The effects of lime on adsorption and desorption of phosphate in five Colombian soils

The effects of lime (applied in the field) on the amounts of total and isotopically-exchange-able phosphate adsorbed from solutions were measured in five soils. The total amount of phosphate adsorbed without lime was in the range 200 to 1700 μg P per g of soil at 0.05 μg P cm⁻³ of solution. Lime diminished the amount of phosphate adsorbed at all concentrations of solution in an oxisol and a dystropept; in an ultisol and another dystropept, lime tended to increase sorption at small concentrations and diminish it at large concentrations; in a dystrandept that contained spheroidal allophane and a great deal of organic matter, lime increased adsorption at all concentrations up to 1 μg P cm⁻³. Lime increased the proportion of added phosphate that was isotopically exchangeable in the oxisol and one dystropept, had no effect in the other dystropept, and diminished the proportion in the ultisol and dystrandept.
Adsorbed phosphate was subsequently desorbed by suspending the soils in solutions without phosphate. After desorption the quantity of exchangeable phosphate in all soils was closely correlated with aluminium extracted by ammonium oxalate; buffer power was correlated in all except the dystrandept, in which it was larger per unit of aluminium than in the other soils; possibly the cause was aluminium associated with organic matter. In all soils lime diminished buffer power allowing a specific amount of exchangeable phosphate to maintain a larger concentration in solution. The beneficial effects of lime on exchangeable phosphate after desorption were consistent among soils, despite inconsistent results when the phosphate was adsorbed.