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

[摘要] 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.

[发布机构] Stellenbosch University