Enhancing soil penetration resistance measurements and interpretations: Normalising for moisture variability to better assess land use effects

Cone penetration resistance (PR) is an important index of soil physical restriction to root growth in agriculture. Spatio-temporal variability of soil water content (SWC) makes PR data less comparable. Our objective was to normalise spatio-temporal PR measurements to a standard SWC, thus allowing comparison across New Zealand’s agricultural regions. We geo-referenced 497 measurements of PR and SWC from two depths (0–15 and 15–30 cm) in multiple agricultural regions between 2002 and 2007. Power, exponential, and logarithmic models were used for fitting PR as a function of SWC. A power function relating PR to gravimetric water content (GWC) provided the best-fit model for normalising PR for variation in SWC. Low relative errors (<10 %) and absolute errors (<0.2 MPa) were obtained at both depths for GWC values of 0.16–0.26 g g−1. We recommend GWC of 0.16–0.26 g g−1, with 0.20 g g−1 preferred for its proximity to the midpoint and its associated lower relative error (5 %). Specific models for different land use categories were unnecessary, while soil-depth-specific models for different soil orders (Allophanic cf. Non-Allophanic) improved the PR normalisation. Normalised PR values (GWC = 0.20 g g−1) for pastoral soils were higher than for cropping soils, particularly at 0–15 cm. About 65 % and 75 % of the sampled sites had normalised PR values exceeding 2.5 MPa at 0–15 and 15–30 cm depths, respectively, indicating the potential for adverse effects from widespread soil compaction. This study highlights the importance of normalising PR data for soil moisture variation to better assess potential adverse effects of land use and management on soil.