Estimating Herbicide Partition Coefficients from Electromagnetic Induction Measurements

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Abstract
A potential method for reducing pesticide leaching is to base application rates on the leaching potential of a specific chemical and soil combination. However, leaching is determined in part by the partitioning of the chemical between the soil and soil solution, which varies across a field. Standard methods of measuring the pesticide‐soil partitioning coefficient (Kd) are too expensive and slow for routine field mapping. Therefore, alternative methods for mapping Kd must be found if variable application methods are to be successful. We investigated the use of noncontacting electromagnetic induction measurements as surrogate measures of Kd. We measured the partition coefficient for atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine), apparent electrical conductivity by electromagnetic induction (Em), and mass fraction of soil organic carbon (foc) on a 250 by 250 m grid with a 25 m spacing. Both Kd and foc were lognormally distributed, while Em was poorly described by either a normal or lognormal distribution. Maps of the measured parameters showed similar spatial patterns, having low values on well‐drained soils and high values on poorly drained soils. Correlation coefficients between Kd and Em and Kd and foc were 0.575 and 0.686, and showed distinct spatial patterns. Spatial structure as indicated by correlograms indicated that each parameter was spatially dependent to distances of about 80 m. Simple relationships of Kd = 176 foc and Kd = exp(0.0336 Em) were found between the data. Maps of Kd estimated from foc or Em were similar to measured Kd, but more diffuse. Electromagnetic induction measurements failed to predict the observed high K6d values. The advantage of using Em measurements to map Kd is that it is a rapid, easy, and inexpensive method once it has been calibrated.

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