Geoscience Reference
In-Depth Information
Jaynes et al. (1995) used EC
a
measured by electromagnetic (EM) induction to estimate the
fraction of organic carbon (
f
oc
) and herbicide-soil partition coefficient (
K
d
) for atrazine within a
32 ha field in Iowa. The correlation coefficient between
K
d
and EC
a
was 0.57
and between
f
oc
and
EC
a
was 0.69. These results suggest that EC
a
maps could be used to predict herbicide
K
d
variability
within a field. The objectives of this study were to determine the relationship between EC
a
and
f
oc
and between
f
oc
and the
K
d
of three different herbicides (EPTC, metribuzin, and metolachlor) in soil
samples taken from three fields in Colorado and to examine whether such a relationship could be
used to map the differences in binding of these herbicides to the soil.
15.2
MAteRIAlS And MethodS
15.2.1 s
i t e s
Three fields irrigated by center pivots were used in this project. Two were located near Wiggins,
Colorado (designated as Wiggins 1 and Wiggins 2), and one near Yuma, Colorado (designated as
Yuma). Wiggins 1 and Wiggins 2 are 71 and 52 ha, respectively, and are located a few kilometers
apart. The soils in these two fields include coarse-loamy, mixed, superactive, mesic Ustic Haplar-
gids; mixed, mesic Typic Ustipsamments; mixed, mesic Ustic Torripsamments; and coarse-loamy,
mixed, superactive, mesic Aridic Argiustolls. Yuma is 57 ha and includes fine-loamy, mixed, super-
active, mesic Pachic Argiustolls; fine-loamy, mixed, superactive, mesic Pachic Argiustolls; and fine-
loamy, mixed, superactive, mesic Aridic Argiustolls.
15.2.2 ec
a
M
e a s u R e M e n t
a n d
s
o i l
s
a M P l e s
Measurements of EC
a
were taken in spring of 1999 and 2000 using the Veris 3100 Soil EC Mapping
System (Veris Technologies, Salina, KS) (Farahani and Buchleiter, 2004). Sample sites were iden-
tified in each field by selecting areas from delineated EC
a
zones (Farahani and Buchleiter, 2004).
Samples were taken from the top 0.3 m in 1999 from Wiggins 1 and Wiggins 2 and in 2000 from
Yuma. The number of samples taken were 28, 38, and 20 from Wiggins 1, Wiggins 2, and Yuma,
respectively. Samples were air-dried, ground, and sieved through a 2 mm mesh. Soil organic mat-
ter was determined by the method of Sherrod et al. (2002). A validation set of ten additional soil
samples was taken from each field in spring 2003 (Figure 15.1). The sites were chosen in areas of
distinct EC
a
zones that had not been previously sampled. The samples were taken and handled as
described previously.
15.2.3 h
e R b i c i d e
-s
o i l
b
i n d i n g
The binding of analytical-grade EPTC, metribuzin, and metolachlor was determined by batch-
slurry equilibration with 10 g of soil and 10 mL of 0.05M CaCl
2
. The herbicides were introduced
into the CaCl
2
solution at a final concentration of 1 ng/mL and allowed to equilibrate with the soil
for 24 h. After equilibration, the system was centrifuged, and the concentration of the herbicide
in the 0.05M CaCl
2
was partitioned into toluene and was measured by using a gas chromatograph
equipped with a mass spectrophotometer.
The herbicide-soil sorption coefficient (
K
d
) was calculated as
K
d
= (herbicide sorbed to soil (μg/g))/(herbicide in solution (μg /m L))
(15.1)
K
oc
(the soil organic carbon sorption coefficient) for each herbicide was calculated as
K
oc
=
K
d
/
f
oc
.
(15.2)
f
oc
is the soil organic carbon mass fraction that was measured for each soil sample:
