Kinetics of phosphate sorption-desorption as influenced by soil pH and electrolyte

The effect of pH and background electrolytes on phosphorus (P) sorption-des orption kinetics was studied in three contrasting benchmark soil series of India. Air-dried soil samples were equilibrated for 6 days in sodium chlori de (NaCl) and calcium chloride (CaCl2) containing 20 and 40 Ing P kg(-1) so il at pH values 4.25-8.00. Phosphate sorption was minimum in the pH range, 5.50 to 6.75. The phosphorus desorption rate aswell as quantity of P desorb ed in six extractions was maximum at pH 5.50 followed by 6.75, 4.25 and 8.0 0, in that order. This behaviour of P suggested that the amount of tightly held P was greater at low pH, decreased sharply in the middle pH range (5.5 0-6.75) and then increased with rise in pH. The Elovich model was found sup erior to the others to describe P desorption in the soils studied. The valu es of the Elovich (B) and parabolic (R) parameters showed that the proporti on of P desorbed by the soil in NaCl was 1.5 times more than that in CaCl2.