DESORPTION AND PLANT-AVAILABILITY OF PHOSPHATE SORBED BY SOME IMPORTANT MINERALS

Experiments were conducted to study the desorption characteristics and plant-availability of phosphate sorbed by some important variable-charge minerals including kaolinite, goethite and amorphous Al oxide. Phosphate desorption from the complexes of goethite-P, kaolinite-P and Al oxide-P by equilibration with 0.02 M KCl, resin or some commonly used chemical extractants was slow compared to desorption from a permanent-charge mineral (montmorillonite). However, rice plants were not observed under P deficiency in a pot trial with a phosphate-mineral complex as the only P source for both the permanent-charge mineral and the variable-charge minerals at either 50% or 100% sorption saturation with the exception of goethite-P at 50% saturation. In the exceptional goethite-P treatment, plant P concentration (1.0 g kg(-1)) was on the threshold of P deficiency. From 15% to 31% of the applied P was recovered by the plants within a growing period of three months, depending on sorption saturation and mineral type. Both the dry matter yield and P uptake decreased with decreasing sorption saturation for all the tested complexes except for Al oxide-P-100 (100% saturation). In the case of Al oxide-P-100, Al toxicity may have occurred, for poor root growth and high Al concentration in the plants were observed. The effect of sorption saturation on the yield and P uptake of plant was obvious for kaolinite and goethite but not very significant for montmorillonite. Based on the recovery of applied P, the plant-availability decreased in the following order: kaolinite-P-100 > goethite-P-100 > Al oxide-P-50 > montmorillonite-P-100 > montmorillonite-P-50 > kaolinite-P-50 > goethite-P-50. Fractionation of the sorbed P before and after plant uptake showed that most of the P uptake originated from the resin-exchangeable P fraction in montmorillonite-P complex, but came mainly from NaOH-extractable fractions in goethite-P complex, whereas all the resin-P, NaHCO3-P and NaOH-P fractions in kaolinite- and amorphous Al oxide-P complex made a contribution to P uptake.