Rates of Weathering and Soil Formation on Granite in Rhodesia

Abstract

AbstractRates of weathering and soil formation on granite were studied in two areas of Rhodesia using small watersheds. The Juliasdale watershed of 0.91 km2 is hilly and has a mean annual rainfall of 1,220 mm. The Rusape watershed of 7.33 km2 is more nearly level and has a mean annual rainfall of 920 mm.Streamflow was measured in the two catchments over a period of at least 7 years. Streamwater, rainwater, soil, and rock samples were analyzed for Ca, Mg. K, Na, Si, and Al.The rate of granite weathering was calculated using Barth's equation which links the rate of weathering to the amount of an element removed in solution per unit time, its concentration in the rock and its concentration in the weathered product. Sodium and Ca were the elements used in the Barth equation. The weathering rates in the Juliasdale and Rusape watersheds were estimated at 400 and 150 kg/ha per year, respectively, or 15.4 and 5.8 mm/1000 years. The concepts of C. E. Marshall and J. F. Haseman as well as S. W. Buol include changes in bulk density during the weathering process and give soil formation rates more acceptable than those of Barth. Thus, the rate of soil formation was found to be 11.0 mm/1000 years in the Juliasdale watershed and 4.1 mm/1000 years in the Rusape watershed.