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a Dep. of Crop Sci., North Carolina State Univ., Raleigh, NC 27695
b Dep. of Soil Sci., North Carolina State Univ., Raleigh, NC 27695
c Plant Sci. Dep., South Dakota State Univ., Brookings, SD 57007
* Corresponding author (Ron_Heiniger{at}ncsu.edu)
Received for publication June 21, 2001. While site-specific nutrient management has the potential for improving crop yields, the cost of intensive soil sampling is usually greater than the benefits gained. Apparent soil electrical conductivity (ECa) has been used successfully to measure soil salinity, clay content, and, in the laboratory, nutrient concentrations. This study was initiated to determine if ECa could be used to measure nutrient concentrations in the field. Fifteen field sites with 12 different soil series were studied in three topographic areas of North Carolina in 1997 and 1999. Soil samples and ECa measurements were taken at the same locations in the field and analyzed for P, K, Ca, Mg, Mn, Zn, Cu, pH, cation exchange capacity (CEC), percentage humic matter (HM), and percentage sand, silt, and clay. Nutrient concentrations and soil properties were compared with ECa using correlation and principal components (PC)–stepwise regression analysis. Few significant direct correlations were found between ECa and the selected nutrient elements (R2 < 0.50). Correlations improved when soil series were analyzed separately within a field. The results indicated that salinity, soil texture, or soil moisture were masking the response of ECa to changing nutrient levels in the soil. While the PC–stepwise regression analysis found that ECa was often a key loading factor, changes in soil texture, CEC, and HM resulted in field-specific relationships between ECa and nutrient concentrations. The primary value of ECa in measuring nutrient levels lies in its ability to identify small changes in soil texture, CEC, or HM that, in turn, indicate where differences in nutrient levels occur.
Abbreviations: CEC, cation exchange capacity • DGPS, differential global positioning system • ECa, apparent soil electrical conductivity • ECs, the electrical conductivity of the soil particles • ECwc, the electrical conductivity of the mobile soil solution associated with large, continuous pores • ECws, the electrical conductivity of the soil solution associated with discontinuous pores • HM, humic matter • PC, principal component • 
s, the volumetric content of soil particles • w, the total volumetric content of water in the soil • ws, the volumetric soil water content of the small, discontinuous pores
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