A STUDY OF A CLIMOSEQUENCE OF SOILS DERIVED FROM VOLCANIC ROCK PARENT MATERIAL IN SANTA CATARINA AND RIO GRANDE DO SUL STATES, BRAZIL

FRANCESCO PALMIERI, Purdue University

Abstract

Ten soils formed from basic volcanic rocks were studied on a 450 km climosequence transect in Santa Catarina and Rio Grande do Sul States, Brazil, to learn how soil properties vary with climate and to provide a data base for the developing Brazilian soil classification system. From east to west the altitude drops from 1250 m to 270 m, the annual temperature increases from 14.1(DEGREES)C to 19.3(DEGREES)C, and the annual precipitation decreases from 2460 mm to 1850 mm. The soils were in the Cambisol, Terra Bruna Estruturada (a new class), Brown Latosol, Terra Roxa Estruturada, and Dusky Red Latosol soil classes of the Brazilian soil classification and in the Dystrochrept, Haplumbrept, Paleudult, Haplohumox and Haplorthox great groups of the U.S. Soil Taxonomy. As soil sites get warmer and drier, kaolinite and hematite contents increase, halloysite and goethite contents decrease, the crystal size of kaolin minerals (kaolinite and 0.7 nm halloysite) increases, soil color changes from brown to dusky red, soil structure changes from well developed blocky to ultra fine granular (massive-like), and surface area and soil swelling decrease. The soil materials mainly had isotic plasmic fabric (isotropic) and had few or no argillans visible in thin section. Gibbsite and halloysite form immediately adjacent to each other in the weathered basaltic rocks. Maghemite occurred in 2.5YR and redder soils. Upward in the solum organic carbon and goethite contents increase, hematite decreases, the soil color becomes yellower and crystal size of kaolin minerals increases. These Fe-rich soils sorbed large amounts of phosphate. Acccording to prevailing ideas, tropical and subtropical soils composed mainly of kaolin and iron-oxide minerals have low surface activity and low shrink-swell potential. These soils shrink enough to open subsoil cracks 10 mm wide in the summer and have a moderately high swelling index (COLE = 0.042). Typically kaolin minerals are described as having 6 to 43 m('2) g('-1) surface area but in these soils it is about 300 m('2) g('-1). Surface area was inversely related to the mean crystallite dimension of kaolin minerals and directly related to COLE values and to phosphate sorption. Soil shear strength measured by the Swedish fall-cone increased as the soil structure changed from ultra fine granular (massive-like) to blocky structure and with increasing halloysite and goethite contents.

Degree

Ph.D.

Subject Area

Agronomy

COinS