Teachers Guide #4.3– Soil bugs refer to FACT CARD 4.3 Soil bugs
An Introduction to soil bugs
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 plants and organisms
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
bacteria.
Soil organisms and organic matter decomposition
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
Teachers Guide #4.3– Soil bugs refer to FACT CARD 4.3 Soil bugs
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.
A closer look at who lives underground
The mighty bacteria
Bacteria
http://interactive.usask.ca
Actinomycetes
http://interactive.usask.ca
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.
Fabulous fungi (5)
Teachers Guide #4.3– Soil bugs refer to FACT CARD 4.3 Soil bugs
Fungi (4)
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.
Arthropods
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
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
Teachers Guide #4.3– Soil bugs refer to FACT CARD 4.3 Soil bugs
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
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 and organisms.
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).
Organic matter decomposition
A simplified nutrient cycle
Teachers Guide #4.3– Soil bugs refer to FACT CARD 4.3 Soil bugs