Soil colour variation between topsoil and subsoil horizons in a plinthic catena on the Mpumalanga Highveld, South Africa

Abstract

The South African plinthic catena is characterised by a grading of soils from red through yellow to grey (bleached) soils down a slope. The colour sequence is ascribed to different Fe-minerals stable at increasing degrees of wetness. Soil survey data (422 points) was obtained from three soil surveys on the Mpumalanga Highveld. For each survey point, the topsoil and subsoil horizon Munsell^® colour was determined. The data was compared for topsoil horizons overlying different subsoil horizons. A distinct trend was observed in that orthic A horizons overlying yellow-brown apedal B horizons exhibited a marked lighter colour (similar to E horizons) compared to the underlying B-horizon as well as orthic A horizons overlying red apedal B horizons. This implied that topsoil colour is not indicative of subsoil horizon colour – especially in soils of the Avalon, Clovelly, Glencoe and Pinedene forms. The lighter-coloured A horizons also exhibited a marked lower clay content compared to the underlying B horizon. The lighter-coloured A horizons on high chroma B horizons have major implications for the conceptual classification of the soils as well as description of hillslope hydrology and mapping of soils from aerial photographs and satellite images.

Keywords: E horizon, Munsell value, orthic A horizon, red apedal B horizon, yellow-brown apedal B horizon

South African Journal of Plant and Soil 2013, 30(1): 47–51