Micromorphological evidence of redox activity in the soft plinthic B horizon of a soil of the Bainsvlei form in an arid bioclimate

Abstract

In South Africa, the pedogenic relevance of soft plinthic B horizons in the border zone between arid and semi-arid bioclimates is often questioned because the prevailing climate is not conducive for fluctuating water tables as required for their formation. These horizons are therefore interpreted as being paleo features, a plausible possibility as the paleo climate of South Africa varied considerable from that experienced today. The genetic activity of the soft plinthic B horizon in the Bainsvlei soil form is questioned even more as it occurs under a red apedal B horizon which is regarded as freely drained. A micromorphological study was therefore carried out on a Bainsvlei Amalia fine sand soil profile located in an arid bioclimatic zone to determine redox activity in its soft plinthic B horizon. Undisturbed clods, impregnated blocks and optical thin sections were studied. Only a few Fe-Mn nodules of varying colour were present in the soft plinthic B horizon. However, mottles of biological and chemical origin with high and low chroma were prominent features. Grey and red mottles and to a lesser extent yellow and black mottles were randomly distributed in the soil matrix. Many of these mottles were concentric and their colours related closely to the position of the macropores displaying a juxtapositioned sequence from grey to yellow to red to black. In other instances the sequence was reversed. Both sequences are typical of redistribution of Fe-Mn under spatially varying redox conditions and hence plinthite formation. Another indication of active plinthite formation was the diffuse transitions between the mottles which increased with depth. Thus sufficient micromorphological evidence evolved to assume recent redox activity of low intensity in the soft plinthic B horizon of this particular Bainsvlei soil.