The occurrence of bleached topsoils on weakly structured subsoil horizons in the Western Cape and Mpumalanga Provinces of South Africa

Le Roux, Johannes Lukas (2015-12)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

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.

AFRIKAANSE OPSOMMING: Verbleiking is ‘n bogrondverskynsel wat erken word op familievlak in die Suid-Afrikaanse grondklassifikasiestelsel. Die benaming van hierdie verskynsel spruit voort uit die vaalgrys kleure wat hierdie bogrondhorisonte openbaar in die droëgrondtoestand, as gevolg van die verlies van grondpigmente soos Fe oksiedes en organiese materiaal uit die boonste gedeeltes van die grondprofiel. ‘n Aantal teenstrydighede aangaande beide die voorkoms van gebleikte bogronde as deel van swak gestruktureerde grondprofiele in Suid-Afrika, sowel as die beskrywing van hierdie verskynsel in die nasionale grondklassifikasiestelsel, het oor die afgelope paar jaar te voorskyn gekom. Hierdie teenstrydighede beklemtoon die beperkte kennis wat daar bestaan aangaande die pedogenetiese oorsprong van gebleikte bogronde op swak gestruktureerde ondergronde in Suid-Afrika. Weens die implikasies van hierdie bogrondverskynsel vir gebruiks- en grondklassifikasie doeleindes, beoog hierdie studie om die eienskappe van gebleikte bogronde en sommige van die swak gestruktureerde ondergronde wat hul oorlê, te bepaal, om sodoende die genetiese oorsprong van hierdie gronde onder sulke toestande te probeer verklaar. As gevolg van grondkleur se belangrikheid as die enigste eienskap wat gebruik kan word om gebleikte bogronde te identifiseer, is aspekte aangaande die meting en uitdrukking van hierdie verskynsel ook as belangrik geag. Altesaam 26 grondprofiele, verspreid oor die Wes-Kaap Provinsie en die Mpumalanga Hoëveld, is versamel. Grondkleur is eers visueel bepaal tydens veldwerk deur gebruik te maak van die Munsell grondkleurkaarte en later ook deur middel van n spektrofotometer in die laboratorium. ‘n Verskeidenheid grondchemiese en –fisiese eienskappe is ook bepaal vir elk van die versamelde horisonte. Die gekose profiele moes van so ‘n aard wees dat beide rooi/geelbruin apedale B of rooi/geel neokutaniese B horisonte ‘n klassifikasiemoontlikheid kon wees. Profiele met en sonder gebleikte bogronde is ingesluit. Tydens die versameling van grondmonsters is dit waargeneem dat verbleiking op die Hoëveld landskap-gedrewe is, met gebleikte ortiese A horisonte wat slegs voorkom op geelbruin apedale ondergronde in die laer hellingsposisies van hierdie landskap. As ‘n gevolg van hierdie waarneming, is grondversameling op die Hoëveld uitgevoer langs katena-transekte af. In die Wes-Kaap was daar geen merkbare verhouding tussen verbleiking en landskapposisie nie en die ondergronde van gebleikte profiele het bestaan uit beide rooi en geel swak gestruktureerde horisonte. Die ondersoeke na grondkleur het bewys dat Fe oksiedes die hoof grondpigment is in die rooi en geel gronde wat versamel is en dat gronde geneig was om rooier te raak soos wat die Fe inhoud van die monster toegeneem het. Verskille ten opsigte van die wyse waarop hierdie uitgedrukte kleure geregistreer word, is tussen die visuele en spektrofotometriese bepalings waargeneem. Oor die algemeen het die oog die grondkleure as helderder en meer chromaties waargeneem en gevolglik was visuele kleurbepaling minder sensitief ten opsigte van gebleikte grondkleure. Die meerderheid van die chemiese en fisiese grondeienskappe wat bepaal is, het nie verskil tussen die Wes-Kaapse en Hoëveld profiele nie en het ook geen verwantskap met verbleiking getoon in enige van die twee areas nie. Die Wes-Kaapse profiele het egter n groter waterdispergeerbare kleifraksie (WDC) getoon, met die gebleikte profiele in die area wat as nog meer onstabiel as die nie-gebleikte variante bewys is. Die grond Fe inhoud in beide die Wes-Kaap en die Hoëveld het proporsioneel dieselfde vlakke van kristallyne en swak-kristallyne Fe oksiedes bevat, met die gebleikte profiele in geheel wat meer swak-kristallyne Fe oksiedes besit het. Hierdie tendens was meer prominent in die Hoëveld bogronde en is toegeskryf aan natter grondtoestande wat variërende fases van Fe reduksie en oksidasie tot gevolg gehad het. Die swak-kristallyne Fe oksiedes tesame met waarnemings in die veld aangaande landskap en grondverhoudings, dui daarop dat Fe reduksie die meganisme is waardeur gebleikte bogronde op swak gestruktureerde ondergronde in die Hoëveld ontstaan. Die sterk verwantskap tussen verbleiking en WDC in soortgelyke profiele in die Wes-Kaap dui daarop dat klei-eluviasie 'n groot rolspeler in hierdie grondprofiele is. Daar kon egter in hierdie studie geen bewyse gevind word wat aandui hoe klei-eluviasie gebleikte grondkleure veroorsaak nie. Verder kon Fe reduksie se bydrae tot verbleiking in die Wes-Kaap gronde nie uitgesluit of vasgestel word nie. Vir grondklassifikasiedoeleindes word daar voorgestel dat gebleikte ortiese A horisonte as n familie-kriterium in die natter weergawes van die geelbruin apedale profiele ingesluit word. Verder, gegrond op die onstabiele klei fraksie in die Wes-Kaap profiele, stel ons voor dat hierdie swak gestruktureerde rooi en geel ondergronde as neokutaniese B horisonte geklassifiseer word en dat in somige gevalle, ‘n gebleikte bogrond ‘n aanduiding kan wees van ‘n meer onstabiele grondprofiel.

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