Soil
Micro-organstructure
isms
Small workers play big role on every farm
by Kathleen King,
CSIRO
T
he ability to grow high yielding crops and
pastures greatly depends on the health and
number of micro-organisms in the soil.
Healthy soils are the result of a range of
interactions between air, water, particles,
nutrients, organic waste and micro-organisms.
M i c r o - o rganisms play a major role in
improving soil structure and fertility by helping
to release and return plant nutrients to the soil.
These organisms are most active in plant litter
and dung within the top 10 centimetres of soil.
Activity levels are strongly influenced by the
distribution and quality of organic matter which
is used as a food source and habitat by soil
organisms. Moisture, temperature and aeration
also affect activity levels.
Soil organisms comprise invertebrate animals
and microbes. Large invertebrates range from
fly larvae and beetles to earthworms.
Microarthropods, comprising springtails and
mites which are about two millimetres long, are
another important group. Nematode worms are
smaller still and need a film of water lining the
soil pores to move through. The smallest soil
organisms are bacteria and fungi with some
Dung beetles rapidly colonise cow pats. The buried dung breaks up faster underground because the soil
provides a more stable environment for decomposition. The cow pats are also immediately colonised by
bacteria and fungi.
bacteria being as small as 0.001millimetres.
But size is no indication of an or g a n i s m ’s
importance in an ecosystem because the
smallest organisms occur in the largest numbers
and are very active.
A better comparison of importance across
organism groups is to consider their biomass,
which is calculated by multiplying the number
of organisms and their average liveweight.
Soil and litter microbes comprise 75 per cent
of all living non-plant tissue in intensively
grazed improved pastures.
Sheep comprise 17% and invertebrate
animals only 8% of the total biomass on
pastures grazed by 20 wethers per hectare. In
biomass terms, the microbes were equivalent to
88 sheep/ha. The usefulness of these organism
is measured by determining their metabolic
Intentionally
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FARMING AHEAD No. 64 - April 1997
Micro-organisms...
activity or respiration rate. This allows for
d i fferences in size as smaller animals have
higher levels of activity per unit weight.
Temperature regulation is also important
because invertebrates and microbes reduce their
metabolic activity in cold weather.
Soil organism roles
The overwhelming majority of or g a n i s m s
that colonise and consume organic matter are
beneficial and are often referred to as
decomposer organisms. Their three main
functions are to break down organic matter, recycle nutrients and enhance soil structure.
Dead organic matter is mainly derived from
uneaten plants, dead roots and the excreta of
grazing domestic animals.
Newly formed organic matter is immediately
colonised by fungi, bacteria and yeasts growing
on the surface of dead leaves and in dung.
Invertebrates such as dung beetles and
earthworms consume and bury this material.
Organic matter breaks down faster underground
because the soil provides a more stable
environment for decomposition.
Nutrient cycling
Nutrient cycling is the end product of
decomposition.
In general, decomposer invertebrates do not
degrade much organic matter in the
biochemical sense as their digestive systems
have few enzymes capable of releasing
chemically bound nutrients. This is the main
job of the microbes which have the central role
in nutrient cycling.
Microbial enzymes can break these bonds,
releasing the mineral elements which become
available for reuse by plants or other soil
organisms.
Invertebrates enhance microbial activity by:
• Fragmenting organic matter, increasing its
surface area for colonising microbes.
• Burying organic matter, promoting faster
decomposition rates.
• Selectively grazing and rejuvenating old
microbial colonies.
• Enhancing nutrient release from microbial
tissue through feeding activities.
• Distributing microbial spores through the
soil, either in the gut or on their skin.
Enhancing soil structure
Soil structure is determined by the size and
arrangement of soil particles and pores. A
network of tunnels or pores is created by the
burrowing of larger invertebrates in the topsoil
layer.
Macropores lower soil bulk density, improve
aeration, drainage, water infiltration, soil water
availability and reduce erosion. Plant roots find
their way more easily down these channels.
