PHOSPHATE ADSORPTION BY CLAYS FROM BRAZILIAN OXISOLS, RELATIONSHIPS WITH SPECIFIC SURFACE-AREA AND MINERALOGY

Oxisols derived from clayey sediments, sandstone, mafic rocks and schi st were sampled in the ''Triangulo Mineiro'' region, Minas Gerais Stat e, Brazil, to study the relationship between the phosphate adsorption and mineralogy and specific surface area. The specific surface area of the iron oxides estimated from N-2 adsorption by the soil clays befor e and after dithionite-citrate dissolution of the Fe-oxides ranged fro m 45 to 110 m(2) g(-1). Specific surface area of the iron oxides corre lated to the goethite: goethite + hematite ratio (r = 0.694). Specifi c surface area estimated by differential X-ray analysis did not show g ood agreement with BET-N-2 adsorption values. Phosphate adsorption max ima for the soil clays (< 2 mu m) ranged from about 70 to 160 mu mol P g(-1) of clay. Stepwise regression analysis showed phosphate adsorpti on for the total clays to be primarily related to goethite, gibbsite, the sum of Fe plus Al oxides and also amorphous Al oxides, A mild trea tment for concentrating the soil iron oxides showed small effect in th e phosphate adsorption for most soils. It showed the importance of gib bsite and crystalline Fe-oxides in the adsorption, with goethite expla ining most of the variation occurring in P adsorption after the treatm ent. Correlation studies showed the adsorption capacity per unit speci fic surface to be somewhat dependent on variations in the iron oxide m ineralogy of the clay samples. Clay color showed good correlation to t he goethites:(goethite + hematite) ratio in soils but no direct relati onship between phosphate adsorption and clay color was detected.