Mapping soil compaction in 3D with depth functions

Soil compaction is a form of physical degradation that causes soil densification and distortion that can lead to changes in three-dimensional soil structure variability. For this reason we proposed a method for mapping compaction in three-dimensional space (3D), based on simple soil-specific depth functions. This method, denoted the ‘top down’ method, is based on a multi-step approach that starts with a framework for selecting the best depth function, considers which shape best describes the soil-specific variation by depth and ends with an interpolation of the coefficients of the function across the field. Subsequently, for each interpolated point this function is solved in order to estimate compaction in 3D space. The top down method was evaluated using a cone resistance dataset, collected at the CULS (Czech University of Life Sciences) farm in Lany. The accuracy of this method in predicting cone-index variation in the 3D space was compared with the generally accepted 3D ordinary kriging interpolation, using descriptive statistics of the predicted values and cross validation. We found that the top down method better represented observed information and generally performs better than 3D kriging with a far smaller sample size. The cross validation results suggest that ordinary kriging can predict cone-index obtaining a value of root mean squared deviation (RMSD) of 0.54 at 10% soil profiles excluded, and 0.57 at 50% exclusion; on the contrary the top down mapping method obtains an RMSD value of just 0.47, at 10% exclusion, and 0.53, with 50% exclusion. We also found that the depth function method was more representative of the observed variability in the predicted dataset, avoiding the smoothing filtering effect typical of kriging interpolation. We then created a cone-index map of the CULS Lany field with a vertical resolution of 10 cm using the top down method. The map can be seen as a series of three videos where it is possible to observe the cone-index pattern in three-dimensions.