Gênese do contraste textural e da degradaçao do horizonte B de um podzólico vermelho-amarelo da Planicie Costeira do Rio Grande do Sul

The origin of soil textural contrast and B horizon degradation of a red yellow podzolic soil was evaluated in 1990 and 1991 in a catena near the city of Porto Alegre, located in the Coastal Plain of Rio Grande do Sul State, through the study of morphological, physical, chemical and mineralogical properties. A lessivage process was confirmed by the increasing ratio of fine clay/total clay, and substantial waxiness in the illuvial B horizon; however the clay and Fe-DCB losses in the elluvial horizons were greater than the gains in the illuvial horizons. The depositional hypothesis was eliminated, due to the occurrence of similar soils on different landscape positions, and the uniform fine and medium sand / total sand ratio in the soil profiles of the catena. A high textural, porosity and soil density anisotropy was verified between the E and EB horizons, which is responsible for the lateral water transport and seasonal water excess in the EB and B/E horizons. The monitoring of soil humidity and Fe 2+ in solution indicated alternation of reduction and oxidation processes in these horizons, whereas chemical and mineralogical analyses indicated a selective dissolution of hematite with the removal of iron from the system. Thermodynamic analyses showed that the vermiculites with interlayered hidroxi-Al are more stable than the kaolinite in the soil and that the quantity and degree of interlaying are greater in the EB horizon. These results indicate a contemporary process of kaolinite dissolution, by ferrolysis, in the transitional horizons, responsible for the degradation of the B horizon.