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Technical Report

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herbicides, fate, water, solubility, soil-herbicide-water interactions, leaching, degradation, flooded soils


The wide use of soil-applied herbicides necessitates an understanding of their fate in soil if we are to use them in the most beneficial manner. All herbicides have some degree of water solubility and the field performance of many are dependent on ample, timely availability of soil moisture. It is important that different aspects of soil-herbicide-water interactions be thoroughly investigated. An excess use of water for irrigation purposes can result in the downward movement of herbicides in the soil profile. Subsoils are less adsorptive and a decreased degradation potential exists when herbicides move below the top 15 cm. In sandy soils, or other areas where extensive irrigation is planned it is prudent to attempt to choose a herbicide with high adsorptive capacity and low water solubility. Leaching of metribuzin, metolachlor, and fluometuron was an important dissipation process for each of the chemicals over the winter months when degradation was slow. Pendimethalin dissipation was greater in alternatively flooded and dried soil than with soil water content at 1/3 bar tension or with a continuous flood. In a laboratory degradation study, over 59% of the oxadiazon persisted after 20 weeks. In a greenhouse study its biological activity was reduced and its persistence increased when applied below the soil surface.

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