The role of ferrolysis in the genesis of selected soils of the Eastern Free State

Plinthic and duplex soil forms make up a substantial part of the soils under cultivation in South Africa. The tendency of these soils to occur either in isolation, or in association with one another, leaves the impression that ideal conditions for the formation of each occur independently but are closely related. This implies that ideal conditions for the development of each vary over short distances. Little research has been done on the duplex-plinthic soil association but a relationship between the two soil groups is implied in literature. A proper understanding of soil genesis may therefore contribute to the better classification, interpretation and evaluation of these soils for sustainable land-use purposes. The hypothesis is that the redistribution of Fe-Mn and degradation of silicate clays are important processes involved in the formation of soils with either a duplex or plinthic character. The objective of the study was to establish the role of ferrolysis and redistribution of Fe-Mn in the genesis of the duplex-plinthic soil association. The catena concept; which describes a sequence of soils of about the same age, occurring under similar macroclimatic conditions and derived from the same parent material; but with different characteristics due to variation in topography and drainage; was adopted. A toposequence of 10 representative profiles was selected in the Eastern Free State. The soils were described, sampled and photographed. Representative composite and undisturbed samples were analysed for several chemical, physical and morphological soil properties. The selected toposequence commences at the crest with a profile of the Hutton soil form. Soils of the Westleigh, Longlands, Avalon (3), Kroonstad (3) and Estcourt forms follow down slope. The characteristic red colour grades to yellow-brown in the soft plinthic soils on the mid slope and grey duplex soils in the valley bottom. The change in colour dominates morphology in the midslope but changes to be dominated by texture differentiation in the valley bottom. Signs of redox activity prevail across the toposequence and its prominence increases drastically with depth in the profiles and down slope in the catena. Subsoil acidification indicates the presence of an early stage of redox activity. Fe-Mn redistribution, present as mottles and concretions, and colour changes are indications of intermediate redox activity. The texture differentiation present indicates an environment supporting ferrolysis to the full.