The influence of rock fragments on field capacity water content in stony soils from hard sandstone alluvium

Worldwide, rock fragments (RFs) are generally considered inert with respect to bulk soil hydraulic properties, such that all soil water retention properties predicted by national pedotransfer functions (such as S-map) are based on the volumetric fraction of the fine earth (<2 mm fraction) only. Research findings contradict those assumptions, but studies commonly focus on porous RFs, and rely on repacked cores and lab studies, leaving uncertainty as to how low porosity RFs characteristic of common strongly indurated lithologies affect soil in the field. We address this question by examining soil water storage in 52 pits excavated into stony soils on the Canterbury Plains, New Zealand, which are formed in 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 × 30 cm pit was excavated in 10 cm increments to a depth of 60 cm. From each increment, soil samples were collected and analysed to determine the volumetric size distribution of RFs, the water content of the fine earth and the water content of the RFs themselves. Our results indicated that RFs could influence the fine earth bulk density, porosity, and soil chemistry. RFs could also retain water: 2–20 mm RFs (0.07 m³ m⁻³) retained twice as much water as >20 mm RFs (0.03 m³ m⁻³). The water retention of the hard sandstone was low compared to other lithologies, but the volumetric abundance of RFs in the stony soils we sampled meant that they accounted for ~10% of the water retained to a depth of 60 cm at field capacity. Our results demonstrate that ~13 mm of water retained by RFs at field capacity is not currently considered in water budgets and nutrient leaching predictions, which may be relevant to best practice land management.