The P(i) soil test for phosphorus (P), which uses an iron oxide impregnated paper, was evaluated in three field experiment on lateritic soils in south-western Australia fertilised with triple superphosphate, North Carolina rock phosphate, Queensland rock phosphate, and in one experiment, Calciphos. Soil samples were collected February to March from 1985 to 1988. The P(i), Colwell, Bray 1, calcium acetate lactate (CAL) and Truog soil P tests were used. Soil test P values were related to yields of triticale (x Triticosecale) or oats (Avena sativa), barley (Hordeum vulgare) or dry herbage yields of subterranean clover (Trifolium subterraneum). The Colwell soil test, which is commonly used in Australia, and the Pi soil test were almost equally predictive, but showed considerable error in prediction of yield. For each soil test and plant species the relationship between yield and soil test P differed with fertilizer type and year. For combined data for all sites, fertilizers and years, the CAL soil test was the most predictive and the Truog soil test was least predictive of plant yield.
