Process and Mechanism for the Development of Physical Crusts in Three Typical Chinese Soils

To compare the development of physical crusts in three typical cultivated soils of China, a black soil (Luvic Phaeozem), a loess soil (Haplic Luvisol), and a purple soil (Calcaric Regosol) were packed in splash plates with covered and uncovered treatments, and exposed to simulated rainfall. Meshes covered above the surfaces of half of soil samples to simulate the effects of crop residue on crusting. The results indicated a progressive breakdown of aggregates on the soil surface as rainfall continued. The bulk density and shear strength on the surface of the three soil types increased logarithmically as rainfall duration increased. During the first 30 min of simulated rainfall, the purple soil developed a 7–8 mm thick crust and the loess soil developed a 3–4 mm thick crust. The black soil developed a distinguishable, but still unstable, crust after 80 min of simulated rainfall. Soil organic matter (SOM) content, the mean weight diameter (MWD) of soil aggregates, and soil clay content were negatively correlated with the rate of crust formation, whereas the percentage of aggregate dispersion (PAD), the exchangeable sodium percentage (ESP), and the silt and sand contents were positively correlated with crusting. Mechanical breakdown caused by raindrop impact was the primary mechanism of crust formation in the black soil with more stable aggregates (MWD 25.0 mm, PAD 3.1%) and higher SOM content (42.6 g kg−1). Slaking and mechanical eluviation were the primary mechanisms of crust formation in the purple soil with low clay content (103 g kg−1), cation exchange capacity (CEC, 228 mmol kg−1), ESP (0.60%), and SOM (17.2 g kg−1). Mechanical breakdown and slaking were the most important in the loess soil with low CEC (80.6 mmol kg−1), ESP (1.29%), SOM (9.82 g kg−1), and high PAD (71.7%) and MWD (4.6 mm). Simulated residue cover reduced crust formation in black and loess soils, but increased crust formation in purple soil.