4.4 Herbicides and Life in the Soil
Pesticides are an essential
in modern agriculture. In Australia
over $800 million is spent each year on pesticides, with 65% of this expenditure
Many herbicides are applied
directly to the soil, which has lead to questions about their impact on the soil
biota - the Life in Soil.
Soil biota consist of a
diverse range of organisms; both flora (microorganisms) and fauna (animals).
These organisms carry out key soil functions and are grouped by both body
size and function (see Figure 1). For
example, the function of nutrient cycling is carried out by microflora, eg
bacteria, microfauna, eg protozoa and mesofauna, eg collembola (springtails).
All the organisms involved in
a specific function are grouped and termed a functional group, eg nitrification
is carried out by nitrifying microorganisms.
Agronomic practices which
interfere with specific organisms may reduce the output of a functional group
but may not cause it to cease. However,
this may result in a new population balance which could lead to plant pathogenic
organisms becoming dominant.
An understanding of the
impact of different herbicides on various soil functions will help farmers work
in harmony with the soil biota and minimise the impact of herbicides on these
The following information
draws on Australian and international research.
A majority of herbicide
research is laboratory based. This
is because field fluctuations in temperature and moisture and the variability in
physical and chemical properties of soil may influence the soil organisms to a
greater extent than the application of a pesticide. Direct transfer of results from lab research to field
situations may not be appropriate.
Physical versus chemical weed control
The appropriate use of
herbicides may be less destructive to the soil environment and soil organisms
than traditional weed control techniques of cultivation and stubble burning (see
For this to hold true, the
following management practices need to be in place:
the retention of stubble/organic residues, a
major carbon source for soil organisms
low rates of chemical application
a ‘recovery period’ for soil biota is
allowed between herbicide applications
repeated application of the same herbicide
within a short period is avoided.
Are all chemicals OK?
Appropriate chemical use with
stubble retention may be better for soil biota than cultivation and burning, but
it does not mean that it is ‘good’ for soil biota.
In general, soil biota may be affected by herbicide applications either
directly or indirectly. A direct effect is when the chemical kills the organism or
inhibits its activity. Indirect
effects include changes in soil temperature and moisture due to removal of
weeds, the addition of weed residues with low C:N ratios and changes in the
populations of predators and prey.
Details of the impact of some
widely used agricultural herbicides are contained in Table 3.
In many cases there are no
definitive answers and only limited work has been carried out in Australia.
Each herbicide needs to be considered separately.
Table 3: A summary of
findings for a range of chemicals tested in Australia or overseas
General Findings from
Australian and Chemical and Group International Research
Reduced ability of microorganisms to grow up to six to eight weeks Group
B - Inhibitors of after
application (microbial stress). May
result in reduced nitrogen
the enzyme acetolactate
mineralisation. May cause increased incidence of plant diseases such
synthase - ALS inhibitors.
as Rhizoctonia and Take-all. Some herbicides also reduce colonisation
by mycorrhizal fungi. No
information on long term effects.
Under long term field applications generally no effect on soil biota
Group C - Inhibitors of
providing organic matter was continually returned to the soil. Short
term effect on soil bacteria especially nitrogen fixers and those
involved with nitrogen cycling. May
increase the chances of some
fungal, viral and nematode induced diseases.
General Findings from
Chemical and Group
No indication of long term effects.
It has been found to interfere
Group D - Inhibitors of
with legume/rhizobia symbiosis. May increase the chances of some
fungal, viral and nematode induced diseases.
Negative effects have been recorded for nitrogen fixation by rhizobia
Group I - Disrupters of plant
and reduction in colonisation
by mycorrhiza. Long term effects
variable and dependant on soil type and environment.
Reduced rate of stubble decomposition when sprayed on to stubble
Group L - Inhibitors of
rather than on to the soil containing stubble.
fixing bacteria are inhibited even at very low concentrations.
Glyphosate No indication of
any long term effects. Nitrogen fixation by legumes
Group M - Inhibitors
has been shown to be
reduced in some crops. Any negative
effects of EPSP
are dependent on soil type.
In recent laboratory and
field studies carried out in Australia, a range of herbicides were applied as
single applications at a range of rates. The
following information details the key results.
Short term versus long term impact
The impact of a herbicide on
soil biota may be reversible or irreversible.
