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This study concerns the indurated horizons of some volcanic ash soils in the Interandine Valley of the Ecuador. These indurated horizons were cropped out by erosion on the piedmont of the sierras, under a subarid to subhumid climate, and then they have been called cangahua. The soils within they are intercalated are composed of several various pyroclastic deposits. The soil profiles show one, two or several indurated horizons. The present observations concern a soil climo-toposequence near Bolivar, viz: downward a soil profile with 2 cangahua horizons; at the midslope a profile with only one horizon of weathered tuff; a little above, another soil with a weakly weathered consolidated ash fall layer; and finally, at the summit on the paramo, an andosol profile with two horizons of weathered tuff; as well as three cangahua samples from ot...
Indurated soils, called tepetates by the farmers, can be seen in the region of Xalapa, Veracruz, Mexico. They are located in the lower part of a topoclimatic sequence of soils developed on volcanic materials. This sequence, located on the slope of the Cofre de Perote volcano toward the Gulf of Mexico, stars at an altitude of 3,500 m. with a successive appearence of andosols and, lower down ferralitic soils and, at an altitude af 1,200 m., tepetate soils. The representative soils were studied within the framework of the morphopedological map of the region. The ferralitic soils are observed in the altitudinal hand between 1,200 and 1,500 m. where there is a subtropical climate with only a short dry season. The tepetate soils appear in the zone where the dry season is 5 to 6 months long. This work presents the changes from ferralitic soil...
The tepetates of Mexico Highlands are indurated horizons with fragipan pr0perties. Tepetates appear among pyroclastic deposits (T1, T2, T3). A soil profile located near Texcoco, Mexico, including tepetate layers was studied. Mineralogical and microscopical features of this profile were studied using a series of methods. This report the main results of the study. Tepetates were characterized by several features: first, the nature of their parent material; second, a close matrix microstructure and third, their colloidal fraction. The latter is conformed mainly by laminated 1:1 to 2:1 mixed layer minerals made of flaky layers. These features can explain the fragipan properties of tepetates and most of their mechanical behaviour. (Résumé d'auteur).
In the sedimentary basin of Rieti (Central Italy) a Mollic Fragiudalf was studied in order to verify, through the analysis of differential dissolution kinetics of Si and Al, wether the presence of this two elements could be involved in determining the fragic characters of its deep horizons. The reuslts of the main physico-chemical analysis confirmed the fragic features of the deep horizons Bt3x and Ctgx which have been already observed in the field study. Differential dissolution of Si and Al was performed using 0,5 N NaOH at 25°C, and the values of silica and alumina obtained after each of ten consecutive extractions were cumulated to trace the cumulative curves of dissolution. Slope changes of the curves, which indicate changes in dissolution rate, allowed os to recognize three mineralogical phases with different degree of solubility...
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