Phosphate absorption and phosphate fractions in field soils of varying histories of phosphate fertilization

Abstract

Phosphate adsorption isotherms were determined by allowing soils to react for 18 hr with phosphate solutions of varying concentrations. Small or no reductions in adsorption maxima were found for red-brown earth samples, although a large total amount of phosphate had been applied over a considerable period of years. Some instances of lower adsorption were found, however, in the lateritic and brunizem soils which had received large, recent applications of phosphate fertilizer. Apparently applied phosphate did not permanently occupy adsorption sites but was converted to other forms which were independent of clay surfaces. Phosphate in these samples with varying histories of phosphate fertilization was determined as "aluminium-, iron-, and calcium-bonded" fractions according to the procedure of Chang and Jackson (1957). All these fractions were found to have been increased by phosphate addition, and the total gain due to fertilizer could usually be accounted for in those fractions. No quantitative relation between any of the fractions and the adsorption isotherms was apparent. The samples were also extracted with the Bray P1 reagent, which is extensively used in a rapid procedure to measure "available" phosphorus. Phosphorus removed by this extractant was approximately a constant proportion of both the "aluminium bonded" (soluble in ammonium fluoride) and the "iron-bonded" (soluble in sodium hydroxide) in fertilized samples and a lower but again nearly constant proportion in unfertilized soils. Amounts of phosphorus extracted by the Bray P1 reagent and the ammonium fluoride appeared to be in fair agreement with the phosphate status of the soils for crop growth.