The soil texture is determined according to the proportion of clay to silt to sand, and it is possible to break down a sample into the precise amounts of each component. Clay is smaller than < 2 μm, silt particles are 2-50 μm and sand particles are larger than 50 μm. The smaller the size of a particle the greater the relative surface area for adsorption. Clays - and there are many types of clay with different properties - exhibit a large external surface area and some exhibit a large internal surface area between layers. With most colloids negative charges predominate and thereby attract positively charged Cations. The ability to hold onto and exchange cations on the clay or humus surface is called the CEC.