The primary salinity, sodicity, and alkalinity status of South African soils
Nell, Johannes Petrus
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Unequalled by any other region in the world, South Africa hosts some of the oldest known salt deposits in its geological material. The weathering of rocks is the primary source of soluble salts entering natural waters, sediments, and soils. Geological material is in most circumstances an important soil formation factor, but for salt-affected soils its effect is probably overshadowed in many areas by rainfall and position in the landscape. Rainfall in particular and fog seem to be a controlling factor often overriding lithological control in the development of salt-affected soils. Certain minerals and rocks are also more vulnerable to chemical reaction than others. Rhyolite with a low weathering potential is for example a non-extreme or non-active parent material and dolerite with a high weathering potential an active parent material. South African soils do not have a severe primary salinity and sodicity problems, the reason is probably that salts less soluble than gypsum such as calcium carbonate, which is commonly found in South African soils, are considered insoluble and hence are not considered to cause salinity and sodicity. Extremely high salinity, sodicity and alkalinity values occur along pans and riverbanks in arid areas in South Africa. The geological units resulting in most salt-affected soils are in declining order: WhitehilI Formation ≈ Knersvlakte Subgroup >Gladkop Suite >Sundays River Formation >Enon Formation >Garies Subgroup >Kirkwood Formation> Port Nolloth Group >Nyoka Formation >Prince Albert Formation. The groundwater units resulting in most salt-affected soils are in declining order: Tanqua Karoo >Richtersveld >Knersvlakte >Ruensveld >Hantam> Namaqualand >Algoa Basin >Bushmanland Pan Belt >Bredasdorp Coastal Belt >Intermontane Tulbagh-Ashton Valley. There is a strong relationship between rainfall, salt occurrence and salt movement. As rainfall increases the salinity, sodicity and alkalinity decreases because of the depletion of basic cations and anions. Salts predominantly move with water. The natural force is usually rainfall, mist, and fog. Regular and high rainfall in the eastern part of South Africa causes a continuous leaching and the transport of leached constituents out of the soil system into the ground water system. On the other hand, erratic and low rainfall combined with high evaporation in the west of South Africa result in the accumulation of salts in the soil profile. It should not be assumed that all salt-affected soils will always show definite and predictable associations with present day climate. The relationship between climate and saltaffected soils are made more difficult to determine, because practically all areas have suffered climates in the past different from those prevailing at present. The three most important topographic conditions that have an influence on salt-affected soils in South Africa are probably pans (wetlands in arid areas), marine terraces, and Karst landforms. Topography can greatly affect the movement of water and salts through soil. This is, to a certain extent a result of gravity, which directly influences water and salt movement and partly as a result of topography's influence on soil development. Nearly 60% of South African soils are non-saline, 23% slightly saline, 5.1% saline, 1.4% moderately saline, 0.4% strongly saline, 3.8% saline-sodic (nonalkaline), 6.3% saline-sodic (alkaline), and only 0.4% can be considered as sodic. Transient salinity, or salinity not influenced by groundwater processes and rising water table, is the predominant salinity type in South Africa, and not dryland salinity. Saline and/or sodic soils in South Africa mostly occur only in relatively small areas due to localised factors, making the mapping on a national scale problematic. Quartile values and not average values are best to use for salt-affected soils to present the data, because the majority of the data is strongly positively skewed, with large differences between median and average values. The use of the outlier definition in its statistical meaning for salt-affected soils is problematic. It is, therefore, better to use outlier in the sense that it means to be an observation that deviates markedly, but for obvious and/or explicable reasons, from the other members of the population and as such is representative of typical variability in a natural situation.