Geology and groundwater regions to quantify primary salinity, sodicity and alkalinity in South African soils

Abstract

Although anthropogenic salinisation and sodification has been researched extensively, little is known about the primary salinity in South Africa. This paper therefore aimed to determine the primary salinity, sodicity and alkalinity conditions for South African soils, based on geological units and groundwater regions, and analysed soil pH_water, electrical conductivity (EC) and exchangeable sodium percentage (ESP). Soils in the Whitehill Formation (Ecca Group) geological unit were the most saline (median EC = 2720 mS m^-1) and the most sodic (median ESP = 79.8). Based on the groundwater regions, soils in the Tanqua Karoo were the most saline (median EC > 400 mS m^-1), whereas those in the Richtersveld were the most sodic (median ESP > 20). Soils in the Richtersveld Subprovince and in the Eendoorn granite geological units, as well as soils in the Richtersveld, Ghaap Plateau and Western Kalahari groundwater regions were the most alkaline (median pH > 8.5). The highest EC values were found in soils occurring in the geological units of the arid western part of the Northern and Western Cape. For the groundwater regions, soils in closed basins, pan environments and in intermontane areas had the highest EC values. The arid areas of the Northern Cape and Western Cape were also the most sodic. The most alkaline geological units occurred in the Namaqua Sector of the Namaqua-Natal Province, and in the Richtersveld, Ghaap Plateau and Western Kalahari groundwater regions.