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Recent advances in apparent soil electrical conductivity (ECa) sensor technology have provided the opportunity to rapidly map soil spatial variability for site-specific management. However, characterizing (or identifying the causes of) the ECa variability has remained difficult, with ECa and soil property (such as soil water and clay content) relationships showing a wide range of varying strength across fields. In this study, our objective was to characterize the main soil properties that alter ECa and evaluate the temporal variability of ECa and soil property relationships using multi-year measurements (1998 to 2003) in three center-pivot irrigated fields in eastern Colorado. Results show that for the irrigated sandy and non-saline fields, ECa maps may be viewed as surrogate maps for soil water content (θw), clay, cation exchange capacity, and/or organic matter content due to their strong correlations (with correlation coefficients between 0.66 and 0.96). The experimental linear functions of ECa versus soil properties changed over time when soil solution concentration (ECw) changed considerably. That finding was supported by theory (the dual-pathway ECa model) showing the relationship between ECa and soil stable properties (such as clay content) to be governed by the status of the soil transient properties of ECw and θw at the time of the ECa mapping. The temporal effect of varying soil temperature on ECa could be significant but was not accounted for due to lack of temperature data. Results collectively suggest that because of the lack of reliability of using empirical ECa and soil property relationships for predictive purposes over time, on-site calibration of ECa versus soil properties of interest are needed at each ECa mapping. [ABSTRACT FROM AUTHOR]