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correlated with clay content on nonsaline soils (Sudduth et al., 2001). Moreover, other findings on
morainic soil in the region have shown strong correlations between EC
a
and clay content (Kor-
saeth, 2005a, 2005b). Such high correlations may be partly explained by the exchangeable cations
associated with clay minerals, which represent an important pathway for EC in soil (Corwin and
Lesch, 2003). In the current study there was a very strong correlation between EC
a
and CEC, and
particularly, exchangeable Ca affected measured EC
a
strongly, but clay content was not correlated
with the exchangeable cations (data not shown). CEC was, however, strongly correlated with SOM
(r = 0.918).
After CEC, soil water content was the most important soil property, influencing measured EC
a
.
It was quite wet at the time of measuring, as the soil had just thawed after the winter frost, and there
had been a period of rain shortly before. A strong correlation between soil water content and EC
a
is to be expected, because the liquid phase is one of three pathways for electrical conductivity in
soil (Corwin and Lesch, 2003), and the results are in agreement with numerous other reports (e.g.,
Khakural et al., 1998; Korsaeth, 2005a; Sudduth et al., 2001).
The relation between soil water content and SOM was very high (Figure 13.1). This may be
explained by the fact that the peaty subregions within the field were mainly located in the hollows,
where the water content is normally higher than on the peaks. High water content is an essential
part of peat formation, because it reduces the oxygen availability in the soil and thereby retards the
decay of organic matter. A logical consequence of higher content of both water and organic matter
in the hollows would be increased EC
a
at such locations, as was actually observed. Clay et al. (2001)
observed the same phenomenon at four different fields in eastern North Dakota. They attributed the
landscape differences in EC
a
to water leaching salts from peaks (summit) to hollows, lower water
contents in peaks than in the hollows (toeslope areas), and water erosion that transported surface
soil from upper to lower areas.
The statistical test of the soil survey map data revealed that there were significantly different
contents of SOM between all the zones tested (Table 13.2). This appears reasonable considering the
obvious coincident of EM
H
and SOM content shown in Figure 13.2. One important soil property,
which is highly related to the SOM content, is the mineralization potential of nitrogen from the soil
organic matter pool. On the investigated field, the most frequent crop in the rotation is spring barley,
and the risk of lodging, due to overfertilization in SOM-rich areas, is considered to be high. Hence,
a map showing differences in SOM content would provide useful information for the farmer (e.g.,
for decisions concerning site-specific fertilization).
Soil water content was more strongly related to EC
a
than was SOM (Table 13.1), but the statisti-
cal test revealed, nevertheless, a lower coincidence between soil water content and the EC
a
-zones
on the soil survey map (Table 13.2). This may partly be related to the difference in the number of
samples analyzed. All 310 samples were analyzed for SOM, whereas only about half the samples
(154) were analyzed for soil water content. The range of the measured SOM was considerably larger
than that of the measured soil water content (Table 13.1). The combination of both larger overall
variation in the measurements and higher degrees of freedom within the EC
a
classes favored SOM
in the statistical test.
For silt and sand content, there were differences between the EC
a
classes, but they were not as
distinct as for SOM. The texture coincided poorly with the EC
a
classes, particularly in the range of
4 to 10 mS m
−1
(Table 13.2). This reflected the fact that soil texture was generally more weakly cor-
related with EC
a
than SOM and soil water content (Table 13.1), and that soil texture had much lower
degrees of freedom in the statistical test, due to fewer analyses, compared with SOM.
13.5 ConClUSIonS
This case study has shown that the standard procedure used in Norway for commercial soil survey
with the EM-EC
a
method may provide maps that show significant differences in soil properties.
