Declining soil quality in South Africa: effects of land use on soil organic matter and surface crusting

Abstract

Soil conservation in South Africa has historically focussed on preventing soil erosion. Effective maintenance of the soil resource requires, in addition to erosion control, an understanding of how land use practices affect more subtle indicators of soil quality. This review outlines how land use in South Africa can rapidly result in a marked reduction in soil organic matter (SOM) content and a greater tendency of soils to crust. Removal of a cover of vegetation whether by ploughing, grazing or burning tends to reduce SOM due to reduced organic matter inputs and enhanced soil microbe activity. Loss of SOM, particularly from the top few centimetres of soil (named here the pedoderm) has a disproportionately large effect on soil infiltrability and nutrient supply. The mineralogy of the clay fraction also has great bearing on the response of soil to land use effects. The unexpected role of quartz in soil dispersion and crusting in South Africa has only recently been unveiled. Apart from SOM effects, land use can lead to subtle changes in soil chemistry. Plantation forestry has resulted in an increase in soil nitrate in many areas, possibly due to greater mineralisation under forests than grasslands. Annual burning in the Kruger National Park bushveld has been shown to increase clay dispersibility and crusting of the pedoderm, which was ascribed to a reduction in electrical conductivity and SOM as well as an increase in the exchangeable sodium percentage. Soil quality is a multifaceted concept. One aspect stands out, however, as critical and that is the conservation and replenishment of nitrogen which is all important for retaining humus and maintaining soil quality.