Geoscience Reference
In-Depth Information
15
Can Apparent Soil Electrical
Conductivity Be Used
to Map Soil-Herbicide
Partition Coefficients?
A Case Study in Three
Colorado Fields
Dale L. Shaner, Hamid J. Farahani, Gerald W. Buchleiter,
and Mary K. Brodahl
ContentS
15.1 Introduction ......................................................................................................................... 241
15.2 Materials and Methods........................................................................................................ 242
15.2.1 Sites ....................................................................................................................... 242
15.2.2 EC
a
Measurement and Soil Samples ..................................................................... 242
15.2.3 Herbicide-Soil Binding ......................................................................................... 242
15.3 Results and Discussion........................................................................................................ 243
References ...................................................................................................................................... 245
15.1 IntRodUCtIon
Herbicides are one of the major pesticide inputs in agriculture. In 2002, atrazine, acetochlor, and
S
-metolachlor were applied to the soil on 62, 25, and 15 percent of the treated hectares, respectively
(Anonymous, 2003). The activity and fate of soil-applied herbicides are affected by multiple factors
including soil organic matter (OM), pH, cation exchange capacity, and texture (Blackshaw et al.,
1994). These factors are spatially variable, and herbicide adsorption to soil can differ considerably
within fields (Koskinen et al., 1994). The soil adsorption of herbicides determines the bioavailability
of the chemical to both weeds and microbes. Liu et al. (2002) found that the efficacy and mineral-
ization of atrazine and alachlor varied across a field depending on soil properties, and weed control
was poor where the herbicides dissipated rapidly.
Herbicides could be applied at varying rates within a field depending on differences in soil prop-
erties as well as weed populations (Williams et al., 2002), but it is difficult to economically measure
the variation in soil properties at the field scale. What is needed is a method that can economically
map the heterogeneity of soil properties that are closely related to herbicide behavior within a field.
In the absence of salinity, apparent soil electrical conductivity (EC
a
) is strongly influenced by soil
texture, including clay content, soil water content, and OM.
241
