Resumo

GUERRERO-ALVES, José P et al. Genesis of a saline-sodic soil in tucupido (guárico-venezuela). Agronomía Trop. [online]. 2007, vol.57, n.3, pp.171-188. ISSN 0002-192X.

Saline-sodic soils, which are rich in salts and sodium, are degraded soils common in zones from subhumid toarid climates. This study was carried out to identifyfactors and processes related with the genesis, undernatural conditions, of a saline-sodic soil (Sodic Haplusterts, fine, smectitic, isohyperthermic) in Tucupido(Guárico-Venezuela), under a semiarid climate. Therewere evaluated morphological, physical, chemical andmineralogical characteristics and properties of the soil.The pH, exchangeable sodium, and electric conductivity(saturation extract) varied between 7.55-8.4; 2-32% and1.0-8.7 dS m^-1 respectively. NaCl was the dominantsalt (> 66% below 0.1-m depth). Bulk density, macroporosity, and saturated hydraulic conductivity varied from 1.8 to 2.2 Mg m^-3, 4 to 5% and 3x10-7 to 1x10-5 m s^-1, respectively. Smectites were the dominating clays, butchlorites and hydrated micas there were present too.High quantities of Ca- and Mg-carbonates were alsofound precipitated in the soil (18%). The conclusionwas that these soils were probably formed by alterationof calcareous shales, formed in sedimentary environments in contact with saline waters rich in sodium, to produce saline-sodic soils during the arid phase at theend of the Pleistocene period. The very low permeabilityof these soils limited salts washing during the wetterHolocene period. High electric conductivities probablywould allow the collapse of interlayer space ofsmectites, and the high content of Ca-carbonates the production of Ca-Al silicate compounds which couldbe acting as cementing agents in the interlayer space ofsmectites, and these processes probably could preventedthe increase of the exchangeable Na% in the soil tovalues higher than 32%.

Palavras-chave : Vertisols; calcium carbonate; smectites; high bulk density.