Impact of strategic deep tillage of sandplain soils on the biological availability of pre-emergent herbicides

Tom J Edwards

Strategic deep tillage, through soil inversion and deep soil mixing, can ameliorate multiple constraints and significantly increase crop production on the sandplain soils of Western Australia. For these soils, an increase in herbicide usage is strongly correlated with a decrease in regular tillage, and how the two interact is imperfectly understood. As a result, current herbicide strategies and rates of application are designed to perform optimally in a minimum tillage cropping system. Understanding the fate and phytotoxicity of these herbicides following deep tillage is critical to developing optimal herbicide strategies. Two field experiments were established in Esperance (Esperance TJM) and Geraldton to evaluate a range of common pre-emergent herbicides following deep mixing or inversion, and to compare the risk of crop toxicity in these soils with soil that has remained under minimum tillage. Both strategic tillage methods significantly changed the soil surface composition and reduced the organic matter content. Metribuzin, trifluralin and diuron treatments recorded incidence of reduced crop (wheat) emergence and vigour following tillage. Further, metribuzin and diuron treatments reduced yield by a greater extent on both the soil inversion and deep mixing treatments (p<0.001) compared to the control soil treatment at both sites. The increased risk of herbicide phytotoxicity persisted beyond the first year and significant reductions in wheat emergence and vigour were recorded in the third growing season at the Esperance site. These results reflected that the influence of deep tillage on the toxicity of herbicides is site specific and highly dependent upon the interactions of soil properties and rainfall, with the type of herbicide and its associated chemical properties. The physical composition of the soil (percentage of sand, silt, clay and organic matter) and the chemical properties of the soil (pH, nutrient levels, cation exchange capacity) will strongly influence the soil adsorption of herbicides. Soil samples (0-10cm) were collected from control (no tillage) and following deep mixed and inversion tillage at three experimental sites: The Esperance TJM site and Geraldton site and an Esperance E1 site. At all three experimental sites, the implementation of tillage reduced the measured soil-liquid partition coefficient (Kd) of trifluralin and diuron. The Kd coefficient provides a measure of the adsorption of the herbicide by soil, impacting the concentration that is biologically available to the crop. An increase in the biological availability of trifluralin and diuron following strategic deep tillage explains the increased incidence of crop toxicity following application of these herbicides in the Esperance and Geraldton field experiments. A detailed screenhouse bioassay was developed to determine if the soil changes from strategic tillage could be directly related to herbicide bioavailability. Intact cores maintained the integrity of the condition of the field soils with and without tillage. The cores from both field sites demonstrated that soil inversion reduced the effective dose of trifluralin required to reduce growth by 50% (p<0.01). Therefore, there is compelling evidence that the reduction of organic matter in the topsoil from deep tillage can increase the bioavailability of trifluralin.

Impact of strategic deep tillage of sandplain soils on the biological availability of pre-emergent herbicides

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