NATURE AND AVAILABILITY OF RESIDUAL PHOSPHORUS IN LONG-TERM FERTILIZED PASTURE SOILS IN NEW-ZEALAND

The nature and availability of phosphorus in long–term fertilized pasture soils was investigated in a series of field trials, which included liming, N fertilizer and cultivation and involved monitoring plant p uptake and changes in topsoil (0–7.5 cm) P fractions for 2 years (1982–83). Liming increased soil organic P mineralization. This was indicated by significant decreases in extractable organic P and concomitant increases in microbial biomass P in the limed soils, although these changes in soil P had no significant effect on pasture yield and P uptake. On the other hand, N fertilizer increased pasture yield and P uptake but had little effect on the amounts of P in the different soil P fractions. In the cultivated soils, increases in plant-available inorganic P were attributed to the release of P during decomposition of plant residues, while the maintenance of fallow conditions decreased amounts of microbial P in these soils. © 1990, Cambridge University Press. All rights reserved.