.
Protozoa belong to soil microfauna. All protozoans are
heterotrophic, and usually obtain their food through some form
of ingestion followed by intracellular digestion. They are either bacterivores or fungivores. A few are
capable of alternating between a heterotrophic and autotrophic
mode of nutrition, depending upon the resources available.
Finally, some are internal or external parasites of animals.
Some species actively prey on live bacteria, whose population
they control. They are commonly found near roots and other
places where bacteria congregate. Typically when bacterial
populations increase (i.e. following rainfall) protozoan
populations also increase. Because protozoa have lower N
requirements than many bacteria, they excrete excess N obtained
from the consumption of bacteria as ammonia (inorganic N). Hence
protozoa (together with nematodes) are important in enhancing
mineralization in soil by releasing the N taken up by bacteria.
subdivision of this group into different phyla is based on how
they feed and move. The general trend in the taxonomy is to
distinguish between those
(1) that move by "cilia" ciliates
(the largest soil protozoa),
(move by flagellum), and
(3) amoebas (move by
Considerable number of animal and human diseases
are attributed to infection by protozoa, but mainly by those
that are waterborne, rather than soilborne.
Nematodes (commonly known as threadworms or roundworms) belong to
soil microfauna. They are found in almost all soils in
surprisingly large numbers. Some are plant parasites that infest
roots, some are predators that feed on other nematodes or
bacteria, fungi, and protozoa. Like protozoa, nematodes have
lower N requirements than many bacteria. Soil compaction
generally reduces the populations of nematodes, which need
adequate space between soil aggregates to move around.
Dr. T. Loynachan, Iowa State University: website
18 movies on soil biology.