EFFECT OF CITRATE AND TARTRATE ON PHOSPHATE ADSORPTION BY AMORPHOUS FERRIC HYDROXIDE

Curves for P adsorption by ferric hydroxide in the presence of citrate and tartrate were prepared. From the curves, it appeared as though ta rtrate at 10(-4) M has no effect on P adsorption up to 750 mu mol P g( -1) ferric hydroxide adsorbed. However, observation of flocculation pa tterns show that even when the curves are identical, there is a differ ence in the way P is adsorbed in the presence of tartrate and citrate in terms of the type of site that is attacked. Assessment of phosphate - citrate competition using P-32 and C-14 showed that citrate is less readily adsorbed by ferric hydroxide than tartrate and that citrate d issolves adsorption sites more readily than tartrate. The implications of this study are that root exudates containing a greater amount of t artrate would cause more efficient use of added P by plants since tart rate occupies the high affinity sites and therefore reduces phosphate fixation. Plants which produce root exudates high in citrate content w ould be able to utilize more fixed P than plants which produce exudate s lower in citrate content because citrate dissolves sites at which P is held.