Andosols are formed on young volcanic ashes with a thick organic Ah-horizon, sometimes overlying a brown, cambic B-horizon. Typical for young soils is the low density of the topsoil, high pore volumina and a high water capacity. Clay is mainly derived by the weathering of volcanic glasses and consists of allophane and imogolite. Under humid conditions weathering is rapid, leading to high nutrient delivery. In semiarid mountains the subsoil is frequently cemented by SiO₂, forming a hard pan (e.g. Mexico: Tepetate). Andosols have a highly variable charge and provide therefore a high CEC at high pH-values. At low pH-values, P-fixation is common.