Predicting field capacity in undisturbed stony soils

An increasing number of studies around the world are showing that a long-held assumption that rock fragments (RFs) are inert with respect to water retention is incorrect. Yet very few pedotransfer functions (PtFs) account for water held by RFs or the effect RFs have on the water retention of the fine earth. The few PtFs that incorporate the water content (WC) of RFs have relied upon measurement methods that may not be representative of field conditions. This indicates a gap in research regarding the characterisation of the water holding behaviour of stony soils in situ using soil volumes that adequately represent the soil. We address this gap in research by developing PtFs that predict the field capacity WC of stony soils using soil water storage measurements from 52 pits excavated into stony soils on the Canterbury Plains, New Zealand. These soils comprise sediment derived from a Mesozoic hard sandstone. The soils at each site were watered to saturation, and then after two days of drainage (a proxy for field capacity), a 30 x 30 cm pit was excavated in 10 cm increments to a depth of 60 cm. Matric potential was measured in situ for each increment, and soil WC was calculated from samples taken back to the laboratory. Our results showed it was possible to accurately predict the field capacity WC of stony soils using only explanatory variables that could be easily measured or estimated from a minimalistic field survey. An existing PtF calibrated on NZ soils (logit PtF), which was constructed on the assumption that RFs had no effect on WC at FC other than reducing the volume of the fine earth, performed worse than our models. By modifying the logit PtF, we conclude that its poorer performance stems from its inability to account for deviations from 1) the matric potential it assumes for field capacity (-10 kPa), 2) water held by RFs, and 3) the effect of RFs on the water retention characteristics of the fine earth. Our results demonstrate that even the low porosity RFs measured in this study can significantly affect model performance, but by including two variables (depth and volumetric proportion of RFs) that are routinely measured or estimated in most soil sampling projects, it is possible to improve prediction accuracy in established models.

Automated summary

An increasing number of studies have shown that the assumption that rock fragments are inert with respect to water retention is incorrect, highlighting a gap in research. By developing new pedotransfer functions based on measurements from stony soils in New Zealand, it was possible to accurately predict the water content of these soils, demonstrating the importance of considering rock fragments in water retention models. The existing model, which did not account for the effects of rock fragments, performed poorly compared to the new models, indicating the need for more comprehensive models in soil water retention studies.