Organic carbon content in the diagnostic horizons and materials of South African soil forms

Soil classification aims to group similar soils to facilitate communication and management. Management of soil organic carbon is gaining international recognition in an attempt to manage atmospheric carbon in addressing global warming. This paper explored the soil morphological and analytical data from a land type survey of South Africa, as augmented by data from other soil surveys in the country, to establish if soil classification, especially the soil form, can indicate the expected soil organic carbon content and thus enable generalisations to be drawn. Average organic carbon in the diagnostic topsoil horizons followed a predictable pattern: organic O (9.51% OC) > humic A (3.69% OC) > melanic A (2.59% OC) > vertic A (1.49% OC) > orthic A (1.38% OC). However, the organic carbon content also differed within the same diagnostic horizon between different soil forms. These differences could largely be related to the nature of the overlying topsoil, where topsoils with higher organic carbon contents typically resulted in subsoils with higher organic carbon. The results presented here can thus be used to semi-quantitatively determine the organic carbon contents of selected soil forms.