Factors that govern the formation of melanic soils in South Africa

Abstract

Soils with a melanic A horizon cover an area of approximately 2.3 million ha or 2% of the territory of South Africa. Little detailed information is available on the factors that govern the formation of melanic topsoils rather than other diagnostic A horizons. The objective of the present study was to interpret information obtained from land-type survey memoirs, regarding climate, relief, and parent materials, associated with melanic soils to elucidate the pattern of their distribution in South Africa. The following trends were recorded. Most melanic soils formed from either mafic igneous (37.3%) or sedimentary rocks (44.0%). Dolerite (22.7%) and sandstone (21.5%) constituted the most common parent lithology. The large contribution of sedimentary rocks is a reflection of the large aerial extent of these specific geological strata rather than preferential formation from these rock types. On the other hand, the large contribution of mafic igneous rocks, especially dolerite, indicates preferential formation of melanic horizons from these rocks, which do not occur widespread. Melanic soils were not restricted to a specific topographic position and were common in crest (22%), midslope (32%), footslope (16%), and valley bottom positions (30%). They were associated with gentle slopes or flat-lying areas. Climate was identified as a dominant soil-forming factor and as many as 75% of the melanic soils have formed under climatic conditions, which are characterised by contrasting seasons. A mean annual precipitation of 550–800 mm and an aridity index of 0.2–0.5 (semi-arid) were particularly conducive to their formation. Soils developed from parent materials, characteristic for melanic horizons, and at the same terrain morphology units did not develop melanic characteristics in areas with an annual precipitation of <500 mm, probably due to the lack of organic matter (OM) addition/preservation.