Herbicide residues are an important cropping issue, but research indicates when understood and managed they appear to have minimal adverse or long-term impacts on soil quality and soil health.
A current GRDC investment involving NSW Department of Primary Industries (DPI) is investigating links between herbicide residues, soil functions and crop productivity. Dr Michael Rose, Soils Project Officer with NSW DPI based at Wollongbar Primary Industries Institute, manages the NSW arm of the comprehensive research.
All aspects of cropping, including fertiliser use, soil disturbance (via cultivation and sowing), and herbicide application can impact on soil characteristics like biological activity, Dr Rose notes. However, he stresses that Australian and international research has found soils to be resilient overall. In most cases, if used judiciously, herbicides have only a temporary impact on soil quality.
Dr Rose points out that over 50 herbicide active ingredients are used in Australian cropping systems. Glyphosate is the most widely used herbicide, and one focal point of the research.
Residues of glyphosate and its primary breakdown component, aminomethylphosphonic acid (AMPA), are commonly found in soil after use. But even when applied experimentally at rates much higher than labelled use patterns, effects of it appears to be low to non-existent in most soil types and environments.
Studies have shown that earthworms are unaffected by glyphosate/AMPA residues at levels found in crop surveys. In contrast, cultivation greatly reduces earthworm numbers. Studies show that soil biological activity and microbe populations were not adversely affected by typical herbicide residue levels present in cropping systems.
In most situations, glyphosate will not have an adverse effect on soil fertility. Glyphosate and AMPA bind to soil particles at the same sites as phosphorus. In rare cases, such as in soils with very low clay and low organic matter contents, high soil phosphorus availability can make soil-borne glyphosate residues available to crops.
Dr Rose emphasised the importance of adhering to herbicide label instructions, and taking a conservative approach for plant back periods.
For example, when follow-up rain is not received after 2,4-D fallow application, residues can remain at higher levels than expected which may impact sensitive emerging crops. More details of the research can be found on the NSW DPI website www.dpi.nsw.gov.au/agriculture/soils/biology
