Effect of liming and air‐drying on the adsorption of phosphate by some acid soils

Abstract

SUMMARYThe effect of liming the A and B horizons of a number of acid soils on the subsequent adsorption of phosphate by soils retained moist or allowed to dry was investigated. Air‐drying increased the phosphate adsorption capacity but the reason was not clear.When A horizons were maintained moist, incubation with lime for six weeks increased phosphate adsorption by four samples and had no effect on another. When A horizons were air‐dried, the effect was considerably reduced or reversed. For B horizons, which had considerably greater phosphate adsorption capacities than A horizons, liming decreased phosphate adsorption irrespective of whether the soils remained moist or were dried. The relative decrease in adsorption was, however, greater when the soils were dried.In a more detailed study using one acid soil it was shown that incubation of the soil with lime for six weeks had no effect on phosphate adsorption by moist A and B horizons but after 36 weeks incubation liming decreased adsorption by the moist samples. If soils were dried then liming decreased phosphate adsorption after six or 36 weeks incubation. Such relative effects of liming on phosphate adsorption were confounded by the fact that air‐drying greatly increased the phosphate adsorption capacity of the unlimed soil. The drying effect was at least partially reversible following rewetting of the soil.Results were interpreted in terms of the lime‐induced increase in soil pH causing (i) the surface charge conferred on soil oxide surfaces to become more negative (thus decreasing phosphate adsorption) and (ii) the precipitation of exchangeable Al as hydroxy‐Al polymers resulting in the formation of new, highly active, adsorbing surfaces (thus increasing phosphate adsorption). Phosphate adsorption by moist limed soils can, therefore, be increased, decreased or unaffected depending on the relative magnitudes of these two processes. It is suggested that after liming, and/or air‐drying, crystallization of amorphous materials progressively decreases their surface area and adsorbing capacity. Thus, liming tends to decrease phosphate adsorption when the soils are dried.