Mapping of penetrometer resistance in relation to tractor traffic using multivariate geostatistics

The traffic of agricultural machines can cause soil compaction and high variability of soil structure, both along normal lines and along those parallel to the field plane. The aim of this work was to investigate the potential of an electronic penetrometer, a GPS, a GIS and geostatistical techniques for mapping soil compaction.In July 2003 soil cone penetrometer resistance was measured using a semi-automatic electronic penetrometer in a sandy-silt soil (Vertic Xerochrept) of inland Sicily where a three-year rotation wheat (Triticum durum Desf.)–wheat–tomato (Solanum lycopersicum L.) was practiced. The measurements were carried out along three parallel 3-m long transects, from the soil surface to a depth of 0.70 m.A multivariate geostatistical approach, including exploratory analysis, variography, stochastic simulation and post-processing of simulations, was applied to produce thematic maps of penetrometer resistance and probability exceeding a critical value, in correspondence of a different number of tractor passes. Penetrometer resistance variation resulted erratic at the surface but showed high spatial correlation between data measured at different depths. The probabilistic maps of compaction risk showed that the soil volume, exceeding the penetrometer resistance of 2.5 MPa, critical for root growth, at the probability level of p > 0.40 increased from 20% to 40% after five tractor passes.