A reversible impact is one where levels of microbial growth are returned
to ‘normal’ over a period of time after a singe application. The duration of reversible
impacts has been found to vary with:
Results from both field and
laboratory research have shown symptoms of microbial stress (reduced new
microbial growth), even at recommended rates, up to six to eight weeks following
the application some herbicides, eg Glean®, Logran® (see Table 4).
These effects were found to be soil type dependent.
For example, in sandy soils with low organic matter and low microbial
activity, the negative effects of these herbicides were found to be greater.
Table 4: The impact of
different chemicals on microbial biomass (MB) and carbon dioxide production (MA)
No impact on MB &
Negative impact on
Positive or no impact
positive impact on
MA both MA & MB
In similar experiments, the
herbicides Ally®, Hoegrass® and Paraquat® were applied directly onto soil
without any stubble cover at two and five times the recommended rate.
Micro organisms were found to be under stress and not functioning
properly six weeks after the application. In
most situations this low level of functioning continued up to nine weeks.
However, when the chemical
was applied directly to the soil or to growing plants, the stress time for soil
organisms was reduced.
Multiple applicationsof the
same or different herbicides within the six to eight week window is a common
practice in broadacre agriculture. The
research has shown that it takes six weeks for the microbial activity to return
to normal. Consequently, multiple
applications before the end of the six to eight week recovery period would be
expected to reduce soil biota function, especially if organic carbon food
sources were also limited.
The duration of this reduced
biota function may be less significant than the timing.
The majority of herbicides in broadacre cropping are applied around
seeding. At germination the plant
is at its most vulnerable, as the root system is still developing.
Reduction in biota functions, such as nutrient release and disease
suppression, may reduce the early vigour of a crop or leave it more susceptible
to root diseases. Application of
another herbicide prior to the biota recovering from the first herbicide may
accentuate the undesirable effects. More
research work is needed in this area.
Impact on functions
Some herbicides were found to
alter the balance between bacteria and fungi populations near the plant litter.
For example, some SU herbicides and Hoegrass® increased the proportion
of fungi near litter. This could
result in the growth of opportunistic pathogenic organisms near the stubble,
increasing the chance of root diseases development.
Organic Matter Breakdown
Application of Ally® and
Hoegrass® at twice the recommended rate reduced the ability of bacterial
populations to use some of carbon substrates available from decomposing residues
and near the growing root. This
could result in a slower rate of stubble breakdown and associated nutrient
mineralisation. Table 5 illustrates
the significant reduction in cellulytic bacteria and fungi after herbicides have
Table 5: The number and
percentage of microorganisms which can breakdown cellulose (a major part of
straw) found near wheat straw after the application of herbicides
% remaining population/g
stubble of control stubble
Control 389000 100%
Ally® 3640 0.9%
Other work has shown that the
nitrifying bacteria, responsible for the transformation of ammonia nitrogen to
plant available nitrate, are the most susceptible to herbicide applications.
Modified applications of
fertilizer N may be appropriate to compensate for reduced N mineralisation
during the first six to eight weeks following herbicide application.
Herbicide effect on
mycorrhizal colonisation is two-fold. Firstly,
herbicides affect the mycorrhizal fungi itself and, secondly, herbicides reduce
root growth affecting the available root length for mycorrhizal colonisation.
Graph 2: Changes in levels of
mineral nitrogen after application of a chemical to the soil
Results are shown as a
percentage of the mineral nitrogen available under the control. The larger percentage suggests high levels of mineralisation
by soil organisms.
Pesticides as food
Microorganisms feed on simple
carbon compounds and agrochemicals, therefore all pesticides, to a lesser or
greater extent, can be used as a food source by the soil biota.
Different chemicals will be used by different microbes, therefore
populations of appropriate microbes must be present if the pesticide is to be
degraded by microbes.
It may take time for the
populations of appropriate microbes to build-up. During this period these pesticides may harm other soil
organisms and temporarily or permanently alter the balance of biota populations
present in the soil.
Sulfonylureas have been found
to persist longer in alkaline soils. However,
it is coming to light that in some alkaline soils, sulfonylurea residues are
becoming less of a problem. This is
thought to be due to an increase in the number of organisms which breakdown
these chemicals in an alkaline environment.
Management practices that could lead to the build-up of these microbes
are as yet unknown.
Microbial degradation of
herbicides is an important method of breakdown. As the majority of microbes in southern Australian soils are
located in the top 10cm of soil, movement of herbicides beyond this zone may
result in long-term persistence of herbicides.
4.4 Herbicides and Life in the Soil
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