LONG-TERM KINETICS OF PHOSPHATE RELEASE FROM SOIL

Long-term phosphate (P) desorption from soil is described using two discrete P ''pools'' in the soil: one available and one strongly fixed pool. The P release kinetics for ea ch pool a re described with a first-order rate equation. A new desorption method is used with hydrous iron oxide inside dialysis tubing acting as a P sink. The widely used iron-impregnated filter paper desorption method overestimates initial P desorption by a factor of up to 4 and underestimates the quantitative progression of desorption as a function of time. P desorption continued with substantial rates for periods longer than 1600 h. A wide range in P desorbability was observed: 15-70% of oxalate-extractable P (P-ox) desorbed after 1600 h. P desorbability decreased with increasing Fe-ox + Al-ox content of the sample. The relative size of the quickly desorbing pool increased with increasing initial degree of P saturation alpha(0) = P-ox[Fe-ox + Al-ox] of the soils. This fact is of direct importance for the estimation of P losses from phosphate-rich soils. This study furthermore provides evidence that all oxalate-extractable P potentially is desorbable: no irreversibly fixed P-ox exists.