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

[摘要] English: Plinthic and duplex soil forms make up a substantial part of the soils under cultivation inSouth Africa. The tendency of these soils to occur either in isolation, or in associationwith one another, leaves the impression that ideal conditions for the formation of eachoccur independently but are closely related. This implies that ideal conditions for thedevelopment of each vary over short distances. Little research has been done on theduplex-plinthic soil association but a relationship between the two soil groups is impliedin literature. A proper understanding of soil genesis may therefore contribute to thebetter classification, interpretation and evaluation of these soils for sustainable land-usepurposes.The hypothesis is that the redistribution of Fe-Mn and degradation of silicate clays areimportant processes involved in the formation of soils with either a duplex or plinthiccharacter. The objective of the study was to establish the role of ferrolysis andredistribution of Fe-Mn in the genesis of the duplex-plinthic soil association. The catenaconcept; which describes a sequence of soils of about the same age, occurring undersimilar macroclimatic conditions and derived from the same parent material; but withdifferent characteristics due to variation in topography and drainage; was adopted.A toposequence of 10 representative profiles was selected in the Eastern Free State. Thesoils were described, sampled and photographed. Representative composite andundisturbed samples were analysed for several chemical, physical and morphological soilproperties.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 followdown slope. The characteristic red colour grades to yellow-brown in the soft plinthicsoils on the mid slope and grey duplex soils in the valley bottom. The change in colourdominates morphology in the midslope but changes to be dominated by texturedifferentiation in the valley bottom. Signs of redox activity prevail across thetoposequence 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 ofredox activity. Fe-Mn redistribution, present as mottles and concretions, and colourchanges are indications of intermediate redox activity. The texture differentiation presentindicates an environment supporting ferrolysis to the full.

[发布机构] University of the Free State