Clay contents of soils, containing varying amounts of allophane, have been calculated from measurements of surface areas of separated fractions using adsorption of nitrogen, acetic acid, and water-vapour as methods of measurement. The clay contents obtained in this way were greater than the amount of clay which could be extracted from the soil at either pH 3 or pH 9, and generally much greater than the amounts of clay given by a standard method of mechanical analysis using sodium hexametaphosphate as dispersing agent. Acetic acid adsorption, although giving greater surface areas than nitrogen adsorption, gave rather similar figures for clay content, but water-vapour adsorption, which gave still greater surface area values, led to clay contents in rather poorer agreement with those obtained from the nitrogen surface areas. Because the presence of organic matter affects both nitrogen and acetic acid surface area values, these are best determined on a separated clay fraction, and on the residual silt plus sand fraction from the same soil sample, following a uniform procedure for pretreatment with hydrogen peroxide.
展开▼
