Soil classification groups to quantify primary salinity, sodicity and alkalinity in South African soils

Abstract

This study aimed to quantify the primary salinity, sodicity and alkalinity of South African soils based on soil classification groups and rainfall classes, by using available analysed data. The 73 soil forms used by the Soil Classification Working Group were organised into 11 groups based on the occurrence of specific horizons. Soil analyses of 12 552 profiles for pH_water, 11 712 profiles for electrical conductivity, and 12 174 profiles for exchangeable sodium and cation exchange capacity were used. Median and not average values best represented salt-affected soil data on a national scale, because most data was strongly positively skewed. Primary soil alkalinity and sodicity were more common than primary salinity in terms of the different South African soil groups. None of the soil groups were saline, only the prismacutanic group was sodic, while the calcic, alluvial/aeolian, neocutanic and prismacutanic groups were alkaline when the median value or highest value in a profile was used as indicator. Sufficient data was not available, even for this relatively large dataset of 648 prismacutanic B-horizons, to proclaim a precise exchangeable sodium percentage value as a criterion for prismacutanic or solonetz soils in South Africa. Salinity, sodicity and alkalinity decreased as rainfall increased due to the depletion of cations and anions. The incidence of salt-affected soils in the soil classification groups decreased in the order: calcic ≥ alluvial/aeolian > prismacutanic > vertic > pedocutanic/red structured > neocutanic > hydromorphic ≥ lithosols > plinthic > apedal > podzolic. This information serves as an indicator of the primary sodicity and alkalinity of South African soils and can be utilised to deduce the expected sodicity and alkalinity of soils from soil survey data, to monitor anthropogenic sodicity and alkalinity impacts, and as an input to map the primary sodicity and alkalinity of South African soils.