Micromorphology of surface crusts in the Knersvlakte, South Africa |
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| Authors: | Sarah-Jane C Fox Anthony J Mills Rosa M Poch |
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| Institution: | (1) Department of Soil Science, Stellenbosch University, Stellenbosch, South Africa;(2) Department of Environment and Soil Science, University of Lleida, Rovira Roure 191, Lleida, Spain |
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| Abstract: | Soils in the Knersvlakte are particularly prone to crusting and have lower inherent infiltrability than other soils across
western southern Africa. Micromorphological techniques were used to examine the structure and porosity of soil crusts in the
Knersvlakte to ascertain why crusting is so intense in this region. Quantile regression using boundary lines was employed
to examine the relationships between infiltrability and soil properties for all samples (n = 67). This analysis showed that
infiltrability is potentially maximal at low waterdispersible ‘clay plus silt’ content and low silt content (r2 = 0.72 and 0.64; respectively, n = 67) (Figure 2). The strength of crusts, pH, EC, clay mineralogy, and water-dispersible
clay, silt and ‘clay plus silt’ content were compared, and a pore analysis using optical microscopy was undertaken on images
of six soil thin sections (n = 6) (circular and parallel polarizers). Pore analysis was further undertaken on five horizontal
slices of equal dimensions taken through each soil thin section. The porosity samples with low infiltrability (< 100 mm·hr−1, n = 4) had greater crust strength, lower porosity (both total and in the least porous slice) and greater water-dispersible
‘clay plus silt’ and silt content than the porosity samples with high infiltrability (/s> 100 mm·hr−1, n = 2). The porosity samples with low infiltrability showed a trend of lower pH and greater water dispersible clay percentage.
Porosity varied within the porosity samples due to the presence of dense clay/silt bands (< 0.5 mm in width) with relatively
few air vesicles. The porosity samples with horizontal slices of low porosity (but large numbers of air vesicles) had low
infiltrability, while those without slices of low porosity (and relatively few air vesicles) had high infiltrability. We conclude
that the intense crusting and resultant low infiltrability of soils in the Knersvlakte appears to be related to the formation
of thin, dense clay/silt bands in the pedoderm. |
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| Keywords: | Infiltrability optical microscopy porosity air vesicles |
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