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

There has been very little study of the P adsorption and buffer capaci ties of semiarid soils of northeast Brazil. The objectives of this stu dy were to assess the effect of soil properties on P adsorption, buffe r capacities, and requirements of some benchmark soil profiles. Phosph orus adsorption data of the soils were adequately described by the Tot h and Redlich-Petersen isotherms. Phosphorus adsorption maxims varied from 117 to 350 mg kg(-1). Averaged across the horizons of the profile s, adsorption capacities followed the order: Non-Calcic Brown > Lake s ediment > Cambisol > Planosol > Lithosol > Alluvid. Adsorption capacit y increased with soil depth in the Non-Calcic Brown, Planosol, and Cam bisol, 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 mo st important soil properties affecting P adsorption and buffer capacit ies were oxalate and dithionite Al, silicate Al, day, and bicarbonate inorganic and organic P (HCO3-Pi and HCO3-Po) which explained 81% of t he 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 ads orption by the soils was attributed to ligand exchange with hydroxyl g roups on the clay edges and the interaction with adsorbed cations in t he Stern and diffuse double layer.,It was, therefore, concluded that A l oxides played a more dominant role than Fe oxides on P adsorption an d buffer capacities of these soils, even though dithionite-citrate ext ractable Al level were three to ten times lower than dithionite-citrat e Fe.