LONG-TERM EFFECTS OF LIME ON SOIL-PHOSPHORUS SOLUBILITY AND SORPTION IN 8 ACIDIC SOILS

Measurements of phosphorus (P) sorption, isotopically exchangeable, KC l soluble and extractable P (Bray(1)) were carried out on limed and un limed soils from eight pasture experiments on the Northern Tablelands of New South Wales at intervals of 1, 2 and 3 years after lime applica tion. Lime increased soil pH by a minimum of 0.5 to a maximum of 1.55 units, and there were corresponding decreases in soluble aluminium and manganese. Lime decreased P sorptivity in every soil and at every sam pling, but decreases were usually largest at the first sampling. They were attributed to the pH-induced increase in surface negative charge and the smaller increases in calcium concentrations of these freely dr ained soils, compared with undrained potted soils, of a previous glass house experiment. Isotopically exchangeable P was increased by the hig hest lime rate (5 t/ha) in all but one soil at the first sampling, whi le soluble P was increased by both lime rates in all soils. Increases in exchangeable P tended to decline at successive samplings, but incre ases in soluble P sometimes increased and sometimes decreased with tim e. In general, lime-induced increases in soluble P were consistent wit h decreases in P sorptivity, although the primary cause of the increas es was probably the dissolution of iron and aluminium phosphates. All these changes were conducive to the increased plant availability and u ptake of soil and fertilizer P.