Maintaining proper soil pH can limit atrazine carryover in a no-till, corn-soybean rotation, preventing potential soybean yield losses. That's the message from University of Missouri agronomist Peter Scharf.

“A number of chemistries in use today are pH-sensitive, and are labeled as such,” says Scharf. “From our experiments, however, the product with which producers are most likely to see carryover is atrazine, as it appears to be sensitive to both high and low soil pH values.”

“No-till presents some unique challenges relative to the soil pH and soil-active herbicides,” he says. “Typical lime rates can produce very high pH in the top inch of soil where herbicides are active. Then, if nitrogen is surface-applied, its acidifying effect can produce very low pH in that same top inch.”

In Scharf's experiments, research plots represented a range of pH values from 4.9 to 7.6 in the top inch of soil. “In each soil type, we planted corn and soybeans, applying one of four herbicide treatments,” he says. “The following year, we rotated crops in the plots, then watched for any carryover effects.”

During each two-year experiment cycle, Scharf found no evidence of carryover effects on corn yields from the previous year's soybean herbicides. However, soybean yields were affected by atrazine applied to corn the year before.

Scharf recommends making smaller, more frequent lime applications than would be typical in a conventional-tilled system. “This will help prevent very high pH in the top inch of soil, where atrazine is mainly active and will further reduce the likelihood of carryover.”

In no-till systems, determine where soil pH differences occur within a field, then correct them using a variable-rate liming application program, Scharf says.