Variation in penetrometer resistance with soil properties: the contribution of effective stress and implications for pedotransfer functions

Soil mechanical resistance to penetration by roots can potentially contribute to the spatial and temporal variability in root and shoot growth. Functions that accurately relate penetrometer resistance to soil properties are important tools for assessing the contribution of soil mechanical resistance (SMR) when the temporal and spatial variability in SMR cannot be readily measured. Although effective stress can make a significant contribution to SMR, the role of texture and compaction on the contribution of effective stress to SMR has not been explored and functions that are currently used to describe the relation between SMR, water content/potential and other soil properties do not contain terms explicitly linked to effective stress. The objectives of this study were to assess functions that included terms that would be compatible with effective stress and to subsequently develop a pedotransfer function to quantify the dependence of SMR on soil properties. Soil resistance was measured on disturbed and undisturbed soils with a range of textures, organic carbon (OC) contents and bulk density after equilibrating the soils at different water potentials (ψ). The SMR decreased with decreasing ψ at the lowest ψ in coarser-textured soils and the effects persisted into medium-textured soils at the higher level of compaction. These effects were attributed to a decrease in effective stress. The function found to be most successful in describing SMR in both disturbed and undisturbed soils was of the form:SMR=a∗(ψb)−c∗ψwhere a, b and c were functions of texture, OC content and bulk density. The dependence on soil properties was different in the disturbed and undisturbed soils. Sensitivity analyses indicated that the variation in the second term with soil properties was compatible with expectations regarding increasing discontinuity in water films or the development of microcracks with decreasing ψ. The study suggested that pedotransfer functions for SMR should be adjusted to account for the possible reduction in SMR at low water contents or ψ due to decreasing values of effective stress.