Abstract
Seven oxisols and ultisols of Puerto Rico were analysed for inorganic P fractions, organic P and P adsorption characteristics. Reductant-soluble or iron occluded P was the predominant inorganic form followed by Fe-P adsorbed P, Ca-P, and Al-P. The adsorbed P (NaHCO3 extractable) was apparently associated with iron and aluminium oxides and its removal prior to fractionation caused a decrease in the Al-P and Fe-P f actions. The decrease in Al-P and Fe-P fractions as a result of the removal of adsorbed P was most pronounced on the soils that had received fertilizer P additions within two year prior to sampling. Organic P, as determined by three different methods constituted 25 to 50 per cent of the total soil P. Adsorption isotherms indicated that the soils had a high retention capacity for phosphorus. Amorphous or non-crystalline forms of aluminium oxides were more correlated with P adsorption characteristics than those of iron.
