Sorção e dessorção de fósforo em solos tropicais do Mali

Abstract

Phosphorus (P) strongly interacts with soil particles, undergoing the process of specific adsorption. Due to this high capacity of soils to adsorb P, especially tropical soils, the availability of this element for uptake by plants is extremely low. Soils have different sorption capacities, which depend on their mineralogical, chemical and physical characteristics. Thus, the objective of the study was to evaluate, through isotherms, the sorption and desorption of P in soils from Mali/Africa compared to soils from Brazil. For this purpose, four soils from Mali were selected according to clay content, namely: sandy soil (8% clay), two medium texture soils (26% and 27% clay) and clayey soil (47% clay). Three soils from Brazil with 6%, 23% and 40% clay were used. From the soil samples, in the air-dried fine soil condition, 0.3 g subsamples were taken to carry out the sorption experiment. The soil:solution ratio was 1:100, containing the following concentrations of P, in the form of monopotassium phosphate (KH2PO4): 0; 2.5; 5.0; 7.5; 10; 15 and 20 mg L-1. These solutions were prepared in electrolyte solution of sodium chloride (NaCl) (0.015 mol L-1). The reaction time with the soil, in the adsorption, was 24 hours in agitation. In the desorption, 30 mL of NaCl (0.015 mol L-1) was added to extract the adsorbed P. The concentration of P in the equilibrium solution, in both steps (adsorption and desorption), was determined using the colorimetric method. In the soils of Mali and Brazil, chemical and textural analyses, sulfuric attack and evaluation of pedogenic iron (Fe) and aluminum (Al) contents were carried out. The adsorption data were modeled according to the Langmuir and Freundlich equations, while the desorption data were submitted to regression analysis. In addition, a correlation study of the chemical and textural properties of the soil with the constants of the models was carried out. The Langmuir model was the most suitable for the adjustment of P adsorption data in soils. Both soils showed similar behavior in the medium and clayey texture classes. However, in the sandy texture class, Brazilian soil adsorbed three times more P than Mali soil, with CMAP of 153.8 mg kg-1 and 47.4 mg kg-1, respectively. In the evaluation of the Fe and Al oxide forms, the soils of Mali had higher levels of Fe oxides than the Brazilian soils. On the other hand, the latter showed a higher content of Al oxides. In the correlation study, potential acidity, H non-exchangeable, total and amorphous Al oxide content and clay content were positively correlated with CMAP. The available and remaining P content correlated negatively. The sorption capacity of P in both soils was similar, except for the sandy textured soil, whereas desorption was higher in the medium textured soils of Mali.