Relações entre formas de ferro e sorção de fósforo em latossolos sob cerrado do Planalto Central

Abstract

Samples from the top layer of three Oxisols, members of a toposequence over basalt, from the Central Plateau of Brazil, were collected according to their color and landscape position, aiming to identify and characterize the mineralogical components of the clay fraction and to relate the P-sorption parameters with some soil properties. The pedomateriais were physically, chemically and mineralogically characterized, before any treatment. After liming, a detailed study was performed, through which the phosphorus adsorption maximum (PAM) and the phosphorus desorption index (PDI) were determined, as well as the liming influence upon these parameters. Two liming levies were adopted: level zero (CO) without liming, and level one (Cl) with lime to raise the original soil pH to 5.5-6.0. The Cl level was determined by the neutralization curve. The adsorption-desorption studies were performed on the whole soil ( < 2mm) with and without liming, on the concentrated iron oxide minerais, and on the silicates and gibbsite fractions, in order to learn more about these complexe processes. In light of the obtained data, the following conclusions can be made: 1. The dominant mineralogical assemblage in the clay fraction revealed a close association with soil color and landscape position, i.e., hematite was associated with gibbsite in the reddest pedon located in the summit position, while goethite was associated with kaolinite in the yellower pedons located in the médium and lower part of the landscape. 2. Among the minerais in the clay fraction, the Fe-oxides seem to be the major responsible ones for sorption reactions, and the yellower soils (Gl and G4) had higher adsorptive and lower desorptive power of P than the reddest pedon (G6). The probable smaller mean crystallite dimension of goethite, which is more abundant in yellower soils, that of hematite, the greater amount of DCB-extractable Al, the higher specific surface area and the existence of layers of Al compounds presenting lower crystallinity degree in these yellower pedons seem to be the main critical factors for this different behavior. 3. The different P adsorption observed between the yellower pedons (Gl and G4) imposes caution about generalizations concerning similar behavior inferred with basis only on color and iron oxides mineralogy, due to the adsorption phenomenon complexity. 4. Liming had variable influence on studied soils with relation to P adsorption, maximizing it on the more goethitic ones (Gl and G4) and minimizing it on the more hematitic pedon (G6) ; however, the desorption was increased in all cases, being this effect more intense on soils Gl.and G4.