The world underground is an amazing frontier of bizarre organisms
and complex interactive networks. Though some people may find it
hard to believe, our lives depend on the minuscule creatures living
in the soil beneath our feet. Without fungi, for example, we would
be smothered by thousands of years of fallen trees, branches and
other dead plants.
There are many important functions in the soil that are dependent
on the organisms that have made it their home. Nutrient cycling,
pesticide decomposition, soil structure improvement and pest control
are but a few. More often than not, it is actually not individual
species that are contributing to overall soil health but the `interactions’
between the many species that are important.
‘Symbiotic relationships’ between
The dictionary describes symbiosis as ‘a close union of two
unlike organisms that live together in a mutually beneficial intimate
association’. In 1885, the scientist A B Frank discovered
that certain fungi form massive webs of hyphae called mycorrhizae
and live in a symbiotic relationship around the roots of certain
plants. Via these mycorrhizae the roots are able to scavenge nutrients
and water from a lot further afield than just the immediate vicinity
of the roots. The mycorrhizae can therefore be thought of as an
extension of the roots. In return, the plant roots will share carbon
and energy captured from the air and sun with the fungi.
Other important ‘symbiotic relationships’ exist between
plants and organisms such as those of the nitrogen-fixing bacteria.
Certain plants or trees (a special type known as legumes) share
the carbon and energy they have obtained through photosynthesis
in return for the much needed nitrogen that only certain specialised
bacteria are capable of obtaining from the atmosphere. White clover
is an example of a legume that is host to such bacteria. In New
Zealand, clovers are a very important pasture plant species (heavily
relied upon by pastoral farmers), as not only do grazing animals
find them very tasty, but they also add significant amounts of nitrogen
to the soil free of charge. In the garden, it is important to know
that incorporating legumes into bed rotations is a good idea, but
one must remember that the bacteria will only ‘fix’
(or scavenge nitrogen from the atmosphere) when they do not already
have a supply of nitrogen at their ‘fingertips’. Indeed
if the soil has plenty of nitrogen already (e.g. after a recent
nitrogen fertiliser application) then the bacteria will not need
to bother fixing atmospheric nitrogen, but instead will just use
up what is already around them in the soil.
The rhizosphere is the area of the soil immediately adjacent to
the plant roots and it is where large amounts of organisms congregate.
Just as we may linger around the kitchen if we are hungry and know
a good meal is about to appear, the rhizosphere is where the bacteria
can find large amounts of dead root cells and root excretions (produced
by plants) to feed upon. The bacteria, in turn, release substances
that can improve plant growth or protect the plant from pests. Indeed
some plants release special substances hoping to attract specific
Organic matter decomposition is the main process that recycles
nutrients back into the soil as well as releasing water, heat and
CO2 back into the atmosphere. Soil bugs are
the decomposers that break down dead plants and animals, releasing
nutrients back into the soil so that plants can make use of them
again to grow. They can therefore be thought of as the drivers of
the carbon cycle.
Decomposition of dead organic matter begins with large soil organisms
like earthworms, arthropods (ants, beetles, and termites), and gastropods
(slugs and snails). These organisms breakdown the organic matter
into smaller pieces that can be decomposed by smaller organisms
like fungi and bacteria. There is no doubt that the effects of soil
microorganisms ‘soil bugs’ on the soil system far outweigh
their size. Pick up a pinch of topsoil and you may be holding a
billion microscopic decomposers!
Though soil organisms are important for improving a soil’s
structure (they release certain glues that bind soil aggregates
together), under certain conditions (for example after tillage when
lots of oxygen is mixed with the soil) they can actually decompose
more organic matter than is desirable. The sudden release of nutrients
as a result of their activities will be beneficial for a short while,
but may become a problem further down the track if the organic matter
ends up being broken down at a faster rate than it is being added.
This process explains why newly broken down sod was initially so
productive for early settlers in many parts of the world and why
the ‘slash and burn’ agriculturalists have to move on
to ‘fresh’ areas once the soil becomes ’spent’.
It is now widely known that reduced tillage is a quick way to build
up soil organic matter due to the fact that organisms cannot break
the organic matter down so quickly.
By permission Saskatchewan Interactive
Bacteria are one of the simplest forms of life – they are
single celled with a length of only a micrometre or so. Soil bacteria
outnumber all other soil organisms by far. In a healthy soil their
numbers can exceed 109 or one thousand billion per gram of soil
with over 20 000 different species present. The weight of all the
bacteria in one acre of soil can equal the weight of a cow or two!
