Vegetation-Environment Relations on Sodic Soils of Zimbabwe Rhodesia

SUMMARY (1) Patches of soils with an abnormally high concentration of exchangeable sodium ('sodic soils') are common in some areas of Zimbabwe Rhodesia, and are often severely eroded. Plant growth on these soils is stunted and plant cover is small. Distinct differences in species-composition (particularly of woody species) are evident from one site to another, although any one site has relatively little variation in vegetation. (2) Samples of soil and of vegetation were taken from forty-one relatively undisturbed stands of vegetation on sodic soils, distributed over about 60 000 km2 mainly in the gneissic-granite regions of the country. The variation in soil and in vegetation, as well as their interrelations, were examined with principal components analysis and reciprocal averaging. (3) Plant root growth on these soils is limited to a relatively shallow surface layer by a highly sodic and largely impermeable B horizon. A number of plant communities were identified, and were found to be associated with the depth of the A horizon, soil-texture differences, and the ratio of the electrical conductivity of the soil solution to the concentration of exchangeable sodium. (4) A developmental sequence in these soils is proposed which interprets the soil conditions associated with different plant communities in relation to known soilforming processes acting on sodic soils.