Vegetation-Environment Relations on Sodic Soils of Zimbabwe Rhodesia

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Abstract
Patches of soils with an abnormally high concentration of exchangeable Na (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. Samples of soil and of vegetation were taken from 41 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 and their interrelations were examined with principal components analysis and reciprocal averaging. 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 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 Na. A developmental sequence in these soils is proposed which interprets the soil conditions associated with different plant communities in relation to known soil-forming processes acting on sodic soils.

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