Results And Discussion

Apparent electrical conductivity values ranged from 45.5 to 81.1 mS m-1 and were spatially dependent (Figure 11.1). The spatial dependence in ECa was best fit with a spherical model (r2 = 0.91, residual mean square (RSS) = 26.9). Areas of the field exhibiting high ECa values were slightly higher in elevation and had been subjected to erosion (tillage, wind, and water), and a portion of the topsoil had been lost. Areas of the field exhibiting low ECa values tended to be depositional areas of the field.

The correlation between ECa and EC1:1 was not strong (r2 = 0.22), likely because there was a small range of EC1:1 values (0.27 to 0.45 dS m-1) in these nonsaline soils. Bulk soil ECa is affected by a number of soil properties including depth of topsoil, clay content, water content, and salt content (Rhoades and Corwin, 1990; Johnson et al., 2001). Laboratory EC1:1 is more strongly correlated with dissolved salts. In spite of the weak correlation between ECa and EC1:1, the predicted EC1:1 map (Figure 11.2) is visually similar to that for ECa (Figure 11.1). The spatial dependence predicted for EC1:1 was best fit with a spherical model (r2 = 0.91, RSS = 1.9E-8).

Clay content varied twofold (range 279 to 797 g kg-1) and was strongly correlated with ECa (r2 = 0.92). Others have also demonstrated a strong correlation between ECa and clay content (Kitchen et al., 2003). As noted above, high clay content was related to landscape position. Predicted clay content was best described by an exponential model (r2 = 0.81, RSS = 46.7). Predicted clay content was positively correlated with ECa with areas predicted to have a high clay content (Figure 11.3) being areas exhibiting high ECa values (Figure 11.1).

Soil NO3-N values ranged from 0.7 to 24.7 g kg-1 and were correlated with ECa values (r2 = 0.86). In these nonsaline soils, NO3-N is a major anion, and ECa has been shown to have great potential in monitoring NO3-N dynamics during the growing season (Eigenberg et al., 2002). The correlation between predicted NO3-N and ECa was negative with low NO3-N values in areas of the field (Figure 11.4) exhibiting high ECa

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