Relationships between cone penetration resistance (PR), soil moisture (SM), and bulk density (BD) were derived for: (i) cultivated (ripped) and uncultivated, hardsetting and non-hardsetting, field soils; and (ii) repacked cores of the uncultivated soils. Each of the soils supports commercial Pinus
plantations in the coastal lowlands of south-east Queensland, Australia.
Penetration resistance was positively correlated with bulk density and negatively correlated with soil moisture for all soils. In the uncultivated soils, penetration resistance was less sensitive to bulk density than typically reported in the literature, or than observed in the cultivated soils where a wider range of bulk density values was studied. In both the cultivated and the repacked soils, penetration resistance was more sensitive to soil moisture at higher bulk density, and more sensitive to bulk density at lower soil moisture.
It was not possible to fit the same models to uncultivated, repacked, and cultivated soils, and therefore not possible to compare relationships for each statistically. Relationships between penetration resistance, bulk density, and soil moisture were best described by additive models in the uncultivated soils and multiplicative models in the cultivated soils. For the repacked soils, models had to be developed relating penetration resistance to bulk density for each soil moisture class separately.
The study demonstrated that: (i) relationships between penetration resistance, bulk density, and soil moisture were insufficiently sensitive to predict responses in the penetration resistance of field soils tochanges in soil moisture, as might occur temporally, or bulk density, as might occur with compaction or reconsolidation after cultivation; and (ii) repacked soils could not be used to simulate the relationships between penetration resistance, bulk density, and soil moisture for cultivated field soils. Therefore, penetration resistances measured at different times in studies in which either bulk density or soil moisture are expected to change cannot be easily compared. In these situations, which include compaction and consolidation studies, both penetration resistance and bulk density, or bulk density alone, should be used to monitor change.
Relationships between penetration resistance, soil moisture, and bulk density, together with moisture characteristic drying curves for individual soils, were used to define relationships between penetration resistance and matric suction. These relationships define a soil characteristic that may be useful for: (i) explaining varying responses of different soils to drying; (ii) explaining various Pinus seedling growth responses to cultivation and compaction; and (iii) delineating soils which are functionally hardsetting upon drying.