Development and evaluation of the Pi soil test for plant‐available phosphorus

Abstract

Abstract The Pi test is a new soil test for phosphorus (P), which uses a strip of iron oxide‐impregnated paper as a sink to adsorb P mobilized in a soil‐solution suspension. The paper strips are prepared by immersing hardened filter paper circles in a 10% solution of ferric chloride; then they are dried and exposed to ammonia vapor. The paper is then cut into strips 10 cm long and 2 cm wide. To extract P, 1 g of soil is shaken for 16 h with 40 mL 0.01 M CaCl2 and a Pi strip, which is enclosed in a nylon mesh bag. Phosphorus retained on the strip is dissolved in 0.1 M H2SO4 and measured. The amount of P extracted by the paper strips has been found to correlate well with yield and P uptake by maize (Zea Mays L., Var. Funk 5757) grown on acidic, alkaline, and calcareous soils and in soils fertilized with soluble fertilizers and phosphate rock. To evaluate the suitability of the method to measure plant‐available P contained in a large number of soils, greenhouse experiments were carried out using 18 soils wit...