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4.4 Herbicides and Life in the Soil

by VVSR Gupta, CSIRO Land & Water

Pesticides are an essential in modern agriculture.  In Australia over $800 million is spent each year on pesticides, with 65% of this expenditure on herbicides.

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 functions.

Please note: 

  • 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 Graph 1).

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

Sulfonylureas (SUs)   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.

Triazines   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

photosynthesis at    term effect on soil bacteria especially nitrogen fixers and those

photosystem II   involved with nitrogen cycling.  May increase the chances of some       fungal, viral and nematode induced diseases.

General Findings from Australian and

Chemical and Group

International Research

Trifluralin    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

Tubulin formation   fungal, viral and nematode induced diseases.

Hormonal Sprays    Negative effects have been recorded for nitrogen fixation by rhizobia

Group I - Disrupters of plant    and reduction in colonisation by mycorrhiza.  Long term effects are

cell growth   variable and dependant on soil type and environment.

Paraquat    Reduced rate of stubble decomposition when sprayed on to stubble

Group L - Inhibitors of    rather than on to the soil containing stubble.  Non-symbiotic  nitrogen

photosynthesis at    fixing bacteria are inhibited even at very low concentrations.

photosystem I

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:

  • rate of application

  • type of herbicide

  • place of application - soil, stubble or plant

  • frequency of application

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

Fusilade®   Ally®   Paraquat®

Roundup®   Glean®   Brodal®

Hoegrass®   Logran®

Logran®   Eclipse®

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

Disease Transmission

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 been applied.

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

Bacteria   Fungi

population/g    % remaining   population/g   % remaining

stubble   of control   stubble  of control

Control   389000   100%   335000   100%

Ally®   3640   0.9%   6510   2%

Hoegrass®   38900   10%   313   0.09%

Nutrient Cycling

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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