Soil microorganisms represent the primary method of degradation for many herbicides. These microorganisms include various species of fungi, bacteria, actinomycetes, and algae. The composition of microorganisms (species and abundance) present in a particular soil is influenced by soil type, pH, organic matter content, moisture, etc. Most species of microorganisms are active under aerobic conditions while others thrive under anerobic conditions. Herbicide degradation by microorganisms most often occurs when the microorganisms consume the herbicide molecules as a source of energy. Certain herbicides are more easily degraded by microorganisms than others.
The rate of herbicide degradation by soil microorganisms is often directly related to prevailing environmental and soil conditions. Any condition that impacts the microbial species composition and population can impact the rate of herbicide degradation. Temperature and moisture are two factors that can greatly influence soil microbial populations. The activity level of most soil microorganisms is higher under warmer soil temperatures than under cooler soil temperatures. This partially explains why herbicide degradation tends to slow as soil temperatures drop during the weeks after crop harvest. Minimal herbicide degradation by soil microorganisms occurs when soil temperatures reach approximately 40 degrees Fahrenheit. Herbicide degradation by soil microorganisms usually occurs most rapidly when adequate soil moisture is present. Under extremely dry conditions the rate of herbicide degradation by soil microorganisms 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? In the simplest terms, herbicide degradation simply takes time and moisture. Currently, soil moisture across much of Illinois is more abundant than at this time last year, but the activity of soil microbial populations remains limited at current soil temperatures. Shallow tillage can help distribute herbicide more evenly across a field, and is more likely to help enhance herbicide degradation when soil temperatures are warm and adequate soil moisture is present. Early planting or planting a rotational crop that is very sensitive to the herbicide applied last season might further increase the likelihood of crop injury from herbicide carryover. Ultimately, the susceptibility of the rotational crop determines whether or not persisting herbicide residues will cause any problems. Planting the same crop next season as was planted in 2012 would effectively eliminate the potential for crop injury from herbicide residues. This solution may not be feasible for every situation where herbicide carryover is possible, but it is an option that warrants some consideration. If crop rotation must occur where there is concern for herbicide carryover, delaying planting as long as possible could allow for additional herbicide degradation to occur.