Relationships between climatic indices and soil properties that reflect leaching and weathering in the Natal Midlands, and their use in the assessment of afforestation potential.

Abstract

In the summer rainfall regions of Southern Africa, knowledge of the mean annual effective rainfall (MAER) pertaining to a site is critical for the assessment of that site's afforestation potential. The ability to estimate MAER at a site from soil properties was sought for the forestry regions of the Natal midlands. The S-value (sum of basic cations) expressed per unit mass of clay (a diagnostic property in the South African soil classification system) is currently used for this purpose, but until now this usage has not been validated. Jenny's conceptual model of state factors of soil formation was employed to demonstrate, within a climosequence, the empirical relationships occurring between soil properties and climatic indices. Weather stations from around the Natal midlands were selected on the basis of the reliability of their rainfall and temperature records and 33 of these sites were subsequently incorporated into a conceptual climosequence. At each site, representative soil profiles were excavated, described and intensively sampled. Soil samples were analysed for a variety of properties indicative of the degree of leaching and/or weathering. At each site, indices of climate (mean annual precipitation, drainage from various depths and sub-horizons in the soil profile, and MAER) were calculated using an agrohydrological water-budgeting model (ACRU). Selected soil properties were systematically related to these climatic indices. The results showed that within the climosequence concerned, of all the soil properties considered, the effective cation exchange (ECEC, sum of basic and acidic cations) of the B1 horizon, expressed per unit mass of soil, was best related to the climatic indices.