RELATIONSHIPS OF WATER REPELLENCY TO SOIL PROPERTIES FOR DIFFERENT SPATIAL SCALES OF STUDY

In order to predict the occurrence of water repellency, which is a lab ile property, from field survey data obtained throughout the year, it is necessary to identify predictive relationships between water repell ency and commonly measured soil properties. This paper evaluates these relationships for diverse soil assemblages. These soil assemblages in clude a set of reference soils from the south-west of Western Australi a (an area of 250 000 km(2)), more intensively sampled suites of soils in several smaller soil-landscape associations within the south-west of Western Australia (congruent to 1000 km(2)), soils from single farm s (1-10 km(2)) and transects (congruent to 0.001 km(2)), and single so il profiles (congruent to m(2)). The severity of water repellency was assessed by measuring water drop penetration time in seconds (WDPT) an d was related to intrinsic properties of soils using log-transformed d ata. For the set of soils from the West Midland Sandplain the type of land use was also considered as a variable. There is a general tendenc y for WDPT to increase as organic matter content increases and decreas e as the content of fine mineral material increases (clay, silt, very fine sand). However, there is no single soil property that is able to predict WDPT adequately. Furthermore, reliability of prediction decrea ses as the area of sampling increases. There appear to be no systemati c differences in the capacity of organic matter from pasture or crop t o induce water repellency, but increments of organic matter under bush increase water repellency at a greater rate than does organic matter from crop or pasture.