Properties of bleached topsoils on apedal subsoils : analysis from the land type profile database

Carstens, Marilee Elizabeth (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Bleached topsoils that occur on red and yellow-brown apedal subsoils are poorly understood and taxonomically they are not distinguishable from their non-bleached counterparts. Bleaching of soils is an important pedological indicator since it can reflect a soils water status. Bleached topsoils are more prone to erosion and degradation than their non-bleached counterparts, thus recognising these features is important. Bleaching is also identified by its colour, thus precise and objective colour measurement procedures are required to correctly identify them. The two mechanisms responsible for bleaching are proposed to be iron (Fe) reduction and clay dispersion. The overall aim of this study was to use the data available in the Profile Database to understand the spatial and geomorphic distribution of bleached apedal profiles as well as assess their lithological, chemical, physical, spectral and subsoil colour properties to provide clues on their genesis. This will allow the diagnostic criteria for their correct identification to be based on scientific understanding and also to provide a scientific basis for these bleached soils use and protection. The study made use of pre-existing data from the Agricultural Research Council (ARC) – Institute for Soil, Climate and Water (ISCW) – Soil Profile Information System. Data from 725 soil profiles, with complete chemical and physical analysis, that contained red and yellow-brown apedal and neocutanic subsoils were selected for the study. Subsamples of the A and B horizon from each profile were collected from the soil store of the ISCW. Soil colours were measured both visually with Munsell colour charts and spectroscopically with a Konica-Minolta spectrophotometer. This colour data was used to classify the soils into bleached and non-bleached categories by following the criteria outlined in the South African soil classification system. The effectiveness of visual colour measurement guidelines as outlined by the Munsell colour system and Food and Agricultural Organization (FAO) was evaluated by comparing visual measurements made in the laboratory and in natural daylight (outdoor) conditions. From the results there seems to be no great difference between soil colour measurements made in visual natural daylight and laboratory conditions. Visual colour measurements were also correlated to spectroscopic colour measurements. It was found that the spectrophotometer tended to make soil hues redder in 32% of the observations when compared to visual laboratory and natural daylight observations. Spectroscopic chroma observations showed 0% total agreement with both visual laboratory and natural daylight observations. When compared, the spectrophotometer tends to designate lower chroma values to soils than both visual laboratory and natural daylight colour measurements would. This means the human eye tends to make soil colour more colourful than the spectrophotometer. The weak relationships between soil pigmenting properties and spectroscopically measured colour components showed that soil colour is a complex expression of both physical and chemical soil components and thus cannot be related to individual soil properties. The wide geographical spread of these soils might be the cause of poor soil property-colour component relationships observed. The geographical location of the soils used for this study did not seem to play a significant role in the occurrence of bleached topsoils in different landscapes. The occurrence of topsoil bleaching seemed to be significantly related to parent materials, with the frequency of bleaching being highest in siliceous lithologies and lowest in mafic lithologies. This might explain why bleached topsoils showed the tendency to develop in soils with low reducible Fe and exchangeable magnesium percentage (EMP). Clay movement from the A to B horizon showed no significant trends in terms of bleaching, which was also the case for exchangeable sodium percentage (ESP). In this study the results for clay movement and ESP thus does not support clay dispersion as a possible mechanism for topsoil bleaching. Bleaching tended to increase with a decrease in base saturation, with the highest incidences of bleaching being on dystrophic soils, which in turn could also relate to climate and soil acidity. The highest occurrence of topsoil bleaching took place on yellow-brown apedal subsoils (66%) and in the Avalon soil form (79%). These results might provide evidence for topsoil bleaching to occur in soils with wetter water regimes, since Avalon soils are usually found in wetter landscape positions. It is recommended that the South African Soil Classification Working Group should consider adding bleaching as a family criteria to soil forms containing yellow-brown apedal subsoils.

