The soil is alive! The earth beneath our feet is filled with an amazing
frontier of bizarre minuscule creatures of varied biology. While some
people may find it hard to believe, our lives actually depend on these
minuscule creatures living in the soil.
Some of the most important soil organisms include fungi, bacteria, protozoa,
nematodes and arthropods. Individual species do not directly contribute
to general soil health, it is rather the interactions between the many
species that are important.
Who lives underground?
Importance to soil
Bacteria on root hair
Bacteria are the only organisms on earth that can fix atmospheric
nitrogen into ammonia, the form of nitrogen that plants can absorb.
These bacteria are often integrated into the plant tissues (e.g.
roots) of legumes (e.g. clover, beans), and form symbiotic relationships
in which both parties mutually benefit, i.e. plants get nitrogen
produced by the bacteria and the bacteria gets a place to live and
food (carbon) produced by the plant through photosynthesis.
Bacteria are tiny (can only be seen under light (x1000) or
electronic microscopes) single- celled life forms.·
Cyanobacteria (photosynthetic bacteria) secrete a polysaccharide
that binds and holds soil particles together.
Their numbers can exceed 1 thousand billion per gram of soil
with over 20 000 different species.
The carbon cycle
Bacteria and fungi help produce carbon dioxide (CO2) from dead organisms
so that plants can use it for photosynthesis.
The organelles that make energy for your body (i.e. mitochondria)
and the site of photosynthesis on green plants (i.e. chloroplast)
were originally free-living bacterial cells.
Bacteria decompose dead organisms, freeing nutrient components that
are trapped to make them available for other organisms.
There are more bacterial cells on a human body than body cells.
Bacteria help clean up our wrong doings, such as oil spills, pesticides,
and other toxic materials, by returning them to their natural state
in the environment while adding much needed fertility to the polluted
A special group of bacteria that forms branched thread like filaments
and gives soil its distinctive scent.
The above graphics are by permission of Saskatchewan
They decompose dead leaves and wood that cannot be broken down by
The fungal lineage includes common mushrooms, rusts, smuts,
puffballs, truffles, morels, moulds, and yeasts.
Fungi, both free-living and as a part of lichens (symbiotically
associated fungi with algae or cyanobacteria, fungus and algae), contribute
to soil stability by binding soil particles with hyphae (fungal cells).
They provide numerous drugs (e.g. penicillin), foods like mushrooms
(i.e. fruiting bodies, which are reproductive structures that
produce spores), anti-rejection drugs (e.g. cyclosporine) used
for organ transplants, and air bubbles in bread, champagne, and
Mycorrhizal fungi improve the host plant’s efficiency at obtaining
nutrients and water while increasing the surface area associated with
the plant root fungus, allowing the plant root more reach to explore
the ground for nutrients and water.
Some fungi (i.e. mycorrhizal fungi) develop a symbiotic relationship
with most plant roots except brassicas. Some plants can’t
survive to maturity without the fungi and are referred to as mycorrhizal-obligates.
Help spread beneficial microbes, control disease-causing organisms
and regulate organism numbers by preying on them.
Importance to soil
Keeps bacterial communities growing by consuming vast numbers·
A group of microscopic animal-like, single-celled creatures
that are present in large numbers near plant roots due to the
abundance of food (i.e. bacteria and organic matter).
Provides nutrients to plant roots and stimulates the rate of decomposition
through increasing bacterial activity, by excreting digested wastes.
Some protozoas tend to live inside the guts of other creatures,
e.g. the protozoa Trichonympha lives inside the termite gut and
helps break down wood (cellulose) from its food but the actual
digestion of wood is done by the bacteria living inside the gut.