Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2001
Publication Date: N/A
Interpretive Summary: Binding of herbicides to soil particles controls the availability of the herbicide for movement through soil. Binding is affected by various soil properties of which soil organic carbon (OC) and clay contents are the most important. For acidic herbicides, like imazamox, the most important factor affecting binding is soil pH. Binding also increases with aging in the field, which would result in decreased availability for degradation and movement. Imazamox persistence has been shown to increase as soil pH decreases and OC and clay contents increase. The increased persistence of imazamox may result in injury to non-target plants by carrying over into years following it's application. Objectives of this research were to determine the influence of soil pH/binding interactions as a function of aging on imazamox soil carryover as indicated by sugarbeet injury and to determine if increasing soil pH with lime would decrease binding of aged residues and potential carryover. We found that although soil pH did not affect imazamox persistence, it appeared to affect its bioavailability, resulting in carryover effects on sugarbeet. For instance, one year after application, imazamox residues at low pH were more bioavailable than at high pH, as indicated by the significantly lower sugarbeet root yield at low pH. We found that raising soil pH with lime reduced sugarbeet injury from carryover of imazamox, but did not totally prevent injury one year after application. More research is necessary to determine to what levels fields have to be limed to decrease carryover problems. Results also suggest that by liming low pH soils, farmers will increase the sugarbeet yield potential of the soils, even in the absence of herbicide residues.
Technical Abstract: Long-term carryover of some imidizolinone herbicides has been observed in soil that is below pH 6.5, resulting in significant sugarbeet damage. In a field study in Southwest Minnesota, imazamox concentration decreased rapidly in field soil, regardless of pH. Despite similar amounts of imazamox remaining in aged soils at different pH levels, bioavailability differed with pH, which can be explained by pH sorption-desorption interactions. At low pH more imazamox was sorbed than at high pH, but it readily desorbed. At high pH less imazamox was sorbed initially, but it did not readily desorb. Thus, after three months, the remaining imazamox in low pH soil was desorbable and bioavailable, resulting in injury to canola and sugarbeet. Adding lime to aged low pH soil, released bound imazamox residues, which would then be degraded and result in less carryover.