The iron oxide impregnated filter paper test (Pi test) is a recently developed soil test for phosphorus (P) in which the FeO paper acts as an infinite sink for P mobilized in a soil solution. Several papers have been published evaluating the effectiveness of the test for predicting plant availability of P under different soil conditions. The use of FeO paper to predict algal availability of P in water bodies and runoffs has also been studied. The purpose of this paper is to review studies on the use of the Pi test to evaluate plant availability of P in soils, and predict availability of P to algae in an aquatic environment. Phosphorus extracted by the FeO paper is primarily physically bound extractable (resin P) and correlates significantly with Bray I and Mehlich P in acid soils and Olsen P in calcareous soils. Dry-matter yield and P uptake by maize (Zea mays L), kidney beans (Phaseolus vulgaris L), and upland rice (Oryza sativa L) grown in acidic soils correlated well with Pi-P. Likewise, in calcareous soils, Pi-P was as good as Olsen-P in predicting crop response. Field trials have shown that the Pi test is a good predictor of plant yield in soils with wide ranging properties. Compared to the standard method to measure bioavailable P to algae in waters and agricultural runoffs involving lengthy algal essays culturing selenastrum capricornutum with sediment samples, the Pi method is a faster and easier method to estimate P that may be potentially available for uptake by algae.
Menon, R. G., Chien, S. H. P., & Chardon, W. J. (1997). Iron oxide-impregnated filter paper (Pi test): a review of its application. Nutrient Cycling in Agroecosystems, 47(1), 7-18. https://doi.org/10.1007/BF01985714