Spatial Relation of Apparent Soil Electrical Conductivity with Soil Properties and Crop Yields at Different Topographic Positions.

Geophysical methods such as electromagnetic induction (EMI) technique can measure apparent soil electrical conductivity (ECa) by inducing an electrical current in the soil and can be used for studying the variability of soil physical, chemical, and hydrological properties. The objective of our study were: (a) to determine the variations in ECa at different topographic positions and (b) to compare the ECa correlations with soil properties and corn/soybean yields with different topographic positions and for all basins (no topographic separation) to determine whether to model ECa and correlated soil properties on whole field scale or separate models at different slope positions needs to be developed. A field experiment was conducted in 2015 at agricultural research site adjacent to Southern Illinois University’s tree improvement center located on west side of the City of Carbondale, IL. Soil samples were taken after corn harvest and analyzed for various soil properties. Genomics EM38-MK2 was used to measure ECa in horizontal and vertical dipole modes, on transects separated at 15 feet on whole study area. Two modes of operation ECa-H and ECa-V provided four dipole readings ECa-H-0.5, ECa-H-1, ECa-V-0.5, and ECa-V-1. The PROC GLIMMIX and PROC CORR procedures of the SAS statistical software were used for data analysis. Simple cokriging models were developed for mehlich-3 extractable phosphorus (P) at 0-15 cm soil depth and ECa horizontal dipole mode 0.5 m (ECa-H-0.5), and mehlich-3 extractable sodium (Na) at 15-30 cm and ECa vertical dipole mode 1 m (ECa-V-1). Our results showed that ECa-H-1, ECa-V-1 and ECa-V-0.5 were significantly lower at shoulder positions compared to backslope and deposition topographical positions. ECa was significantly correlated with soil properties. Cross validation RMSE of simple cokriging interpolation method was lower for shoulder and backslope compared to all basins together with no topographic position separation model for Mehlich-3 P. Cross validation RMSE was higher for shoulder position compared to all basin with no topographic position separation for Mehlich-Na. Topography is an important factor that needs to be considered while making correlation of ECa with various soil properties and crop yields.