Water holding capacity of ironstone gravel in a typic plinthoxeralf in southeast Australia

Water retention by coarse fragments in the soil is often ignored in agronom ic and water balance studies. Following the calculation of inexplicably hig h water retention by the fine earth fraction, water contents at -20 and -15 00 kPa and apparent bulk density were determined for remnant pisolithic iro nstone gravel samples isolated from soils on the Dundas Tableland in southe ast Australia. The volumetric water content of the ironstone gravel at -150 0 kPa was found to vary between 0.12 and 0.24 m(3) m(-3), while at -20 kPa it varied between 0.16 and 0.36 m(3) m(-3) Available water holding capacity (AWHC) of the ironstone gravel varied between 0.03 and 0.15 m(3) m(-3) Bot h the AWHC and the water content at -1500 kPa of the ironstone gravel showe d significant increases with depth. Magnetic ironstone gravel, found almost exclusively in the A and E horizons, was much denser than nonmagnetic iron stone gravel (average 3.38 vs. 2.64 Rig m(-3)), but had similar water reten tion characteristics. Ignoring the water retention characteristics of the i ronstone gravel would have Led to overestimation of the AWHC of the bulk so il by a factor 1.08 to 1.67 for various horizons. For the combined tap 1.0 m of the soil, ignoring the water held by the ironstone gravel mould have l ed to an estimated AWHC of 162 mm, while in bet it was only 129 mm. Water b alance studies of soils with ironstone gravel clearly need to take into acc ount the water holding characteristics of that gravel.