Identifying the plough pan position on cultivated soils by measurements of electrical resistivity and penetration resistance

Long term tillage has led to soil profile degradation in many cultivated fields. The topsoil is disturbed by plowing. The movement of fine particles from the topsoil to the subsoil and direct pressure from agricultural machinery create an abrupt delineation in the form of a plough pan with very low permeability. The plough pan prevents water infiltrating deeper into the soil profile and reduces the water supply to the lower layers. The plough pan also has a negative effect on the root growth of the crop, leading to a reduced yield. In this paper we discuss the feasibility of using electrical resistivity tomography and penetrometry to identify the presence and the position of plough pans, and also their spatial uniformity, on two fields with different tillage depths. Electrical resistivity measurements were subjected to a comparison with soil physical characteristics, such as soil water content, porosity and bulk density. Standard statistical and geostatistical methods were used. Electrical resistivity tomography seems to be an attractive method that offers a faster and more efficient method than standard invasive soil sampling for obtaining continuous information about the plough pan. It has been shown that the position of a compacted layer within the soil profile can be identified reasonably well by combining electrical resistivity data and penetration resistance data. The semivariogram showed higher variation by orders of magnitude in the topsoil than in the subsoil. This suggests macroscopic homogeneity of the compacted layer formatted in the subsoil in two differently tilled fields. We conclude that a short span between the electrodes should be used (app 10 cm) in order to observe the shallow positioned plough pan clearly.