Direct application phosphate rocks: Do soil phosphorus tests take site influence into account?

Conventional soil phosphor-us (P) tests have been found unreliable where di rect application phosphate rock (PR) is used. However, the iron oxide-impre gnated filter paper (Pi) and mixed anion/cation ion-exchange membrane tests have been suggested as being suitable in this situation. These tests measu re a part of the residual PR that contributes to the soil P status. However , the agronomic effectiveness of a PR depends on its rate of dissolution at a site. A soil test measuring residual PR in soil will be ineffective if t he PR does not dissolve in the field. The dissolution rate is influenced by site properties such as drainage and rainfall as well as soil and PR prope rties. Soil samples from New Zealand pastoral sites where the PR dissolutio n rate was either very low or very high were used to investigate the influe nce of site and soil properties on the amount of PR-P measured by the tests . There was no significant difference in the amount of PR measured by eithe r test between soils from the low and high dissolution rate sites. This ind icates that the impregnated filter paper (Pi) and mixed anion/cation ion-ex change membrane tests would overestimate soil P status where direct applica tion PR was used at a site with low PR dissolution rate.