Soil water in a fragiaqualf

Abstract

The profiles of density and soil water retentivity for a Fragiaqualf, the Tokomaru silt loam, were found to correspond with the morphological features, with a bulk density of up to 1670 kg m-3 and a negligible volume of macropores in the C horizon fragipan. The saturated hydraulic conductivity at undrained sites was as low as 0.1 mm day-1 in the B horizon, although there is evidence that at mole-tile drained sites the effective permeability is much higher. Water movement is non-uniform in the subsoil, apparently due to preferential flow between the large polygonal structure units. Comparison of neutron probe profiles and laboratory soil water retentivity data demonstrated the difficulties in using the latter to estimate the available-water holding capacity of the soil. The main problem was the lack of a clearly defined rooting depth. This was probably due to non-uniform water extraction from the subsoil by pasture roots, because of their preferential distribution between rather than within the large structure units. This made it impossible to correlate the 'permanent wilting point' with a particular soil matric potential.