Size characteristics of soils and sediments with special reference to clay fractions

Abstract

Soils and sediments from south-eastern Australia were examined to determine whether texture B horizons had distinctive and unique particle-size characteristics. Most texture B horizons had higher contents of fine clay (<0.2 µm) than coarse clay (0.2-2 µm), whereas coarse clay was the dominant clay-size fraction in A and C horizons. Sequences of soils in alluvium showed a systematic increase in particle-size differentiation with increasing stratigraphic age and corresponding increases in the proportion of fine clay in their B horizons. Pedologically unmodified, clay-rich sediments generally had lower contents of fine clay and higher contents of silt than B horizons. However, sediments of a lake floor and samples of argillans had clay-size fractions similar to B horizons. The distribution of fine clay in soils, determined here by centrifugal sedimentation, was poorly related to illuviated clay determined microscopically. Detailed particle-size data showed that texture B horizons were more enriched than adjacent horizons by a fraction with upper size limits between 0.25 and 0.5 µm. The greater the profile texture contrast, the more pronounced this enrichment became. The enriched clay-size fractions of these B horizons had the particle-size characteristics of log normal distributions. Within the clay-size range of texture B horizons, the fraction < 0.5 µm was the most directly related to surface area measurements made on dry soil samples. Sediments of a lake floor and samples of argillans had similar clay-size characteristics to soil B horizons. It was concluded that particle-size differentiation in these soils was related mainly to pedogenetic processes within the profile. However, the particle-size characteristics of texture B horizons are not unique. Criteria other than those based on particle-size analysis are therefore required to determine the relative importance of translocation and weathering.