BULK-DENSITY OF SURFACE CRUSTS - DEPTH FUNCTIONS AND RELATIONSHIPS TOTEXTURE

A range of soils from Pleistocene deposits with sandy to sandy loam te xtures, and a group of loess-derived soils with predominantly silty te xtures were subjected to 60 mm of simulated rainfall to form structura l seals. After drying, samples of the surface crusts were collected to determine their bulk densities at a high resolution of depth (0-15 mm ) using an immersion method. The hulk density data obtained for each s oil sample were plotted as a function of depth beneath the soil surfac e. Two models were fitted to these plots. The first was an exponential decay type function as proposed by Mualem et al. (1990), and the seco nd was a sigmoidal type of function assuming that maximum compaction h ad already progressed to some depth below the soil crust surface. All of the results indicated a gradual decrease in the bulk density with d epth below the surface, until convergence with the initial hulk densit y of the undisturbed soil was attained. The maximum bulk densities rec orded for crust segments representing the uppermost 2 mm of the crusts ranged from 1.78 to 1.91 g cm(-3) for soils with silty sand, loamy sa nd or sandy loam textures. Crusts of loess-derived soils showed lower values, ranging from 1.44 to 1.65 g cm(-3). The maximum surface bulk d ensity was shown to be highly significantly correlated with the log of geometric mean diameter of the primary grain size distribution. In mo st cases, both models showed good to very good fits to the measured da ta; the exponential decay function appeared to better represent the in itial stages of surface compaction, and the sigmoidal function the lat er stages of structural crust formation.