Using Soil Electrical Conductivity Mapping for Precision Irrigation in the Columbia Basin.

High resolution soil electrical conductivity mapping is a precision agriculture product which intends to guide seeding, irrigation, fertilization, and other management decision. Electrical conductivity is used as a proxy for a wide variety of parameters, including nitrate and micronutrients, soil pH, texture, and water holding capacity. Farmers need confidence that soil maps accurately reflect field conditions. Mapping by measuring the soil’s apparent electromagnetic conductivity (EC_a) allows generation of high resolution maps, but requires establishing a relationship between the EC_a and soil properties of interest. In practice, correlation between soil characteristics and EC_a must be determined independently for each time the method is employed. In this study, high resolution maps of EC_a were compared to co-located physical samples. To identify confounding effects from soil properties that may result in spurious correlation, principal component analysis were used to describe variability in both the EC_a measurements and soil samples. A conditional sampling approach was used to find all possible cross-correlations. Field methods and data used in this study were commercially available and comparable to methods generally marketed to farmers. We demonstrate that soil chemical properties in the studied fields strongly influence the accuracy of soil EC_a mapping in predicting soil texture. Specifically, high base saturation was identified as a likely confounding factor for mapping soil texture with EC_a in the Columbia Basin. After measurements have been taken, a few simple checks are identified that can help farmers identify if EC_a maps are reliable in fields with potential confounding factors.  The analysis shows that quality control checks can identify problematic measurements and potentially improve data in post-processing. In summary, the study shows that soil mapping by EC_a can be a useful tool for describing within field variability in practical applications, but that a number of conditions and checks must be applied to ensure validity.