Andisols can absorb large amounts of phosphorus rapidly, and then release it slowly, yet the mechanisms by which they retain P and release it for plant growth are poorly understood. Ligand exchange of organic compounds from Al-humic complexes by P and/or Si release - due to breakdown of allophanic microstructure to provide sorption sites - might account for the retention of P, but its extent is not known. We applied a soil column flow-through technique to quantify the release of anions and organic carbon (C) associated with P sorption by two andic soils, and we related the anion release to possible mechanisms for the retention of P. Phosphate (H2PO4- HPO42-) sorption and concurrent anion desorption were obtained by passing a 1-g P l(-1) (32 mmol KH2PO4 in 1 mM CaCl2) solution through the soil columns (25 cm(3)). Total dissolved P, Fe, Al, S, Ca, Mg, K, Mn, organic C and pH were determined in the eluent. Changes in eluent pH and the patterns of the retention of P and corresponding concentrations of Al, Si and organic C in the eluent were similar for the two Andisols. The general pattern and changes in pH of the eluent coincided with changes in the patterns of release of organic C and Si and the rate of P retention. Release of silica accounted for <6% of the P sorbed and had only a minor role in P retention in these two Andisols. Release of organic C, however, accounted on a molar basis for 40% and 83%, respectively, of the P sorbed. Direct measurements of the pH of the eluent and release of anions and organic C concurrent to P retention contribute to rapid assessment of the controlling mechanisms of P retention. The results indirectly confirm the hypothesis of ligand exchange of solution P with organic complexes held on allophanic surfaces. The organic C release, however, is not specifically related to either the fast or the slow P retention phase. The shift in the controlling P retention reaction associated with a change from the fast to the slow P retention phase is clearly indicated by an abrupt change of the pH of the eluent. This shift, in previous studies' identified graphically by a change in slope of the P sorption isotherm, can be identified directly by measuring the pH of the matrix.