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RESEARCH PRODUCT

Over-estimation of efficiency of weathering in tropical “Red Soils”: its importance for geoecological problems

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Abstract Weathering and soil formation rates are regarded as the main criteria of a tolerable soil loss. The efficiency of weathering in the seasonal semiarid tropics has often been greatly over-estimated especially in the geomorphologic literature in which weathering is assumed to be as fast or even faster than surface erosion. Six selected “Red Soils” in two intramontane basins of hyperthermic SW Nepal near the border with India, with 1500–1750 mm annual rainfall (5 humid months), and a “Black Soil” near Baroda, Gujarat, India (3–4 humid months) were studied mineralogically. Two of the “Red Soils” have TL ages between 10 and 30 ka, the “Black Soil” has one of about 10 ka. The yellowish silty parent material of the “Red Soils” is a preweathered soil sediment; it contains only small amounts of easily weatherable primary minerals: around 5% feldspars and 10–15% phyllosilicates, dominantly muscovites. Surprisingly, little pedogenic clay mineral formation could be identified. The illites and kaolinites are predominantly of detrital origin. The few non-regular mixed-layer minerals in the fine clay fraction (