Temporal stability of soil electrical conductivity in irrigated sandy fields in Colorado

The utility of apparent soil electrical conductivity (ECa) remains elusive because of its complex interactions with soil properties. Nonetheless, a significant interest is emerging to utilize the spatial variability in ECa to guide direct soil sampling and develop varying site-specific management. While the spatial variability of ECa is of significant importance, understanding its temporal variability is equally important. That is particularly true if delineated ECa zones are to be used to manage agricultural inputs across the field for multiple years. We present multi-year (1998 to 2002) measurements of shallow (top 0.3 m of soil) and deep (top 0.9 m of soil) ECa from three irrigated sandy fields in eastern Colorado and quantify their degree of temporal change. During the study period, the fields were managed uniformly, including all farm operations and the applied inputs. For each field, soil ECa values were highly correlated between measurement days (for periods of a few days to four years between measurements) with coefficients ranging between 0.71 and 0.92 for shallow and 0.87 and 0.96 for deep ECa, but significant deviations from the 1:1 line (indicative of temporal variability) were exhibited by shallow ECa. In these non-saline and sandy fields, delineating spatial patterns of ECa into low, medium, and high zones was highly stable over time, mainly because they reflect the stable soil properties. Grid-by-grid comparison of the low, medium, and high ECa patterns front different measurement days showed mean matching percentages of 87, 74, and 78 for shallow and 93, 85, and 89 for deep ECa patterns, respectively, for the three fields. When salt concentration and buildup are low (as was the case herein), results suggest that single ECa mapping should suffice to delineate stable low, medium, and high ECa zones without a need for remapping.