CHEMICAL PROPERTIES AND SOIL COLOR OF SOME OXISOLS FROM BRAZIL AND SPAIN IN RELATION TO SULFATE SORPTION

Abstract

Knowledge of the capacity of soils to retain SO 4 2- is fundamental for assessing SO 4 2- leaching and availability. Research was carried out to characterize 18 subsurface horizons from 15 Oxisols, and to investigate their ability to retain SO 4 2-. Yellow Latosols (Y soils) and red Latosols (R soils) from the state of Sao Paulo (Brazil) were studied along with Oxisols from Galicia (Spain) (G soils). Organic C, pH, ECEC, clay percentage, and amounts of Fe and Al extractable with ammonium oxalate, dithionite-citrate, and citrate-ascorbate were measured. The hematite/ (hematite + goethite) ratio of the soil samples was estimated by second-derivative diffuse reflectance spectroscopy. Sulfate sorption was studied by adding 0.4 or 1.6 mM solutions of K 2 SO 4 , initially acidified to pH 3.0, to the soils (ratio of soil:solution, 1:10). The pH-H 2 O values of the soils studied were generally low, with mean values of 4.6, 5.0, and 5.3, for the Y, R, and G soils, respectively. Levels of dithionite-citrate extractable Fe (Fed) in the Y soils (26 g kg -1 ) were significantly lower (P < 0.05) than those in the R and G soils (50 and 67 g kg -1 , respectively). The hematite rich soils (R soils) sorbed significantly (P < 0.01) greater amounts of SO 4 2- than the other two groups of soils (9.0 compared with 6.8 and 6.9 mmol SO 4 2- kg -1 , respectively), in which goethite predominated, although the mean amount of SO 4 2- sorbed per unit of Fed followed the order G soils < R soils < Y soils. The lower amounts of SO 4 2- per unit of Fe d of the G soils compared with the other soils, in spite of containing the greatest amounts of goethite, may be related to their lower clay content, lower degree of Fe substitution by Al, and higher pH values than the other soils studied. Therefore, differences in soil components and properties, related to the dissimilar environments of formation, lithology, and age, may explain these patterns. The results indicate that SO 4 2-retention in Oxisols cannot be predicted on the basis of the dominant type of Fe oxide/oxy-hydroxide (either goethite or hematite) or on the quantity of these minerals in the soils.