Long-term effects of lime on soil-phosphorus solubility and sorption in eight acidic soils

Abstract

Measurements of phosphorus (P) sorption, isotopically exchangeable, KCl soluble and extractable P (Bray(1)) were carried out on limed and unlimed soils from eight pasture experiments on the Northern Tablelands of New South Wales at intervals of 1, 2 and 3 years after lime application. 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 sampling, 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 drained soils, compared with undrained potted soils, of a previous glasshouse experiment. Isotopically exchangeable P was increased by the highest lime rate (5 t/ha) in all but one soil at the first sampling, while soluble P was increased by both lime rates in all soils. Increases in exchangeable P tended to decline at successive samplings, but increases in soluble P sometimes increased and sometimes decreased with time. In general, lime-induced increases in soluble P were consistent with decreases in P sorptivity, although the primary cause of the increases was probably the dissolution of iron and aluminium phosphates. All these changes were conducive to the increased plant availability and uptake of soil and fertilizer P.