Soils and Paleosols in South India as Heritages of the Past.
Arnt Bronger, University of Kiel, Olshausenstr 40, Kiel, Germany
To call soils as heritages of the past, they need an age estimation. A mid-Pleistocene welded F6- paleosol at Stari Slankamen, Serbia, developed in a time span of 140 ka, although pedogenesis was interrupted several times by loess formation, show much greater pedochemical weathering and clay mineral formation than in Holocene loess soils: more than 40% of the feldspars and almost 80% of the micas are decomposed; mainly smectites in the dominating fine clay fraction followed by illites were formed, but no kaolinites. In the Atlantic coastal region of Morocco with a thermic soil temperature regime and a xeric soil moisture regime in the first about 100 ka only weathering of calcarenites to Rendzinas (Typic Calcixerolls) has taken place with small formation of 2:1 clay minerals. In a time span of several 100 ka, however, the direction of weathering goes towards strong kaolinite formation. The efficiency of weathering in the seasonal semiarid to subhumid tropics has often been greatly overestimated. Six selected „Red Soils“ (Typic Dystrudepts to a Typic Hapludalf) in two intramontane basins of hyperthermic SW- Nepal near the border on India, with 1500- 1750 mm annual rainfall (5 humid month), and a „Black Soil“(Vertic Haplustoll) near Baroda, Gujarat, India with 930mm annual rainfall (3-4 humid month) were studied. Two of the „Red Soils“ have TL-ages between 10 and 30ka, the „Black Soil“ has one of about 10ka. Surprisingly, little pedogenic clay mineral formation could be identified. The illites and kaolinites are mostly of detrital (inherited) origin. In the dated Vertic Haplustoll only a small increase mainly of smectites but no kaolinites could be found, although the content of weatherable minerals is high. - In South India nine soils in a climatic sequence from ten to one humid months per year and derived from saprolite of weathered granitic gneiss were examined for recent and relict features. Above a threshold of about 2000mm (6 humid month) in an Udic Rhodustalf deep weathering is a recent process leading to the formation of kaolinites; above 2500mm (10 humid month) in a Typic Rhodudult it leads also to the formation of gibbsite. In Aridic Rhodustalfs (3 to 1 humid months) besides the formation of 2:1 clay minerals still strong formation of kaolinites has taken place. But this process has now almost ceased; instead secondary carbonate is accumulating in the saprolite (Cr) and lower part of the Bt horizons. Earlier we concluded that Alfisols with kaolinites as the dominant pedogenic clay minerals in now semiarid India are relict soils or non- buried paleosols formed in an earlier period of much moister climate. But according to our recent results from Morocco and Nepal besides a climatic change the soil forming factor time is very important: strong pedogenic formation of kaolinites also in the seasonal tropics needs a longer time probably some 100 ka. – Historically, the whole of India was under forest cover except its driest parts. According to official Indian sources 20- 23% is forest, however, the area of true forest is now scarcely 10% of the total area. The extensive deforestation together with the widely used dryland farming system, in which the land is fallowed for 5-7 months, favours soil erosion, especially under monsoon rainfall patterns. As a result the depth of the soil cover in large parts of India south of the Ganga plain is only 50- 100 cm. In an even larger area between Madhya Pradesh, Orissa and Andhra Pradesh, the depth of the soil cover is only 20- 50 cm. This is a serious situation in view of the high age of the Rhodustalfs.