Contribution of soil coarse fragments to plant available water

Stony soils are widespread and used for food production, but the effect of coarse fragments (CFs) on water retention remains poorly understood. This study aimed to assess whether the available water (AW) retained in CFs depends on their size and porosity. CFs from three contrasting sites in southern Brazil, varying in size and porosity, were analyzed. Measurements included their volume, total porosity, bulk density, and water retention at suctions of 1 m and 150 m. AW was estimated as the difference of water retention between suctions of 1 m and 150 m. For each site, linear regression was used to evaluate the relationship of AW in CFs with total porosity, water-filled porosity at 1 m suction (θ 1m ), and volume of CFs. Results showed that AW in soil coarse fragments (CFs) depends almost exclusively on their porosity (R 2 around 0.99), while CF size has a negligible effect (R 2 ≤ 0.10). Compared to CFs total porosity, the CFs water-filled porosity at a suction of 1 m is a slightly better predictor of AW in CFs, but the total porosity of CFs is advantageous because its determination is less laborious and equipment-dependent. The relationship between AW and porosity in CFs depends on CFs parent rock lithology and weathering degree, which implies in calibration demands of a pedofunction type AW (porosity).