These microscopic organisms decompose all kinds of organic matter,
releasing nutrients that plants can use (a process called mineralisation).
Sniff a handful of soil and the familiar, fresh aroma comes from
a substance that scientists have shown is released by actinomycetes,
a particular group of soil bacteria. Actinomycetes are important
in helping to breakdown some of the more resistant compounds in
plant material such as cellulose and chitin. A single gram of soil
can contain hundreds of millions of soil bacteria!
Specialised groups of bacteria are extremely important in soil
since they promote vital biochemical reactions. Some bacteria help
protect environmental quality by degrading compounds that would
otherwise become pollutants. They do this by using the complex pollutive
compounds to support their growth, thereby degrading the pollutants
and improving environmental quality. For example, ammonia can be
turned into nitrate through a process known as nitrification and
this process is brought about by bacteria. Similarly bacteria are
responsible for turning elemental sulfur into the plant available
form of sulfur, namely sulfate.
There are over 50 000 species of fungi. Most are very, very small
and play an essential role in the recycling of nutrients from dead
plant matter. They are capable of decomposing (breaking down) the
parts of dead leaves and wood that generally cannot be broken down
by many of the other soil decomposers, like bacteria. They do this
by releasing specialised chemicals (enzymes) that help them absorb
the nutrients from the organisms they are decomposing! You may have
noticed a lot of mushrooms growing on a lawn where a tree once stood.
This is because many fungi, including those that make mushrooms,
thrive on decomposing wood.
There are many fungi that are helpful, e.g. penicillin and other
antibiotics are made from fungi. Fungi like mushrooms, mildew, mold,
and toadstools are not plants. They don't have chlorophyll so they
can't make their own food. The mushroom is actually just the ‘tip
of the iceberg’ as it is the fruiting body of large underground
networks of fungal hyphae that might be compared to a mass of white
shoelaces. In 28 g of fertile soil there may be as many as 2.3 km
of these thread-like mycelia.
Beetles, spiders, mites, millipedes, and other arthropods have
jointed legs, segmented body parts and no backbones (though they
do have an exoskeleton). Many of them are in the front line of the
army of organisms that decompose organic matter as they shred the
larger plant debris, mixing it with microbes and soil so it is ready
for the next stages of decomposition. Arthropods also have other
uses in the garden as many prey on disease-causing pests and influence
soil structure through their activities in the soil.
Protozoa are a group of tiny one-celled creatures. Many of them
like to eat bacteria so they are common near plant roots where bacteria
like to congregate. Protozoa numbers will go up and down depending
on what bacterial numbers are doing. For example, after tillage
or rainfall bacterial numbers generally increase and therefore so
too do protozoa numbers. Plants can benefit from the protozoa grazing
on bacteria simply because the protozoa have lower nitrogen requirements
than do most bacteria. So when they consume bacteria the protozoa
ooze out the excess nitrogen that they do not require – and
it happens to be in the form of inorganic nitrogen – exactly
the form that plants like best for uptake.
Nematodes tend to get labelled with a ‘bad name’ because
some of them attack roots and cause plant diseases (e.g. potato
cyst nematode). Nematodes are, however, an incredibly diverse group
and most species are actually beneficial. Nematodes are grazers
just like protozoa and they similarly help to release plant available
nutrients into the soil and therefore provide a useful service to
plants by releasing the nitrogen previously taken up by other micro-organisms.
Providing an inviting home for
microorganisms in the garden
Organisms do not merely live in the soil, but are also a component
of soil and can significantly influence both its physical and chemical
make-up. Just how well the organisms perform all these important
functions is affected by how well we manage the land. The more we
mistreat a soil, the more ‘sterile’ it will become and
all that beneficial work that the organisms do will have to be done
by you (or not get done at all)! Compaction of the soil, for example,
has been shown to reduce the populations of beneficial nematodes
It is important to provide an inviting home for the diverse array
of organisms that will be beneficial in your garden. Luckily, to
some extent, this will happen by default as long as you follow the
basic guidelines in the garden to create or maintain healthy soil
(i.e. by making sure there are adequate inputs of organic matter,
and that water inputs and drainage are managed efficiently).