World’s Soil Resources

Figure 9.13 Topsoil pH derived from undisturbed (natural) soils. Source: Beukes, Stronkhorst and Jezile, 2008a.
Soil organic matter
Although data on soil organic carbon in South Africa are limited, fragmented and uncoordinated, general
trends of SOC content can be derived from a range of studies conducted (Barnard et al., 2000; McKean, 1993,
Du Toit and Du Preez, 1993, Le Roux et al., 2005; Mills and Fey, 2004; Prinsloo, Willshire and Du Proez, 1990;
Van Antwerpen and Meyer, 1996; Birru, 2002; Du Preez, Mnkeni and Van Huyssteen, 2010, 2011a, b). A review of
SOC research estimated that approximately 58 percent of South African soils contain < 0.5 percent organic C,
38 percent contain 0.5–2 percent organic C and 4 percent have > 2 percent organic C. These organic C contents
vary greatly as a function of soil types, climate, vegetation, topography and soil texture, and are greatly
influenced by management practices which result in organic C losses such as overgrazing, high levels of soil
disturbance during cultivation, and the use of fire in rangeland management. Soil organic matter losses were
generally associated with dryland cropping, but were less prevalent in irrigated agriculture. Increasing SOM
is a slow process, but it has been achieved by implementing zero/minimum tillage, by mulching and through
reversion of cropland to perennial pastures. Increases have mainly occurred in the upper 300 mm of soil, and
in most instances, have been restricted to the upper 50 mm of soil.
Loss of SOM has been found to result in lower nitrogen and sulphur reserves, but not necessarily in lower
phosphorus reserves. Loss of SOM also coincided with changes in the composition of amino sugars, amino
acids and lignin. It further resulted in a decline of water stable aggregates which are essential in the prevention
of soil erosion.
Rantoa, Du Preez and Van Huyssteen, (2009) used data from the approximately 2 200 modal profiles from
the land type survey to estimate organic carbon stocks in South African soils with reference to master horizons,
diagnostic horizons, soil forms and land cover classes. In summary, the average organic carbon content in the
Status of the World’s Soil Resources | Main Report Regional Assessment of Soil Changes
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in Africa South of the Sahara