101 Question answered
Understanding Herbicide Effects on Soil Microbes: Impact, Recovery, and Management
Yes, but like tillage, it might not be as bad as it sounds.
Spraying herbicide isn’t the same as dropping a nuclear bomb and obliterating all soil life. Herbicides don’t kill off every single microbe and creature in the soil, and even if they did, some microbes are protected inside soil aggregates. Even if you drenched the paddock with herbicide, it would be hard to get them all. Note we are talking about herbicide, not insecticides or fungicides which will directly kill various insects and fungi.
Herbicides do affect soil biology, either directly – killing it or slowing it down, or indirectly by changing soil conditions. This might be from more soil moisture from killing weeds, changing food sources by adding weed residues with low carbon: nitrogen ratios, and changing predator/prey populations because the herbicide had a detrimental effect on one population but not another. For example, fungal growth can increase after applying the insecticide endosulfan because it affects microarthropods that like to eat fungi.
Regularly overapplying herbicides does affect soil biology. It is difficult to point to exact products and their impacts, as there is not much work in Australia, but some things we know are:
- It takes about six weeks for microbial activity to return to normal after a herbicide application. Multiple applications in this 6–8 week period would be expected to reduce soil biota function.
- Levels of Rhizoctonia, Take-all and cereal cyst nematode can rise following crops that had sulfonylurea herbicides applied.
- Trifluralin can interfere with legume nodulation.
- Nitrifying bacteria are quite sensitive to herbicide applications. After herbicide applications there’s a drop in nitrogen mineralisation (microbes make less nitrogen available for crop use). Check out the effect various herbicides have on mineral nitrogen levels about 1 and 3 weeks after application. For Paraquat, Hoegrass® and Ally®, mineral nitrogen drops back in the first week after application.
Herbicide vs tillage
While herbicides reduce microbial activity, they can have less of an impact than cultivation and stubble burning as forms of weed control. This will only be true if:
- Stubble is retained.
- Herbicides are used at the label rate.
- There is a ‘recovery period’ (more than 8 weeks) between herbicide applications.
- Repeated application of the same herbicide within a short period is avoided.
The graph ‘Impact of different farming methods on soil biota populations’ compares how cultivation + burning vs herbicide + good farming practices affect soil biota populations. In each case, cultivation + burning leads to lower populations than herbicide + good farming practices. Herbicides were used at the recommended rates.
While herbicides + good farming practices have less of an impact on soil biology, there is still an impact, but for the moment it’s the best we’ve got. There is work underway for non-chemical weed control, such as using waterjet cutting, lasers, and electrical weeding.
Microbes breakdown herbicides
Herbicides don’t persist in the soil forever because microbes degrade them. Microbes have been around for billions of years so there isn’t much we have made that they can’t deal with in their own time.
Different microbes degrade different chemicals; degradation relies on the right type of microbes being present to do so. It might take a while for these populations to build up in the soil.
Also remember that most microbes are in the top 0 – 10 cm of soil. If herbicides move deeper into the soil, they can persist for longer.
