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

Weathering and soil formation rates an regarded as the main criteria of a t olerable soil loss. The efficiency of weathering in the seasonal semiarid t ropics has often been greatly over-estimated especially in the geomorpholog ic literature in which weathering is assumed to be as fast or even faster t han 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" hav e TL ages between 10 and 30 ka, the "Black Soil" has one of about 10 ka. Th e yellowish silty parent material of the "Red Soils'' is a preweathered soi l sediment: it contains only small amounts of easily weatherable primary mi nerals: around 5% feldspars and 10-15% phyllosilicates, dominantly muscovit es. Surprisingly, little pedogenic clay mineral formation could be identifi ed. The illites and kaolinites are predominantly of detrital origin. The fe w non-regular mixed-layer minerals in the fine clay fraction (< 0.2 mu m) c an be interpreted as resulting from the initial stage of silicate weatherin g. The hematites, however, are mostly of pedogenic origin. Therefore the ru befication is a recent autochthonous process, and by itself is not a reliab le indicator of strong pedogenic weathering. In the dated "Black Soil", onl y a small increase in 2:1 minerals, mainly smectites, could be found, altho ugh the content of weatherable minerals is high. These results support earl ier conclusions from South India, where above a threshold of 2000 mm annual rainfall (6 humid months) deep weathering is a recent process leading to t he 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 re garded as Vetusols. In the Typic and Aridic Rhodustalfs, earlier soil formi ng processes such as deep weathering and strong kaolinite formation have no w almost ceased because of the semi-arid conditions; instead secondary, car bonate is accumulating in the saprolite (Cr) and lower part of the Bt horiz ons. We conclude that. as well as Ultisols, most Alfisols or Lixisols in no w semiarid India are relict soils or non-buried paleosols formed in an earl ier period of much moister climate. Because the rate of compensatory regene ration of soil is now in effect, almost zero, the soil erosion there is a p ermanent Loss of the country's most important natural resource, but it has not been recognized as such because the soils were not identified as paleos ols. (C) 2000 Elsevier Science B.V. All rights reserved.