Apparent electrical conductivity (ECa) 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 ECa observed in this and many other studies (e.g., Khakural et al., 1998; Kitchen et al., 1996). The strong, positive correlation between ECa 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 EMH correlated generally more strongly with the topsoil properties than did EMV (Table 12.2 and Table 12.3), which agrees with the results presented by Korsaeth (2005). The superiority of EMh to EMV 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 EMH than for EMV. With regard to the subsoil, the results at both locations appear to reflect the larger relative weighting of the EMV signal there. An exception was found in the deepest layer at Apelsvoll (60 to 80 cm), where EMH 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 ECa in such soils (Table 12.4). Nevertheless, the regression models that explained most of the measured EMh 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 survey maps made by the EM-ECa technique.

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