Pedological significance of the gravels in some red and grey earths of central north Queensland

Abstract

Red, yellow and grey earths consist largely of quartz, kaolin, and iron oxides and hydroxides, but little is known of the pedological processes that have resulted in their formation. Iron oxides and hydroxides, however, vary markedly according to their soil environments and can be used to study pedogenetic pathways in soil profiles. A detailed mineralogical examination has been made of two soils representing the red and grey end-members of the red-yellow-grey earth soil continuum in the semiarid tropical Torrens Creek area, 200-300 km southwest of Townsville, in central north Queensland. The profiles selected (a Plinthustalf and Haplargid of Soil Taxonomy) have high contents of ferruginous gravels in the solum, and are underlain by mottled, weathered sandstone. Four corresponding layers were sampled from each soil profile. Washed gravels from these samples were sieved into four size classes, and were sorted by hand into morphological groups, and into magnetic and nonmagnetic fractions. Subsamples of the resultant groups of gravels (>2 mm), of the fine earth ( < 2 mm), and of the whole soil, were analysed by X-ray diffraction. Quartz, kaolin, goethite and hematite were found in various proportions in the 205 analysed samples, but maghemite was restricted to the magnetic materials; few other minerals were detected. The mineralogical composition of each morphological group of gravels was found to be independent of the particle-size of the materials in the group. The iron oxide mineralogy of the soils suggests that the red, yellow and grey earths developed under different soil water regimes: free drainage and high organic matter oxidation rates in the red earths yielded hematite, with its strong red-pigmenting capacity, and goethite; profile saturation for relatively short periods of time has resulted in the formation of goethite, characteristic of the yellow earths and of the ferruginous gravels of the grey earths; and saturation of the grey earth profiles for lengthy periods has led to the loss of iron from the soil, which has then assumed the grey colour of the minerals (quartz and kaolin) comprising its matrix. These mineralogical interpretations are supported by field observations of contrasting hydrological regimes in the soils.