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but zones with more than 7 mS/m and less than 3 mS/m were not represented among the soil sam-
pling locations.
12.4 dISCUSSIon
Apparent electrical conductivity (EC a ) correlated strongly with soil texture (Tables 12.2 and 12.3),
as is commonly found in nonsaline soils (Sudduth et al., 2001). This 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). Another pathway is through the liquid phase. Because both water
content and the amount of exchangeable cations usually decrease with increasing sand content, this
may explain the negative relation between sand content and EC a observed in this and many other
studies (e.g., Khakural et al., 1998; Kitchen et al., 1996). The strong, positive correlation between
EC a and ignition loss confirms earlier findings on the same soil type (Korsaeth, 2005). One should,
however, take into account that ignition loss and clay content were to some extent positively intercor-
related (data not shown), so that the result may in fact have been due to variation in clay content.
Measured EM H correlated generally more strongly with the topsoil properties than did EM V
(Table 12.2 and Table 12.3), which agrees with the results presented by Korsaeth (2005). The super-
iority of EM H to EM V in terms of detecting variation in topsoil properties is also reported under
other conditions (Boettinger et al., 1997; Khakural et al., 1998). Such a phenomenon is to be expected
from the sensitivity functions of EM38 (McNeill, 1980), which show that the relative contribution to
the signal from the topsoil is larger for EM H than for EM V . With regard to the subsoil, the results at
both locations appear to reflect the larger relative weighting of the EM V signal there. An exception
was found in the deepest layer at Apelsvoll (60 to 80 cm), where EM H was almost as well correlated
in this layer as in the topsoil. No obvious explanation for this is apparent.
The regression analyses underlined the importance of topsoil clay content for measured EC a in
such soils (Table 12.4). Nevertheless, the regression models that explained most of the measured
EM H included, at both locations, more subsoil than topsoil properties as predictors (Table 12.4).
This shows that both topsoil and subsoil properties must be considered when interpreting soil sur-
vey maps made by the EM-EC a technique.
12.5
ConClUSIonS
When using the EM-EC
a technique to map topsoil properties, EM H is superior to EM V .
On typical morainic soils in southeast Norway, topsoil clay, total sand, and ignition loss
correlate best with EM H .
The EM-EC
a technique is suitable for mapping soil variation on such soils, but both topsoil
and subsoil properties should be considered when interpreting such maps.
RefeRenCeS
Boettinger J.L., Doolittle J.A., West N.E., Bork E.W., and Schupp E.W. 1997. Nondestructive assessment of
rangeland soil depth to petrocalcic horizon using electromagnetic induction. Arid Soil Res Rehabil 11:
375-390.
Corwin D.L., and Lesch S.M. 2003. Application of Soil Electrical Conductivity to Precision Agriculture:
Theory, Principles, and Guidelines. Agron J 95: 455-471.
Hendrickx J.M.H., Borchers B., Corwin D.L., Lesch S.M., Hilgendorf A.C., and Schlue J. 2002. Inversion
of soil conductivity profiles from electromagnetic induction measurements: Theory and experimental
verification. Soil Sci Soc Am J 66: 673-685.
Khakural B.R., Robert P.C., and Hugins D.R. 1998. Use of non-contacting electromagnetic inductive method
for estimating soil moisture across a landscape. Commun Soil Sci Plant Anal 29: 2055-2065.
Kitchen N.R., Sudduth K.A., and Drummond S.T. 1996. Mapping of sand deposition from 1993 midwest
floods with electromagnetic induction measurements. J Soil Water Conserv 51: 336-340.
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