ESTIMATING HERBICIDE PARTITION-COEFFICIENTS FROM ELECTROMAGNETIC INDUCTION MEASUREMENTS

A potential method for reducing pesticide leaching is to base applicat ion rates on the leaching potential of a specific chemical and soil co mbination. However, leaching is determined in part by the partitioning of the chemical between the soil and soil solution, which varies acro ss a field. Standard methods of measuring the pesticide-soil partition ing coefficient (K-d) are too expensive and slow for routine held mapp ing. Therefore, alternative methods for mapping K-d must be found if v ariable application methods are to be successful. We investigated the use of noncontacting electromagnetic induction measurements as surroga te measures of K-d. We measured the partition coefficient for atrazine -chloro-4-ethylamino-6-isopropylamino-s-triazine), apparent electrica l conductivity by electromagnetic induction (E(m)), and mass fraction of soil organic carbon (f(oc)) on a 250 by 250 m grid with a 25 m spac ing. Both K-d and f(oc) were lognormally distributed, while E(m) was p oorly described by either a normal or lognormal distribution. Maps of the measured parameters showed similar spatial patterns, having low va lues on well-drained soils and high values on poorly drained soils. Co rrelation coefficients between K-d and E(m) and K-d and f(oc) were 0.5 75 and 0.685, and showed distinct spatial patterns. Spatial structure as indicated by correlograms indicated that each parameter was spatial ly dependent to distances of about 80 m. Simple relationships of K-d = 176 f(oc) and K-d = exp(0.0336 E(m)) were found between the data. Map s of K-d estimated from f(oc) or E(m) were similar to measured K-d, bu t more diffuse. Electromagnetic induction measurements failed to predi ct the observed high K-d values. The advantage of using E(m) measureme nts to map K-d is that it is a rapid, easy, and inexpensive method onc e it has been calibrated.