Phosphorus is among the macronutrients the least absorbed by plants. Nevertheless, the amount of P-containing fertilizers applied to humid tropical and subtropical soils in relatively high. The reason is the strong interaction between of P and Fe and Al oxides, which are abundant in these soils. The study of these interactions is fundamental to know more about the relations between phosphorus adsorption and soil constituents. The objective of this study was to evaluate how the minerals in the clay fraction, before and after selective dissolution of the iron oxides and of different soils and how some of their mineralogical properties affect the maximum phosphorus adsorption capacity (MPAC). For this purpose, chemical and mineralogical soil properties, were characterized in the clay fraction of the B-Horizons in 20 samples soil from the State of Paraná. In these soils the maximum phosphorus adsorption capacity (MPAC) and the external specific surface area (SSA) were determined in the clay fraction before (MPAC, SSA) and after (MPACd, SSAd) selective dissolution of the free iron oxides. No definite tendency of MPAC was observed with the selective dissolution of the iron oxides. The MPAC values were higher than the MPACd in seven and lower in 13 samples. In the clay fraction, Fe d levels ranged from 25.9 to 108.8 g kg-1; and Al d 6.7 to 30.1 g kg-1. The Al free (Al d) and poorly crystalline forms (Al o) were best correlated with MPAC and MPACd (r = 0.66 and 0.70, respectively). Among the chemical properties analyzed, remaining phosphorus (P-rem) can be used to estimate MPAC.
iron oxides; gibbsite; inner-sphere-complexes
