An evaluation of some aspects of the iron oxide-impregnated filter paper (Pi) test for available soil phosphorus with New Zealand soils

Abstract

Some features of the iron oxide-impregnated filter paper test (Pi test) for soil phosphorus (P) were investigated with a range of New Zealand soils. Soil particles were found to adhere to the filter-paper surface, even after careful rinsing, and contributed 0–85% (mean 30.8%) to the Pi test value. Phosphate-retentive soils high in inorganic P accumulated from fertiliser application contributed most. Ultrasonic treatment reduced the effect but also removed some of the phosphate adsorbed by the iron oxide. Acidity from aging of the hydrous iron oxide impregnating the filter paper, and also that released from soil by exchange with Ca²⁺, lowered the pH value of soil suspensions shaken with iron oxide-impregnated paper strips by 0.5–1.2 pH units. This could increase dissolution of some soil P compounds. For example, some phosphate rock in mixtures with soil was extracted by the Pi test. Particles of phosphate rock adhering to the strips increased Pi values significantly with three of the four soils investigated. In addition, phosphate rock dissolved during extraction of two of the soils which had low suspension pH values.

Keywords:

• adhering soil
• iron oxide-impregnated paper test
• pH
• phosphate rock
• Pi test
• ultrasonic treatment