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.

An Assessment of Gully Erosion in Gombe Town, Gombe State, Nigeria

“…The overall pattern of variation showed a downward increase in the organic contents. Mills and Fey (2003) also found a linear decrease in erodibility with increasing organic content over a range of 0 to 10%. However, there was significant differences at the top and bottom layers of the gully walls (P>0.05).…”

Systematic land-cover change in KwaZulu-Natal, South Africa: Implications for biodiversity

“…Indeed, certain catchments have been closed to new applications and a moratorium has been placed on others, pending further investigations on associated run-off reductions (Thambu D 2014, written communication, August 7). Plantations create acidic soils and an increase in available nitrate, 33 a situation for which many indigenous plant species are not adapted. Shading effects may promote shade-loving or forest species, but these species will be lost during rotational harvesting practices.…”

“…Differing land-use practices may alter soil chemical and physical properties. 33 Ploughing, heavy grazing and burning deplete soil organic matter which affects soil water infiltration, retention and nutrient supply. 38 Dryland cropping, which is increasing in KZN, results in significant losses of soil organic matter.…”

Review and meta-analysis of organic matter in cultivated soils in southern Africa

“…Ecologically, soils provide a range of ecosystem services, such as nutrient cycling, carbon (C) sequestration, water filtration and storage, and are therefore considered an integral part of many natural systems (Doran and Zeiss 2000;Dominati et al 2010). Soil organic matter (SOM), or its associated indicator elements, C and nitrogen (N), are often equated with soil quality (Mills and Fey 2003). Loss of SOM is a global concern as it not only leads to a deterioration of soil quality, but also releases greenhouse gases (GHG) to the atmosphere, resulting in accelerated climate change (Smith et al 2007).…”