THE EFFECTS OF SOIL PROPERTIES ON THE DIFFERENTIAL PHOSPHATE SORPTION BY SEMIARID SOILS FROM NORTHEAST BRAZIL

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Abstract

There has been very little study of the P adsorption and buffer capacities of semiarid soils of northeast Brazil. The objectives of this study were to assess the effect of soil properties on P adsorption, buffer capacities, and requirements of some benchmark soil profiles. Phosphorus adsorption data of the soils were adequately described by the Toth and Redlich-Petersen isotherms. Phosphorus adsorption maxima varied from 117 to 350 mg kg-1. Averaged across the horizons of the profiles, adsorption capacities followed the order: Non-Calcic Brown > Lake sediment > Cambisol > Planosol > Lithosol > Alluvial. Adsorption capacity increased with soil depth in the Non-Calcic Brown, Planosol, and Cambisol, but decreased with depth in the Alluvial and Lake sediment. The soil P buffer capacities followed the trend of P adsorption capacity, and the external P requirements were between 3.4 and 109 mg kg-1 in comparison with 700 mg kg-1 reported for most Cerrado soils. The most important soil properties affecting P adsorption and buffer capacities were oxalate and dithionite Al, silicate Al, clay, and bicarbonate inorganic and organic P (HCO3-Pi and HCO3-Po) which explained 81 % of the variance in adsorption maxima, 70% of P affinity index, and between 66 and 90% of P buffering indices. The effect of silicate Al on P adsorption by the soils was attributed to ligand exchange with hydroxyl groups on the clay edges and the interaction with adsorbed cations in the Stern and diffuse double layer. It was, therefore, concluded that Al oxides played a more dominant role than Fe oxides on P adsorption and buffer capacities of these soils, even though dithionite-citrate extractable Al levels were three to ten times lower than dithionite-citrate Fe.

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