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

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.