EVALUATING IRON-IMPREGNATED PAPER STRIPS FOR ASSESSING AVAILABLE SOIL-PHOSPHORUS

Iron (Fe)-impregnated filter paper strips (Pi) have been proposed as a method for measuring available soil phosphorus (P). A well-defined Pi method has not yet been developed and Pi strips are often prepared wi th different filter papers and procedures. A study aimed at arriving a t a consistent Pi method is thus needed. Four types of Pi strips, prep ared with the two most widely used papers, Whatman No. 50 and 541, fol lowing a procedure that incorporates improvements both proposed in the literature and made in our laboratory, were evaluated for P extractio n capacity and error. Two of the best strips, which are significantly different in P extraction capacity, along with the Mehlich 1 (0.05M HC l and 0.0125M H2SO4) and the Olsen method (0.5M NaHCO3, pH 8.5) were f urther evaluated in a greenhouse experiment involving eight soils plan ted with corn (Zea mays L.). Results indicated that strips prepared wi th both Whatman No. 50 and 541 were appropriate for P extractions as l ong as strips were washed with deionized water after treatment with am monium hydroxide (NH4OH). At room temperatures the strips probably con tain both hydrous Fe hydroxides and oxides in both crystalline and amo rphous forms. Pi P uas well correlated with Olsen P and P uptake in al soils, indicating that Pi is generally applicable in diverse soils. N o obvious advantage was found for the Pi with respect to the Olsen met hod. Both tile Pi and the Olsen method were better extractants with re spect to the Mehlich 1, which was ineffective for extracting P in calc areous soils. Extractable P by Mehlich 1, Olsen, and Pi all correlated highly with accumulated plant available P estimated by eight sequenti al crops in the greenhouse. However, none of the methods could account for all the variation in plant P removal.