The variability in total and extractable soil phosphorus under a grazed pasture

A comparison of the total soil phosphorus (P) and extractable soil P between 224 samples of topsoil from an area of ~27 m2 within a grazed, established grass/clover sward has been made. The values of total soil P displayed an approximately normal distribution around a mean concentration of 1264 mg P kg-1 and were positively correlated to acetic-acid-extractable P which accounted for <2% of the total soil P. The amount of total water-extractable P was much smaller (~0.4% of total soil P) and was not significantly correlated with either the concentration of total soil P or acetic-acid-extractable P. A variable proportion (from less than 5 to 60%) of the total water-extractable P was present in a non-molybdate-reactive form, and there was no apparent relationship between these molybdate-reactive and molybdate-unreactive forms. All variograms showed a positive intercept on the ordinate. For acetic-acid-extractable P, the greatest proportion of variance was attributable to the molybdate-reactive P fraction, while it was equally proportioned between molybdate-reactive and -unreactive P forms in water extracts. The greatest variance usually occurred at the maximum sampling distance (18 m). However, even at the smallest distance (11 cm) the variability in total acetic-extractable P was 2.35 mg P kg-1 and water-extractable P was 0.45 mg P kg-1. Therefore the roots of individual plants within the studied pasture may encounter considerable variability in the concentration of available phosphorus. The potential variability of phosphorus found between rooting zones of different individual plants was greater than that likely to be encountered within the area exploited by any one individual root system.