The relation between water content and water potential is a key soil hydraulic property. The presence of rock fragments in soils can affect this property in bulk soil. In this study, we focused on the relation property determined with a WP4 dew point potentiometer. The objective was to evaluate the hypothesis that breaking large gravel-size particles into smaller fragments is a suitable strategy for accurate WP4 measurements in coarse materials without affecting the retention properties. Ten initial samples of basalt rock fragments with different weathering degrees were collected from the A horizon of an Entisol (Neossolo). Rock fragments initially sized between 38 to 4 mm were successively fractured into smaller fragments (size classes of 4-2, 1-0.25, and <0.25 mm). For each size class, the retention properties (water content versus water potential) were determined with WP4 equipment. No significant effect of fragment size was detected on the determined retention properties. Using supplementary water retention data from a previous study, we found an overestimation of water retention at -1.5 MPa ranging from 0.08 to 0.12 g g-1 when rock fragments were removed, which is of the order of magnitude to the available water capacity of several soils. This study showed that fragments larger than the diameter of the WP4 sample cup could be broken to sizes smaller between 4 and 0.25 mm without affecting the retention properties. This is a suitable strategy to allow accurate WP4 measurements in coarse materials while considering the effect of such coarse material on retention properties.
