Indications of ferrolysis and structure degradation in an Estcourt soil and possible relationships with plinthite formation

Geographic associations of duplex soils, like the Estcourt form, and plinthic soils, like the Avalon and Westleigh forms, such as are found in the Eastern Free State, are recognised worldwide. These soils share redox morphology. In duplex soils, the redox morphology is dominated by a contrast in clay content between the A or E horizon and the B horizon, indicating that degradation of silicate clay minerals dominates. In plinthic soils, mottling due to redistributed Fe-Mn dominates. Both features can be the result of ferrolysis. In order to research this relationship, an Estcourt profile with degrading peds was sampled and analysed. Undisturbed peds of the B- horizon were subdivided for analysis, together with disturbed samples of the A, E and C horizons. The spatial variation of the colours, Fe, Mn, clay content and mineralogy in the profile indicates that both ferrolysis and redistribution of Fe-Mn are active during short periods of reduction in the E and interpedal pores of the B horizon. Degradation of the silicate clay minerals results in ped degradation visible as rounded ped tops and grey material interfingering the B horizon. In this landscape, in soils of the Avalon and Westleigh forms, degraded peds are also present in the lower soft plinthic B horizons and underlying material with the degree of ped degradation visibly associated with the degree of redistribution of Fe-Mn in mottles and nodules. In this landscape, a transitional horizon with properties of both plinthic and degrading structured B horizons covers large areas. This indicates that plinthic and/or E horizons can develop from structured horizons under the current climate.