Maximum Yield Australia/New Zealand March/April 2020 | Page 42
M
icroorganisms living in the
rhizosphere have a symbiotic
relationship with the plants who host
them. Bacteria, fungi, nematodes,
protozoa, and arthropods are all
beneficial microbes connected in the
soil food web and working together
to feed the plants in our garden. As
gardeners, it helps us to learn about
these beneficial microbes and how to
support them, so they thrive in the soil
and work to give us a healthy garden.
BACTERIA
One teaspoon of healthy soil
contains between 100 million and
one billion bacteria. These tiny,
one-celled organisms are a miniscule
0.1 millimetres wide and only slightly
longer in length. They are the first
microbes to digest new organic matter
in the soil. They recycle nutrients,
suppress diseases, and improve the
soil’s ability to hold water. There are
four kinds of bacteria: decomposers,
mutualists, lithotrophs, and pathogens.
Decomposers feed on carbons,
such as root exudates and plant
litter. After digesting them, their
waste becomes food for other soil
organisms in the food web. This way,
nutrients such as nitrogen are kept
in the root zone instead of leaching
out into the groundwater.
Mutualists have symbiotic
relationships with plants. For example,
nitrogen-fixing bacteria make their
home in the root zone of legumes and
certain trees. They convert nitrogen
from the air into a plant-available form.
This nitrogen is then returned to the soil
when the plant drops its leaves.
Instead of consuming carbon
compounds, lithotrophs consume
nitrogen, sulfur, iron, and hydrogen.
They play a role in the nitrogen
recycling process and can be helpful
at degrading pollutants.
While bacterial pathogens can
damage plants, they can be kept in
check with a healthy and diverse
microbial community in the rhizosphere.
Other microorganisms will compete
with the pathogens and keep their
populations and check.
FUNGI
Fungi are another key player in the
soil food web. These microscopic
cells grow strands called hyphae that
weave throughout the soil and bloom
above the soil as mushrooms. They
can be grouped as decomposers,
mutualists, and pathogens.
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Maximum Yield
Decomposers play an important
part in nutrient cycling by breaking
down hard-to-digest materials, such
as cellulose and lignin, and retaining
the nutrients in the soil. They also
help create humus, which is resistant
to degradation and holds water well.
Mutualists have a symbiotic
relationship with plants. They
colonise the root zone and send
out their hyphae to create a vast
underground network. The hyphae
resemble roots and can travel miles
underground. Plants can communicate
with one another through this network
and warn one another of potential
threats. These mycorrhizal fungi will
bring soil nutrients and water back
to its host plant.
Pathogenic fungi, such as Verticillium
and Pythium, can reduce a harvest
or kill plants completely when they
colonise their root zone. However, some
pathogenic fungi help to control pests
and disease. For example, certain
strains will set traps for harmful
nematodes. Some strains create sticky
traps, while others make circular
rings with their hyphae to constrict
and kill their prey.
PROTOZOA
Protozoa are several times larger
than bacteria and can be classified
into three groups: ciliates, amoeba,
and flagellates. Ciliates consume
bacteria, amoebas, and flagellates.
They can eat up to 10,000 bacteria
per day, and then release nitrogen
in a form plants and other soil
organisms will dine on.
“
Fungi are another
key player in the
SOIL FOOD WEB.
These microscopic
cells grow strands
called hyphae that
weave throughout
the soil and bloom
above the soil
as mushrooms.”