Determination of Plant‐available Phosphorus by the Iron Hydroxide‐impregnated Filter Paper (Pi) Soil Test

The use of iron hydroxide‐impregnated filter paper (P i ) strips as a collector for phosphorus (P) in soil suspension offers a new approach to soil testing for plant‐available P. To correlate the P i test with crop response, four soils ranging in pH from 4.5 to 8.2 were incubated with three rates of P either as triple superphosphate (TSP) or central Florida phosphate rock (CFPR), and P was measured in these soils by the P i , Bray 1, Bray 2, double acid, Olsen, water, and resin extraction methods. Two successive crops of maize ( Zea mays L.) were grown on these soils in greenhouse pots, and the dry matter yield and P uptake by maize were correlated with the soil test values. The P i gave highly significant correlation with crop response in all four soils. Across all soils, P source, rates of application and crops, P i gave the best correlation (0.870) with P uptake and dry matter yield. The next best correlation with dry matter yield was that of Olsen extractable P. The P i test showed potential as a tool for prediction of P fertilizer requirements in soils.