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

Abstract

Water retention by coarse fragments in the soil is often ignored in agronomic and water balance studies. Following the calculation of inexplicably high water retention by the fine earth fraction, water contents at −20 and −1500 kPa and apparent bulk density were determined for remnant pisolithic ironstone gravel samples isolated from soils on the Dundas Tableland in southeast Australia. The volumetric water content of the ironstone gravel at −1500 kPa was found to vary between 0.12 and 0.24 m3 m−3, while at −20 kPa it varied between 0.16 and 0.36 m3 m−3 Available water holding capacity (AWHC) of the ironstone gravel varied between 0.03 and 0.15 m3 m−3 Both the AWHC and the water content at −1500 kPa of the ironstone gravel showed significant increases with depth. Magnetic ironstone gravel, found almost exclusively in the A and E horizons, was much denser than nonmagnetic ironstone gravel (average 3.38 vs. 2.64 Mg m−3), but had similar water retention characteristics. Ignoring the water retention characteristics of the ironstone gravel would have led to overestimation of the AWHC of the bulk soil by a factor 1.08 to 1.67 for various horizons. For the combined top 1.0 m of the soil, ignoring the water held by the ironstone gravel would have led to an estimated AWHC of 162 mm, while in fact it was only 129 mm. Water balance studies of soils with ironstone gravel clearly need to take into account the water holding characteristics of that gravel.