Using Soil Apparent Electrical Conductivity to Optimize Sampling of Soil Penetration Resistance and to Improve the Estimations of Spatial Patterns of Soil Compaction

This study presents a combined application of an EM38DD for assessing soil apparent electrical conductivity (EC(a)) and a dual-sensor vertical penetrometer Veris-3000 for measuring soil electrical conductivity (EC(veris)) and soil resistance to penetration (PR). The measurements were made at a 6 ha field cropped with forage maize under no-tillage after sowing and located in Northwestern Spain. The objective was to use data from EC(a) for improving the estimation of soil PR. First, data of EC(a) were used to determine the optimized sampling scheme of the soil PR in 40 points. Then, correlation analysis showed a significant negative relationship between soil PR and EC(a), ranging from −0.36 to −0.70 for the studied soil layers. The spatial dependence of soil PR was best described by spherical models in most soil layers. However, below 0.50 m the spatial pattern of soil PR showed pure nugget effect, which could be due to the limited number of PR data used in these layers as the values of this parameter often were above the range measured by our equipment (5.5 MPa). The use of EC(a) as secondary variable slightly improved the estimation of PR by universal cokriging, when compared with kriging.