I copied this from an article on amending clay soils.
Clay particles, bound end to end and side to side in extensive planes, are stacked in a sandwichlike matrix and held together by electrochemical forces. This platelike stacking of horizontally arranged clay particles results in a large surface area.
Because individual clay particles are negatively charged, they have the capacity to attract and hold onto, or adsorb, positively charged elements (called cations) such as ammonium, potassium, calcium, magnesium, and other trace elements. Clay soils are relatively fertile because of this capacity to adsorb these important plant nutrients. Conversely, the single, uncharged sand particles in sandy soils lack the capacity to adsorb cations and thus they contribute very little to soil fertility.