Edafología. Volumen 7-1. Abril 2000. pág 31-50.

Abstract.

We have studied the ultic rhodoxeralf developed on granite in western Cáceres (Spain) under a Mediterranean climate with a mean annual rainfall of 1276mm and a mean annual temperature of 13.9°C wich correponds to a xeric moisture regime and a mesic temperature regime. They are characte-rised by an argic horizon with sandy clay loam texture and have an acid ph, a low cation-exchange capa-city and a low base saturation. Genetically, two cycles of formation can be see. The weathering creates kaolinite caused by neoformation, from the K- feldspars and the plagioclases, and also illite by way of a transformation of the micas, part of which is trioctahedral-illite. The ilimerisation is evident from the increase in total and fine clay, and the presence of clay and iron coatings. Kaolinite is the type of clay which tends to be illuviated first, and constitutes the coatings. The rubefaction is produced by the iron released in the weathering of the biotite and associated with the clay in the coatings. The first cycle could have given rise to an E horizon. The deforestation and cultivation of the land caused the top layers of the soil be eroded. In the present-day Ap1 horixon, coatings of fine and very fine sand can be seen, which indicate that its origin could have been in a previous E horizon. The Ap2 and Bw horizons show hypo-coatings which show a process of "brownification" in both, and suggest that the former may heve its ori-gin in a previous Bt horizon. Alesser current lixiviation is favourable to the formation of Al-Chlorite from micas in the upper horizons. In the deeper horizons, where white nodules appear, these maintain the form of pre-existing phenocrystals, and are made up of sericite and kaolinite.

Keys words: Soil genesis, Rhodoxeralf, Soil mineralogy