Development of Particle‐size Distributions in Some Alfisols of Southeastern Australia

Particle‐size distributions of 20 soils in southeastern Australia were studied to determine whether Alfisols had consistent characteristics, and to examine the changes in clay and nonclay fractions with soil age. Particle‐size intervals, based on the logarithmic phiscale, were chosen to give adequate details of size distributions. The A horizons of three Alfisols with strong textural contrast from erosional and depositional landscapes (group I) had essentially one size mode in the nonclay range. The Bt horizons had bimodal particlesize distributions with a strong mode in the 0.06‐ to 0.12‐ or 0.12‐ to 0.24‐µm size classes and another in the nonclay range. Other subsoil horizons were less clearly bimodal. Seventeen soils from four river terrace sequences (group II) ranged from unmodified alluvium of valley floors (Fluvents) to soils with strong textural contrast (Us‐talfs) on Pleistocene terraces. The A horizons of these soils generally had one particle‐size mode in the nonclay range. With increasing soil age and textural differentiation, the particle‐size distributions of B horizons were more strongly bimodal with clay‐size populations similar to the Bt horizons of soils in group I. Only one clay‐size population occurred in Bt horizons with an upper size limit close to 2 µm. Although enriched, the fine clay fraction (<0.2 µm) was not identified as a size population separate from coarse clay (0.2‐2 µm). Development of Bt horizons is envisaged here as progressing from an initial translocation of the clay inherited from parent materials to intensive weathering and size reduction of clay particles in response to strong seasonal fluctuations in soil moisture.