Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/13/2000
Publication Date: N/A
Citation: Interpretive Summary: Sorption (binding) of herbicides to soil controls the availability of the herbicide to plants, and for movement through soil. Sorption is affected by a number of soil properties including soil organic carbon (OC) and clay contents. For acidic herbicides, like imazethapyr, the most important factor affecting sorption is soil pH. Sorption is also affected by aging in nthe field, which results in decreased availability for degradation and movement. The increased persistence of imazethapyr in low pH, high OC and clay content soils results in injury to non-target plants by carrying over into years following herbicide application. The objectives of this research were to determine the influence of soil pH/sorption interactions as a function of aging on imazethapyr soil carryover as indicated by sugarbeet injury and to determine if increasing soil pH with lime would decrease sorption of aged imazethapyr residues and subsequent potential carryover. We found that although pH did not affect imazethapyr persistence in soil, it appeared to affect its bioavailability, resulting in carryover effects on sugarbeet. Two years after imazethapyr application, residues at low pH were more bioavailable than at high pH, as indicated by lower sugarbeet root yield at low pH. We found that raising soil pH with lime would reduce sugarbeet injury from carryover of imazethapyr, but would not totally prevent sugarbeet injury two years after application. More research is necessary to determine to what levels fields have to be limed to decrease potential carryover problems. Results of these experiments also suggest that by liming low pH soils, farmers will increase the sugarbeet yield potential of the soils, even in the absence of herbicide residue.
Technical Abstract: Soil pH affects imazethapyr sorption-desorption, which in turn can affect persistence and bioavailability. Long-term imazethapyr carryover has been observed in soil that is below pH 6.5, resulting in significant sugarbeet damage. We found imazethapyr concentration decreased rapidly in field soil, regardless of pH. In spite of similar amounts of imazethapyr remaining in aged soils at different pH levels, there were differences in bioavailability, which can be explained by sorption-desorption. At low pH more imazethapyr was sorbed than at high pH, but it readily desorbed. At high pH less imazethapyr was sorbed initially, but it did not readily desorb. Thus, after three months, the remaining imazethapyr in low pH soil was desorbable and bioavailable, resulting in injury to canola and sugarbeet. Liming aged, low pH soil released bound imazethapyr residues, which would then be degraded and result in less carryover.