ABSTRACT
Organic matter in Vertisols ƒ It is surprising to find dark soils containing a sizeable quantity of organic carbon (sometimes more than 1% in the surface horizon, and 50 to 100 t ha-1 of organic matter in the entire profile) in hot dry climates favouring mineralization (Knowles and Singh 1998). This persistence in Vertisols of organic carbon is because the organic matter is strongly bound to the clay minerals. Up to 72 hours of repeated treatment with hydrogen peroxide at 50° C is necessary to remove about 90 per cent of the carbon originally present (Hyeong and Capuano 2000). The clay then becomes light-coloured. At the same time, the behaviour of smectites is peculiar in these soils (Bornand et al. 1984): • The clay minerals, when saturated with Mg, should normally
have a spacing of 15 Å; here the spacings are about 15.8 Å. • After heating at 400° C, the layers do not collapse to 10 Å as
is usual for the smectites of other soils. The interlayer spacing remains larger.
