Introduction

One of the difficulties in predicting the fate and leaching risk of field-applied pesticides is that their transport properties are affected by soil properties that can vary greatly across the landscape. For example, the affinity of a chemical to sorb to the soil matrix has been shown to vary spatially within a field (Cambardella et al., 1994; Rao et al., 1986; Wood et al., 1987) and even within the same soil map unit (Novak et al., 1997). Yet Loague et al. (1990) found that sorption can be the predominant process contributing to the variability of pesticide mobility across the landscape.

The partitioning of an herbicide between the solution and soil phases is typically represented by Freeze and Cherry (1979):

where 5 is the sorbed concentration on the soil (mg kg-1), c is the solution concentration (mg L-1), and Kd is the partition coefficient (L kg-1). Standard methods for measuring Kd (Novak et al., 1994) are time consuming and require costly specialized equipment. Due to the expense and time required to measure partition coefficients for combinations of specific soils and chemicals, the coefficients are often estimated from related soil properties. In particular, organic carbon content is commonly used because this soil fraction strongly interacts with nonionic herbicides (Bailey and White, 1970). The relationship between soil organic carbon mass fraction, SOC (kg kg-1) and Kd can be expressed as

where Koc (L kg-1) is the coefficient of proportionality. Although Kd is not perfectly correlated with SOC because of additional sorption to the clay fraction (Laird et al., 1992), the Koc approach has proven to be satisfactory for many purposes (Rao and Davidson, 1980). However, if we wish to characterize the spatial variability of Kd over a field or larger unit, even this approach is cumbersome because measuring SOC for a large number of samples is not a trivial task. Thus, alternative, more easily measured surrogate parameters of both Kd and SOC are needed that can be used at the field scale or larger.

Soil electrical conductivity as measured by electromagnetic induction (ECa) has been used successfully to map soil characteristics. The electrical conductivity of a soil is determined by a combination of soil water content, dissolved salt content, clay content and mineralogy, and soil temperature (McNeill, 1980). In many fields, a single property (e.g., salinity) is the primary factor directly controlling soil electrical conductivity. Thus, once the correlation between electrical conductivity and this property is established, an ECa survey can be used to map this soil attribute quickly and cheaply. For example, ECa measurements have been successfully used to measure soil salinity (Cameron et al., 1981; Lesch et al., 1992; Rhoades and Corwin, 1981) and soil water content (Kachanoski et al., 1988); to map groundwater contaminant plumes associated with elevated chloride, sulfate, and nitrate levels (Drommerhausen et al., 1995; Greenhouse and Slaine, 1983); and measure clay content (Williams and Hoey, 1987).

ECa measurements have also been used to determine soil and field properties that it cannot measure directly. ECa has been used to determine soil cation exchange capacity and exchangeable Ca and Mg (McBride et al., 1990), depth to claypans (Doolittle et al., 1994), field-scale leaching rates of solutes (Slavich and Yang, 1990), spatial pattern of groundwater recharge (Cook et al.,1989, 1992), and yield (Jaynes et al., 1995). These studies were successful because the parameter of interest either influenced a soil property (e.g., water content) that affects the ECa reading directly or because the parameter is associated with pedogenic processes that create properties that affect ECa.

Given its utility as a surrogate for many important soil and field properties, ECa may be an easily used, acceptable surrogate for Kd. Data presented in Cambardella et al. (1994); Jaynes et al. (1994), and Novak et al. (1997) will be used to illustrate how ECa measured by electromagnetic induction can be used to estimate Kd for the herbicide atrazine [6-chloro-N-ethyl-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine across an agricultural field.

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