Four soils derived from volcanic ash were studied in a transect on the east slope of the Central Cordillera of Colombia. Average annual temperatures range from 8C at 3,670 m altitude to 24C at 980 m. Only the soils located in the lower part of the transect have B horizons. From highest to lowest elevation the soils are classified a Cryandept, a Humitropept, and two Dystrandepts. The sand and silt fractions of the soils consist of a similar suite of minerals, several of which are high in sodium. Volcanic glass and plagioclase are most abundant; cristobalite, quartz, vermiculite, pyroxenes, hornblende, amphibole, and magnetite are present in smaller amounts. Amorphous material predominates in the clay fraction of the soils, but crystalline minerals such as smectite, vermiculite, chloritic intergrade mineral, imogolite, halloysite, kaolinite, and some primary minerals are also present.
The genesis of the soils is controlled chiefly by climate and by the nature of the parent material. As the environmental temperature increases, the degree of B horizon development increases, clay content increases, organic matter content decreases, degree of weathering of primary minerals increases, and the content of amorphous material, kaolinite, and halloysite increases.