Browsing by Author "Le Roux, Johannes Lukas"
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- ItemThe occurrence of bleached topsoils on weakly structured subsoil horizons in the Western Cape and Mpumalanga Provinces of South Africa(Stellenbosch : Stellenbosch University, 2015-12) Le Roux, Johannes Lukas; Clarke, Catherine E.; De Clercq, W. P.; Stellenbosch University. Faculty of Agrisciences. Dept. of Soil Science.ENGLISH ABSTRACT: Bleaching is a topsoil discolouration phenomenon recognised at family level within certain soil forms in the South African soil classification system. These topsoil horizons derive their name from the pale greyish colouration they exhibit in the dry state which is the result of the loss of pigmentation agents, specifically Fe oxides and organic material, from the upper part of the soil profile. In recent years, discrepancies regarding both the occurrence of bleached topsoils as part of weakly structured soil profiles in the South African soil landscape, and the description of this phenomenon in the national soil classification system, have become apparent. This has accentuated the clear lack of understanding which exists regarding the genesis of bleached topsoil horizons under weakly structured subsoil conditions in South Africa. Based on the land-use and classification significance of this soil feature, this study aimed to provide clarity on the characteristics of bleached topsoils and some of the weakly structured subsoil horizons they overlie to determine by which mechanism these bleached topsoils develop and if the pedogenetic mechanisms are similar across two regions of South Africa. Due to soil colour’s obvious importance as the only feature with which to recognise topsoil bleaching, an investigation into the measurement and expression of soil colour was also deemed to be important. A total of 26 soil profiles were sampled throughout the Western Cape (WC) and Mpumalanga provinces. Colour was visually determined in the field using a Munsell soil colour chart and also by means of a spectrophotometer in the laboratory. A wide variety of soil chemical and physical properties were also determined for each sampled horizon. For comparison’s sake, the selected soil profiles needed to represent profiles that could potentially qualify as having red/yellow-brown apedal B (ferralsols) or red/yellow neocutanic B horizons (cambisols), either with or without a perceived bleached topsoil (achromic). During sampling on the Highveld, bleaching was observed to be landscape related with bleached orthic A horizons only occurring on yellow-brown apedal B subsoil horizons at lower positions along the plinthic catenas. As a result, sampling on the Mpumalanga Highveld was conducted along catenal transects. In the WC, bleached profiles did not follow a noticeable landscape pattern and subsoils comprising both red and yellow weakly structured horizons were recorded. Soil colour investigations proved Fe oxides to be the main pigmentation agents responsible for the expression of red- and yellow colours in the sampled soils, with soil samples also becoming redder with an increase in the Fe oxide content. Discrepancies were detected in the way soil colour was registered through human perception and spectrophotometer measurements. In general, the eye perceived the soils to be brighter and more chromatic and therefore was less sensitive towards detecting bleached horizon colours. The majority of the determined chemical and physical soil properties did not differ between the Western Cape and Highveld soils and did not show any relation to the bleaching phenomenon in either of the locations. In the Western Cape, profiles tended to have a greater water dispersible clay (WDC) phase, with the bleached Western Cape profiles proving to be even more unstable than the non-bleached variants. Iron oxide characterisation indicated proportionally similar amounts of crystalline and poorly-crystalline Fe oxides were present at both locations although in general bleached topsoils tended to have greater poorly-crystalline Fe contents. This trend was more pronounced in the Highveld profiles and was deemed to be indicative of a wetter soil moisture regime and alternating cycles of Fe reduction and oxide precipitation at this location. The reported poorly-crystalline nature of the Fe oxides together with the observed landscape influences, suggest Fe reduction to be the pedogenetic process responsible for bleached topsoil horizons overlying weakly structured subsoils on the Mpumalanga Highveld. The strong association between bleaching and clay dispersibility in similar profiles of the Western Cape suggest clay eluviation to be a common pedogenetic process in these soils. The presented data is this study did not provide an explanation for how clay eluviation results in bleached soil colours and no evidence was obtained to enable conclusive statements regarding the role of Fe reduction and clay eluviation as independent or complementary processes responsible for bleaching in the Western Cape soils. For the purpose of soil classification in South Africa, the inclusion of bleached orthic A horizons as family criteria in wetter variants of the yellow-brown apedal profiles is suggested. Based on the instability of the clay phase in the Western Cape profiles, it is proposed that these red or yellow weakly structured subsoils would be better classified as neocutanic B horizons and that bleached topsoils can in some instances be indicative of a more dispersive profile.