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

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 (<0.2 μm) can be interpreted as resulting from the initial stage of silicate weathering. The hematites, however, are mostly of pedogenic origin. Therefore the rubefication is a recent autochthonous process, and by itself is not a reliable indicator of strong pedogenic weathering. In the dated “Black Soil”, only a small increase in 2:1 minerals, mainly smectites, could be found, although the content of weatherable minerals is high. These results support earlier conclusions from South India, where above a threshold of 2000 mm annual rainfall (6 humid months) deep weathering is a recent process leading to the formation of kaolinites over a long time interval; with 10 or more humid months per year, it leads to the formation of gibbsite. These soils are regarded as Vetusols. In the Typic and Aridic Rhodustalfs, earlier soil forming processes such as deep weathering and strong kaolinite formation have now almost ceased because of the semi-arid conditions; instead secondary carbonate is accumulating in the saprolite (Cr) and lower part of the Bt horizons. We conclude that, as well as Ultisols, most Alfisols or Lixisols in now semiarid India are relict soils or non-buried paleosols formed in an earlier period of much moister climate. Because the rate of compensatory regeneration of soil is now in effect, almost zero, the soil erosion there is a permanent loss of the country's most important natural resource, but it has not been recognized as such because the soils were not identified as paleosols.