Accumulation of phosphorus fractions in yellow‐brown pumice soils with development

Accumulation of soil phosphorus (P) as a result of fertiliser topdressing of pasture on yellowbrown pumice soils in the Taupo region was investigated using samples from a range of sites covering undeveloped land through to highproducing dairy farms. Organic and inorganic soil P were fractionated using a sequential extraction procedure and the amounts in each fraction compared with amounts of total soil P, soil microbial P, and Olsen P. Total soil P was used as a basis for comparing accumulation patterns as this partially integrates the duration and intensity of fertiliser application. The more labile organic P fractions (extracted by 0.5 M NaHC0 3 and O.1 M NaOH) increased curvilinearly with total P. These fractions account for the majority of the organic soil P and so there is a similar pattern for total organic P. All the other P fractions increased linearly with total P. Most of the inorganic P in these soils originates from fertiliser application, the inorganic P levels in samples from virgin sites being very low. Some residual fertiliser P accumulated in the residual inorganic fraction not extracted by alkaline or acid reagents. This form is unlikely to contribute to plant P requirements in the short term. Microbial P is significantly correlated with NaHC0 3 ‐P 0 ( P < 0.001) and the other organic P fractions ( P < 0.01), but none of these regressions can account for more than onethird of the variation in the microbial P data. There are no significant correlations with any of the inorganic fractions. Despite difficulties in the useof the Olsen P test with yellow‐brown pumice soils, the Olsen P values reflect soil P status as they are significantly correlated ( P < 0.001) with all the P fractions except residual P i . Anomalies in interpretation of Olsen P values for yellow‐brown pumice soils are discussed in the light of these results.