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Rain will be Scarce the Next Seven to Ten Days. What is the Fate of my Soil-Applied Herbicide?

A weedy soybean field photographed in western Illinois in mid-June 2012 during a historic drought (photo: Angie Peltier).

This article was written by University of Minnesota/NDSU Extension sugarbeet agronomist Dr. Tom Peters and originally posted in volume 5 of the 2020 NDSU Crop & Pest Report. 

Many producers are planting. Producers have heard the messages from Extension, ag-retailers, and crop consultants to use soil applied herbicides since conditions are also correct for weed emergence. The question is should I apply a PRE herbicide and will PRE herbicides be stable following application, especially if rainfall is spotty the next 7 to 10 days.

Unfortunately, there will be some losses especially if herbicides remain on the surface for more than two weeks. But in general, the herbicides most frequently used today will withstand our environmental conditions since they are not volatile and are adsorbed to soil colloids following application.

Herbicide volatility (evaporation), adsorption and solubility interact with rainfall and effect the fate of soil-applied herbicides. Volatility is the change in herbicide physical state, from a liquid to a gas. Most soil-applied herbicides used by farmers have a medium or low vapor pressure meaning they generally will not volatilize during warm and dry conditions. Adsorption is the attachment of herbicides to soils. Herbicides must be bound to soils or they would easily leach away. Most herbicides are moderately or strongly bound to soils colloids and should not be impacted by our dry conditions. Water solubility is a measurement of how much of a chemical will dissolve in water, and typically is expressed in parts per million. The greater the solubility, the more of the chemical that dissolves in water. Adsorption and solubility are inversely related. Thus, as solubility increases, binding to soil decreases.

Soil type influences herbicide activation. In general, herbicides are more readily available in coarse than medium or fine textured soils. Likewise, soil moisture levels at the time of application and following application influence herbicide performance. A dry soil requires more rainfall to activate soil applied herbicides than moist soils since rainfall must first wet a dry soil surface before significant movement of the herbicide into the soil profile will occur.

To summarize, I am confident herbicide applied this week will be stable and will be activated and control weeds if at least 0.5-inch rainfall occurs in the next ten days. Activation and resultant weed control will be dependent on your soil type and water solubility of the herbicide you select.

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