SYMPOSIA PAPER Published: 01 January 1993

The P Test for Evaluating Bioavailability of Phosphorus


The Pi test is an innovative approach for evaluating the bioavailability of phosphorus (P) in soils. The test makes use of paper strips impregnated with iron oxides as a sink for P mobilized in a soil solution. The amount of P extracted by the strip was increased with increased surface area of the strip and increased time of contact between the iron oxide on the strip and phosphate ions in the soil solution. From soils the amount of P extracted increased with shaking time up to 8 h shaking. Over 8 h shaking, there was little increase in the amount of P extracted. Phosphorus extracted by the strips represents solution plus labile P that is mobilized. Soil : solution ratio had little effect on sorption of P by the Pi strips. Based on these, paper strips with 40 cm2 surface area and 16 h shaking time were chosen for further study.

The Pi test is insensitive to the type of soil. Significant amounts of P can be extracted by the filter paper strips from soils with low to high pH and the P extracted can predict yield and P uptake well at both low and high pH. Phosphorus extracted by the strips correlates well with P extracted by the Bray I, Mehlich I, and Olsen tests in acid soil and with Olsen P in alkaline and calcareous soils. The Pi test has a potential use in monitoring P concentrations in aquatic ecosystems, with preliminary studies showing that the test can reliably estimate dissolved P in water.

Author Information

Menon, RG
International Fertilizer Development Center (IFDC), Muscle Shoals, AL
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Developed by Committee: D18
Pages: 58–67
DOI: 10.1520/STP23871S
ISBN-EB: 978-0-8031-5225-0
ISBN-13: 978-0-8031-1475-3