Bulk density of surface crusts: depth functions and relationships to texture

A range of soils from Pleistocene deposits with sandy to sandy loam textures, and a group of loess-derived soils with predominantly silty textures were subjected to 60 mm of simulated rainfall to form structural 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 bulk density data obtained for each soil sample were plotted as a function of depth beneath the soil surface. Two models were fitted to these plots. The first was an exponential decay type function as proposed by Mualem et al. (1990), and the second was a sigmoidal type of function assuming that maximum compaction had already progressed to some depth below the soil crust surface. the results indicated a gradual decrease in the bulk density with depth below the surface, until convergence with the initial bulk density of the undisturbed soil was attained. The maximum bulk densities recorded for crust segments representing the uppermost 2 mm of the crusts ranged from 1.713 to 1.91 g cm−3 for soils with silty sand, loamy sand 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 density was shown to be highly significantly correlated with the log of geometric mean diameter of the primary grain size distribution. In most cases, both models showed good to very good fits to the measured data; the exponential decay function appeared to better represent the initial stages of surface compaction, and the sigmoidal function the later stages of structural crust formation.