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

•~10% of the water retained in stony soils at field capacity is from rock fragments.•On average, 2–20 mm rock fragments retained twice as much as >20 mm rock fragments.•The water retention of hard sandstone alluvium was low compared to other lithologies.•Rock fragments influenced the fine earth bulk density, porosity and soil chemistry. 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 (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.