Digital mapping of A-horizon thickness using the correlation between various soil properties and soil apparent electrical resistivity

Conventional soil science methods for the estimation of the spatial variations of soil properties within landscapes are destructive, time consuming, and do not allow the estimation of the short range variability. Recent advances in geomatic global positioning systems and sensors offer new possibilities for the mapping of spatially varying soil patterns. Although geophysical techniques offer an alternative to traditional soil sampling methods, the resulting data are still often misinterpreted, especially in terms of the inter-relationships between, for example, soil apparent electrical resistivity (ρ) and several soil physical or chemical properties. Our main objective was to test the suitability of using ρ measurements for mapping the thickness of the organo-mineral A-horizon (ThickA). ρ was mapped (0–0.5 m), using an RM15, in a highly water eroded field plot of 300 m² showing large variations in ThickA, but no significant variations in soil clay and nutrient content. The correlations between ρ and ThickA, the top-soil water content (θ0.1), and the bulk density (BD0.1) were determined at the nodes of a regular grid of 2 m (n = 96). Principal Component Analysis (PCA) was used to identify the impact of the different soil properties on the ρ data. ThickA varied between 0.17 and 0.3 m, and ρ between 1046 and 3864 Ω m. There was a significant correlation between ρ and ThickA (r = 0.56) and also between θ0.1 (r = 0.23) and BD0.1 (r = 0.22). If it was assumed that θ0.1 and BD0.1 were constant over the study plot then there was a correlation between ρ and ThickA with an r of 0.88, thus making possible the accurate mapping of ThickA (MAE = 0.017 m; i.e., 7% of the average value of ThickA). Prediction (MAE = 0.015 m) was only slightly improved through the use of regression kriging of ThickA with ρ recalculated as the covariate.This result showed that apparent soil electrical resistivity measurements could be very successful for estimating A-horizon thickness provided that limited information on top-soil characteristics is included in the mapping process.