AFRIKAANSE OPSOMMING: Gebleikte bogronde wat voorkom op rooi en geel-bruin apedale ondergronde word swak verstaan en taksonomies is hul nie onderskeibaar van hul nie-gebleikte ewivalente nie. Verbleiking is 'n belangrike pedologiese aanwyser aangesien dit die grond se water toestand weerspieël. Gebleikte bogronde is ook meer geneig tot erosie en agteruitgang as hul nie-gebleikte ekwivalente. Verbleiking word geïdentifiseer aan sy kleur, dus word akkurate en objektiewe kleur meting prosedures benodig om hul korrek te identifiseer. Die twee voorgestelde meganismes verantwoordelik vir verbleiking is yster reduksie (Fe) en klei dispergering. Die oorkoepelende doel van hierdie studie was om gebruik te maak van data wat in die Profiel Databasis beskikbaar was om sodoende die ruimtelike en geomorfologiese verspreiding van gebleike apedale grondprofiele te verstaan, sowel as om hul litologiese, chemiese, fisiese, spektrale en ondergrond kleur eienskappe te evalueer om so dus leidrade oor hul ontstaan te kan voorsien. Dit sal die diagnostiese kriteria vir die korrekte identifisering van gebleikte gronde gebaseer laat wees op grond van wetenskaplike begrip wat ook 'n wetenskaplike basis sal bied vir die gebruik en beskerming van hierdie gebleikte gronde. Die studie het gebruik gemaak van reeds-bestaande data uit die LNR (Landbou Navorsings Raad) - Instituut vir Grond, Klimaat en Water (IGKW) – Grondprofiel Informasie Sisteem. Data van 725 grondprofiele, meerderheid wat ingesamel is tydens die Land Tipe Opname, met volledige chemiese en fisiese analises wat rooi en geel-bruin apedale en neokutaniese ondergronde bevat was geselekteer vir hierdie studie. Submonsters van die A- en B-horisonte van elke profiel was ingesamel uit die grond stoor van die IGKW. Grondkleure is visueel gemeet met Munsell kleurkaarte en spektroskopies gemeet met 'n Konica-Minolta spektrofotometer. Hierdie kleur data is dan gebruik om gronde in gebleikte of nie-gebleikte kategorieë te plaas deur om die kriteria te gebruik wat uiteengesit is in die Suid-Afrikaanse Grondklassifikasie Sisteem. Die doeltreffendheid van visuele kleur metings riglyne soos die uiteengesit deur die Munsell kleur stelsel en Voedsel- en Landbou-Organisasie (FAO) is geëvalueer deur om korrelasies te maak tussen verskillende visuele kleur metings kondisies (laboratorium en natuurlike daglig (buite). Die resultate het getoon dat daar geen betekenisvolle verskille was tussen visuele grondkleur metings wat gemaak is in natuurlike daglig en laboratorium kondisies nie. Visuele kleur metings is ook gekorreleer met spektroskopiese kleur metings. Die resultate het getoon dat die spektrofotometer geneig was om grond skakerings rooier te maak in 32% van die waarnemings in vergelyking met die visuele laboratorium en natuurlike daglig waarnemings. Die spektroskopiese chroma waarnemings het 0% totale ooreenstemming getoon met beide van die visuele kondisies se chroma waarnemings. Die spektrofotometer was meer geneig om laer chroma waardes aan gronde toe te skryf in vergelyking met visuele laboratorium en natuurlike daglig chroma kleur metings. Dit mag beteken dat die menslike oog geneig is om grondkleur meer kleurvol te maak in vergeleiking met die spektrofotometer. Die swak verhouding tussen grond pigmenterings eienskappe en spektroskopiese gemete kleur komponente het getoon dat grondkleur 'n komplekse uitdrukking is van beide fisiese en chemiese grondkomponente en kan dus nie betrekking hê tot individuele grondeienskappe nie. Die geografiese verspreiding van die gronde in hierdie studie mag dalk die oorsaak wees vir die swak grondeienskap-kleur komponent verhoudings wat waargeneem was. Die geografiese ligging van die gronde wat gebruik was vir hierdie studie het nie 'n betekenisvolle rol gespeel in die voorkoms van gebleikte bogronde in verskillende landskappe nie. Dit bleik asof daar ‘n beduidende tendens is vir gebleikte bogronde om op sekere moedermateriale te ontwikkel. Silikahoudende gesteentes het die hoogste frekwensie van verbleiking getoon en mafiese gesteentes die laagste frekwensie. Hierdie resultate mag dalk verduidelik hoekom gebleikte gronde geneig is om in gronde met lae reduseerbare yster (Fe) vlakke en uitruilbare magnesium persentasie (UMP) te ontwikkel. Klei beweging van die A tot B-horison het geen beduidende verskille getoon in terme van verbleiking nie, wat ook die geval was vir uitruilbare natrium persentasie (UNP). Die resultate vir klei beweging deur ‘n profiel en vir UNP ondersteun dus nie klei dispergering as die meganisme verantwoordelik vir bogrond verbleiking in hierdie studie nie. Verbleiking het ook geneig om toe te neem met ‘n afname in basisversadiging, met die hoogste voorkoms van verbleiking op distrofiese gronde wat dalk verband mag hou met klimaattoestande. Die hoogste frekwensie van bogrond verbleiking het voorgekom op geel-bruin apedale ondergronde (66%) en in die Avalon grondvorm (79%). Hierdie resultate mag dalk bewyse lewer dat bogrond verbleiking meer geneig sal wees om in natter gronde te ontwikkel, aangesien Avalon gronde gewoonlik geassosieer word met natter landskap posisies. Daar is voorgestel dat die Suid-Afrikaanse Grondklassifikasie Werkgroep dit moet oorweeg om verbleiking as ‘n familie kriteria te erken in grondvorms wat geel-bruin apedale ondergronde bevat.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98369
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