Mineralogy and origin of some volcanic ash soils of continental Chiloé, Chile.

Eduardo Besoain M.1, Mario Peralta P. y Silvana Massaro M.

Four andisols from volcanoes Hornopirén and Hualaihué or Apagado, located in continental Chiloé, X Region (41º 53’S) were studied. The soils are located on the slopes of the volcanoes, on a very steep elevation covered with dense rain forest. The local climate averages 4,000 -- 5,000 mm annual rainfall and a mean annual temperature of 10ºC. The soils were described morphologically and their chemical, physical and mineralogical properties were determined. The horizons are deep and their textural variation suggests various eruptions. All the soils were characterized by a thick dark A horizon with high organic matter content (12-25% organic carbon); high to moderate water retention at 1,500 kPa; high phosphate retention values (>90%), with the exception of the subsurface horizon of Pululil soil. The content of volcanic glass is very high (59-88%) and tends to increase with the depth of the profile; most of the glass contains bubbles. Soil colloids are allophanic, but at the surface Al, Fe-humic complexes predominate. Opaline silica is also found in A-horizons of all pedons. The soils satisfy the andic properties and belong to the Order of Andisols and Sub-Order of Udands.

Keywords: Andisols, volcanic ash, alophane.
1 Universidad de Chile, Facultad de Ciencias Agrarias y Forestales, Casilla 1004, Santiago, Chile.