Morphological, physical, chemical and mineralogical characterization and genesis of an alkaline sodic pedon from Papelón, Portuguesa State

Abstract

Salt-affected soils are a major factor limiting plant growth and productivity of crop plants. Out of the 831 million hectares that the FAO/UNESCO reports worldwide as salt-affected soils more than 50 % are alkaline-sodic soils. This study was conducted in order to identify factors and processes in the genesis of alkaline sodic soil from the alluvial plains of Portuguesa River. Morphological, physical, chemical, and mineralogical attributes were determined. Results indicated exchangeable sodium percentage (ESP) up to 43 %, NaHCO₃ accumulation, high pHs (>8.5), very high bulk densities (up to 2.03 Mg m^-3), both low hydraulic conductivity (< 0.5 mm h^-1) and macroporosity values (<3.4%) in Btn and Cn horizons, CaCO₃  precipitation, mixed mineralogy characterized by clorite, interstratified 10/14 m, quartz, and micas. It was inferred that the soil was produced, first, from materials grounded by glacial ice action in Venezuelan Andes during the arid phase of Pleistocene. Later, during the Holocene humid and warmer climate, sediments were transported and deposited in plains of Portuguesa River and then interacted with NaHCO₃ rich waters. Alkaline sodium salts could be formed by weathering of sodic silicates in rocks from Venezuelan Andes or from the accumulation of organic matter in a reduction environment with the elimination of sulfates as H₂S. The soil evolved to promote clay migration, the substitution of Ca-Mg by sodium in soil exchange complex, CaCO₃  precipitation, nitric horizon formation, elimination of salts in deep drainage, and finally, when total salt quantities diminished  the degradation of soil structure was produced