Efficacy of iron oxide impregnated paper strip compared with two standard methods in extracting P from phosphate rock fertilized soils

The iron oxide impregnated paper strip (Pi-strip) may be superior to conventional chemical extractants as soil tests where poorly soluble phosphate rock (PR) fertilizers are used, because use of acidic extractants overestimate bio-availability of phosphorus (P), whereas alkaline extractants underestimate it. We compared the Pi-strip method against Olsen and Bray 2 methods in three Malaysian soils that were fertilized with triple superphosphate (TSP) or North Carolina PR (NCPR), and sweet corn (Zea mays L.) was grown as the test crop. All the methods extracted less P from PR than TSP-fertilized soils thus indicating the need for separate calibrations of soil P in PR-fertilized soils. When total P uptake by corn was related to P extracted by the three methods, Olsen method underestimated P availability from the NCPR in the three soils tested, Bray 2 tended to overestimate P, particularly at high P rates, but the Pi-strip test extracted similar amounts of P from both NCPR and TSP treatments in two out of three soils tested. Therefore, Pi-P was considered to be the most suitable method for P extraction in PR soils since it was the least sensitive to fertilizer type. Phosphorus concentration of sweet corn fertilized with PR was lower than TSP-fertilized corn, but the agronomic effectiveness of the NCPR used was about 100% based on dry matter yield (DMY). Further work is warranted to compare the Pi-strip method with other conventional methods in a wider range of soils since it shows some potential, and since PRs and their derivatives are becoming economically attractive sources of P in agriculture.