Many herbicides are degraded by soil microorganisms, such as fungi, bacteria, actinomycetes, and algae. Soil type, pH, organic matter content, and moisture influence soil microorganism composition. The rate at which soil microorganisms degrade herbicides depends on environmental and soil conditions affecting the microbial species composition and population, such as temperature and moisture. The activity level of most soil microorganisms increases with soil temperature. Minimal degradation occurs when soil temperatures approach 40 degrees Fahrenheit.
Soil microorganisms need moisture to degrade herbicides rapidly. Under extremely dry conditions, the degradation rate can slow enough to allow the herbicide to persist into the next growing season.
What can be done to minimize the risk of injury to rotational crops from residues applied during the previous growing season?
"Herbicide degradation simply takes time and moisture," Hager said. "Soil moisture across much of Illinois is more abundant than at this time last year, but the activity of the soil microbial populations remains limited under the current soil temperatures."
He added that shallow tillage can help distribute herbicides more evenly across a field and can help enhance degradation, especially when soil temperatures are warm and soil moisture is adequate. Early planting or planting a rotational crop that is very sensitive to the herbicide applied last season increases the likelihood of crop injury from herbicide carryover.
Ultimately, crop susceptibility determines whether persisting residues will cause problems. "Planting the same crop next season as was planted in 2012 would effectively eliminate the potential for crop injury from herbicide residues," Hager said. "This solution may not be feasible for every situation where herbicide carryover is possible, but it is an option that warrants consideration."
Hager suggested delaying planting as long as possible if crop rotation must occur where there is concern about carryover.
Source: University of Illinois