Pedogenesis of bleached topsoils occurring on weakly structured, high chroma subsoils in South Africa

Abstract

Topsoil bleaching is a common occurrence in South African soils. Bleaching is usually associated with the removal of soil pigmenting agents through redox stripping, clay eluviation and/or podzolisation. Thus topsoil bleaching above non-podzolic, high chroma red and yellow, supposedly well-drained subsoils is difficult to explain. These deep, weakly structured, red and yellow subsoils are some of the best agricultural soils in the country, and when climate allows, are farmed intensively. Bleached topsoils are known for their unfavourable physical characteristics, therefore understanding how these soils form is important to guide their taxonomic discrimination so that they can be managed sustainably.Profiles showing bleached and non-bleached (high chroma) topsoils were sampled from two different geographical regions; the summer rainfall eastern Highveld region and the winter rainfall, Western Cape Boland region. These samples were statistically compared to identify differences between bleached and non-bleached variants. Topsoil bleaching was shown to be hydrologically driven in catenal sequences in the summer rainfall region. In the red – yellow – grey topographic sequence, subsoil colour changes lag behind topsoil colour changes resulting in a zone where bleached topsoils overly high chroma yellow subsoils. The significantly higher (p = 0.05) acid oxalate/citrate dithionite bicarbonate Fe ratio (Far) in bleached topsoils adds further evidence to a redox driven system. Bleached profiles from the Highveld show indicators of increased subsoil clay accumulation, but it is proposed this is not a dominant process and is likely to be a result rather than the cause of bleaching.Bleaching mechanisms in the Western Cape are harder to establish. Only water dispersible clay differs significantly (p = 0.01) between bleached and non-bleached variants. Extractable Fe fractions do not provide significant evidence of redox pathways, although Far is slightly higher in bleached topsoils, despite a lower absolute oxalate extractable Fe concentration. Although not significant (p = 0.11), exchangeable Na, is higher in the bleached profiles and this may account for the more dispersible nature of the bleached profiles. These results suggest that topsoil bleaching in the Western Cape red apedal soils should be used as an indicator for poor structural stability and that these soils require more stringent management to prevent degradation.