ABSTRACT
The discovery of negative charge in soils occurred about 1850 in England by a Yorkshire farmer, Harry Stephen Thompson. Constant charge results from structural imperfections, such as isomorphous substitution in mineral structures. Variable charge originates from preferential adsorption of certain ions on particle surfaces. The charge is both negative and positive and is pH dependent. The iron minerals, mainly oxides, gradually coat the surfaces of other mineral particles and alter their charge properties. Properties are dominated by the constant charge and they are called constant charge minerals. The charge of soil minerals comes mainly from the clay fraction. Properties are dominated by the constant charge and they are called constant charge minerals. They are commonly the dominant clay-minerals in minimally and/or moderately weathered soils. Chlorites are an infrequent component of soil clays and are generally inherited from parent materials derived from metamorphic and Igneous rocks.
