Soil-residual herbicides effectiveness has been reduced due to the dry soil conditions throughout most of the state during the 2012 growing season. Associate professor of weed science Aaron Hager warns these conditions also slow the rate of herbicide degradation, increasing the likelihood that herbicide carryover could damage rotational crops.
"Once a herbicide is applied, there is little that can be done to shorten the time it remains active in the soil environment," Hager says. "Herbicide persistence depends on the interaction of the herbicide's characteristics, the soil, and climactic conditions"
Characteristics of the herbicide molecules affect its persistence in the soil. Some herbicides, such as thifensulfuron, have very little soil persistence while others, such as picloram, can persist for several months.
"Soil persistence can also vary among herbicides within a particular chemical family," Hager notes. "For example, within the imidazolinone herbicide family, soil persistence of imazamox is much shorter than that of imazethapyr."
Chemical, physical, and microbial soil properties also influence herbicide persistence. For some herbicides, soil pH is important because it impacts how much herbicide is available for plant uptake and how quickly it is degraded by hydrolysis.
"For example, atrazine is more available for plant uptake when the soil pH is approximately 7.0 or higher," Hager explains. "It also persists longer because the rate of hydrolysis slows at higher soil pH values. The degradation rate of many sulfonylurea herbicides slows with high soil pH values. Imidazolinone herbicides persist longer in soils with low pH values."