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which increase EC
a
. These horizons are also characterized by associated elevations in bulk density
and pH, soil properties positively correlated with EC
a
and negatively correlated with productivity.
At the FICS, strong correlations were not found between EC
a
and individual soil properties at
point sources. This is because EC
a
integrates multiple soil properties, wherein changes in one may
be buffered by corresponding changes in another. In this semiarid environment, EC
a
is most useful
for delimiting overall soil productivity and for defining distinct zones of within-field yield potential.
Therefore, soil and residue sampling based upon EC
a
productivity zones appears to be a useful basis for
(1) zone soil sampling, (2) tracking the temporal impact of farm management on soil productivity, and
(3) assessing soil parameters to calculate fertilizer and herbicide inputs in site-specific management.
18.3.3 P
h a s e
iii: ec
a
v e R s u s
c
R o P
y
i e l d s
(J
o h n s o n
e t
a l
., 2003
b
)
Field-scale
measures of crop yield were evaluated for significant relationships to both EC
a
and the
sampling-site scale soil properties integrated by EC
a
(Phase II) using two years of geo-referenced
yield maps for wheat and corn. Winter wheat yields were negatively correlated with EC
a
(0 to
30 cm) and positively correlated with soil properties, indicative of production potential. Correla-
tions between EC
a
and yield have been corroborated by investigators in other regions (Corwin et al.,
2003b; Kitchen et al., 2003). A wheat-yield response curve for 1999, a high-yielding year, revealed
a boundary line of maximum yield that decreased with increasing EC
a
(Figure 18.5), an effective
basis for identifying yield goals.
Although crop yield maps provide the most realistic picture of yield heterogeneity, they inte-
grate all factors driving crop yields, including weather variations. Productivity zones, based on EC
a
,
distinguish only the soil-based factors underlying yield heterogeneity that can be managed. These
zones offer a basis for three key aspects of site-specific management: (1) yield goal determination,
(2) soil sampling to assess residual nutrients and soil attributes affecting herbicide efficacy, and
(3) prescription maps for metering fertilizer, pesticide, and seed inputs.
No consistent associations were found between EC
a
(0 to 30 cm) and corn yields probably due
to high drought stress in corn during the 2-year study. However, both wheat and corn yields were
positively correlated with EC
a
(0 to 90 cm), a reversal of the negative relationship between EC
a
(0
to 30 cm) and wheat yield. This is likely due to differences in the impact of soil clay content on
7
6
5
4
3
2
1
0.05
0.10
0.15
0.20
0.25
0.30
0.35
EC
a
(dS m
-1
)
fIGURe 18.5
Scatter plot of 1999 winter-wheat yield as a function of apparent soil electrical conductivity
(EC
a
) measured at approximately 0 to 30 cm soil depth. The red line is a “boundary line” of maximum poten-
tial yield defined as yield points falling at the ninetieth percentile of yield frequency for each 0.01 increment
of EC
a
.