The ability of soils to retain water is
improved by the activities of larger soil animals
which mix mineral and organic particles. Soil
structure is also enhanced by organic binding
agents or gums secreted by microbes that
cement soil particles together. Fungal filaments
also bind soil particles into water - s o l u b l e
FARMING AHEAD No. 64 - April 1997
aggregates which helps reduce erosion. The
passage of soil and organic material through the
earthworm gut also assists in the aggregation of
soil particles.
Livestock stocking rates
Overgrazing decreases the numbers of soil
organisms in native and improved pastures.
The litter layer on the soil surface is several
millimetres thick when pastures are moderately
stocked. This layer has a major effect on the
ability of soil organisms to survive and
ultimately benefits the soil.
With moderate stocking rates or g a n i c
material and nutrients such as urine and dung
are also spread more evenly over the surface.
Surface litter acts as an insulation layer
providing protection to the biological activity in
the topsoil. It can also act as a mulch which
slows evaporation.
Soils are less compacted under moderate
grazing, leaving a network of soil pores for
smaller microarthropods and nematodes.
Agrochemicals
Fertilising with superphosphate and sowing
with productive grasses and legumes increases
both the production and quality of feed for
grazing stock.
Some people believe artificial fertilisers
decrease the biological activity of soil. But
there is a gradual increase in numbers of soil
fauna following the application of artificial
fertilisers.
This is probably the result of increased plant
productivity and quality but a few short-term
e ffects of nitrogenous fertilisers on
microarthropods have been reported for forest
soils with elemental sulphur appearing to harm
fungi and protozoa.
Decomposer invertebrate animals increased
considerably in numbers and biomass during a
trial at Armidale, New South Wales, when
native pastures were improved with
superphosphate and phalaris and white clover
were sown.
Springtail numbers on improved pastures
were three times higher than on native pastures.
The trial also demonstrated a substantial
contribution to the transfer rates of nutrients,
for example, phosphorus, sulphur and nitrogen
at a
GLANCE
• Biological activity is vital to
maintain soil structure and fertility.
• A square metre of pasture soil can
contain hundreds of invertebrate
species.
• Overgrazing reduces the number of
soil organisms in native and
improved pastures.
• Soil organisms increase after
artificial fertilisers have been
applied.
Fertilisers maintain nutritious perennial grasses
and legumes that sustain high levels of biological
activity in the soil, provided it does not become
too acidic or saline.
by microarthropods from plant litter to the soil.
Applying lime to improve soil structure or soil
pH can change the composition of the organism
population. But this change rarely occurs until
a pH of about 6.0 is reached. An initial, shortterm, increase in bacterial populations has been
reported with liming.
The use of some antiparasitic drugs can
cause harmful effects on the first soil organisms
to arrive at the scene of fresh dung pats
containing these chemicals.
Dichlorvos, Phenothiazine and Ivermectin
have had adverse effects on dung beetles and
fly larvae.
But the overall effect of the chemicals on
dung decay and the release of nutrients from
dung may be small over the course of a whole
year and is a small ecological price to pay given
the value of these drugs.
The use of herbicides on pastures is low
compared with cropping soils.
Pasture cultivation
Cultivating during pasture establishment and
renovation reduces the number of soil animals
through the physical disruption of their habitat.
Smaller animals with shorter generation
times recover within a single growing season
but it can take up to two years for lar g e r
animals such as earthworms to return to precultivation levels.
Tillage decreases soil structure and microbial
populations but this is probably due to a
depletion in organic matter rather than any
physical effect of cultivation on microbes.
The disturbance of soil organisms during
pasture renovation is less with combined use of
direct drilling techniques and herbicides.
Moderate grazing has little effect on soil
organisms or botanical composition.
Fertilisers maintain nutritious perennial
grasses and legumes that sustain high levels of
biological activity in the soil provided it does
not become too acidic or saline. It would seem
to be a case of what is good for the plant
is good for soil organisms.
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