Geoscience Reference
In-Depth Information
because measuring SOC for a large number of samples is not a trivial task. Thus, alternative, more
easily measured surrogate parameters of both
K
d
and SOC are needed that can be used at the field
scale or larger.
Soil electrical conductivity as measured by electromagnetic induction (EC
a
) 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 fac-
tor directly controlling soil electrical conductivity. Thus, once the correlation between electrical
conductivity and this property is established, an EC
a
survey can be used to map this soil attribute
quickly and cheaply. For example, EC
a
measurements have been successfully used to measure soil
salinity (Cameron et al., 1981; Lesch et al., 1992; Rhoades and Corwin, 1981) and soil water con-
tent (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).
EC
a
measurements have also been used to determine soil and field properties that it cannot mea-
sure directly. EC
a
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 EC
a
reading directly or because
the parameter is associated with pedogenic processes that create properties that affect EC
a
.
Given its utility as a surrogate for many important soil and field properties, EC
a
may be an
easily used, acceptable surrogate for K
d
. Data presented in Cambardella et al. (1994); Jaynes et al.
(1994), and Novak et al. (1997) will be used to illustrate how EC
a
measured by electromagnetic
induction can be used to estimate K
d
for the herbicide atrazine [6-chloro-N-ethyl-N-(1-methylethyl)-
1,3,5-triazine-2,4-diamine across an agricultural field.
14.2 MAteRIAlS And MethodS
Measurements were made in a 32 ha field within the Walnut Creek watershed in central Iowa
(41°58′ N, 93°43′ W). A detailed description of the soils, geology, and farming practices within the
watershed can be found in Hatfield et al. (1999). The landscape within the watershed is character-
ized by gentle swell-swale relief of several meters (Daniels and Handy, 1966). Surface drainage is
poorly developed, resulting in numerous closed depressions or potholes that have been extensively
tile drained in the past 100 years.
Soils within the field were formed from till from the most recent substage of the Wisconsin
glaciation. The toposequence of soils within the field range from well-drained Clarion loam (fine-
loamy, mixed, superactive, mesic Typic Hapludolls), to the somewhat poorly drained Nicollet loam
(fine-loamy, mixed, superactive, mesic, Aquic Hapludolls), to the poorly drained Canisteo silty clay
loam (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquolls) and Harps loam (fine-
loamy, mixed, superactive, mesic Typic Calciaquolls), and ending with the very poorly drained
Okoboji mucky silty loam (fine, smectitic, mesic Cumulic Vertic Endoaquolls) (Figure 14.1).
Prior to planting in 1992, a grid was laid out across a 250 m by 250 m area in the southern half
of the field (Figure 14.1). Detailed information about the grid and soil properties not reported here
can be found in Cambardella et al. (1994) and Novak et al. (1997). The grid spacing was 25 m in
both the easterly and northerly directions. Additional grid points were established at closer spacings
but were not included in this study. Within 1 m of each grid point, three 6-cm diameter soil cores
were taken to a depth of 15 cm and composited for analysis. The mass of organic carbon was mea-
sured using dry combustion methods with a Carlo-Erba NA1500 NCS elemental analyzer (Haake
Buchler Instruments, Paterson, NJ) after carbonates had been removed with 2
M
H
2
SO
4
.
