COMPARISON OF OLSEN AND PI SOIL TESTS FOR EVALUATING PHOSPHORUS BIOAVAILABILITY IN A CALCAREOUS SOIL TREATED WITH SINGLE SUPERPHOSPHATE AND PARTIALLY ACIDULATED PHOSPHATE ROCK

The Pi test for phosphorus (P) is a new method in which strips of iron oxide impregnated filter paper are used as a sink to sorb and extract P from a soil solution. In a greenhouse experiment, the Olsen and Pi tests were compared for their effectiveness in evaluating P availability to maize on calcareous soils. Phosphate rock from Togo, partially acidulated with H2SO4 at 50% acidulation level (PAPR 50% H2SO4) and single superphosphate (SSP) were applied at different rates to a calcareous soil (Vernon Clay, pH 8.2, CaCO3 18.9%) which was preincubated with KH2PO4 to raise plant-available P to different levels. In soils treated with SSP, dry-matter yield of maize correlated equally well with Pi-P and with Olsen-P (r = 0.96***). P uptake correlated significantly with P(i)-P (r = 0.94***) as well as Olsen-P (r = 0.97***). Likewise, in soils fertilized with PAPR, significant correlations were found between dry-matter yield and Pi-P (r = 0.97***) and between dry-matter yield and Olsen-P (r = 0.94***). When all the data were pooled, Pi-P and Olsen-P correlated equally well with both dry-matter weight (r = 0.97***) and P uptake (r = 0.94***). Phosphorus extracted by the Pi test correlated significantly with P extracted by the Olsen test (r = 0.99***).