Characteristics of non-allophanic Andisols derived from low-activity clay regoliths in the Nilgiri Hills (Southern India)

Low-activity clay soils on old planation surfaces of the tropics are genera lly considered as stable end points of soil formation. It is therefore surp rising to find Andisols on them. We characterized the properties of six pro files representative of these soils in the western part of the Nilgiri Hill s (2000-2500 m above mean sea level), Southern India, where the present cli matic conditions are cool (mean annual temperature 15 degreesC) and humid ( mean annual rainfall 2500 mm). Thick (50-80 cm) dark-reddish brown topsoil overlies strongly desilicated yellowish-red materials. This horizon has and ic properties to a sufficient depth and the carbon content requirement of t he melanic epipedon to place these soils in the Andisol order. Our data as well as the history of the Nilgiri Hills suggest that the formation of thes e non-allophanic Andisols resulted from the succession of two main steps. F irst, a 'lateritic' weathering cycle led to the relative accumulation of se condary Al and Fe oxides. Later, the accumulation of organic matter favoure d by a more recent climatic change induced complexation by organic acids of Al and Fe oxides, and the production of enough metal-humus complexes to gi ve rise to andic properties. Such soils, in which secondary Al and Fe oxide s, generally considered as indicators of an advanced weathering stage, are involved in a new cycle of soil formation, are original Andisols.