THE EFFECT OF SOIL-PHOSPHORUS BUFFERING CAPACITY ON PHOSPHORUS EXTRACTION BY IRON OXIDE-COATED PAPER STRIPS IN SOME ACID SOILS

Iron oxide-coated strips can serve as a P sink to continuously remove P from solution. In this way, P extraction is analogous to P absorption by plant roots. Because the relationship between soil P sorption capacity and P extraction by the Fe oxide-coated strips can affect the interpretation of the P test values, it needs to be examined. The effectiveness of the Fe oxide-coated strips in extracting sorbed P, and its relationship with soil P sorption capacity and with the commonly used NaHCO3 P test, were evaluated in acidic soils amended with varying amounts of P. Both the Fe oxide strip and NaHCO3 P tests were correlated with corn (Zea mays L.) dry matter yields to determine their relative effectiveness in describing plant growth response to increased P availability in soils. Iron oxide strip-extractable P, like NaHCO3-extractable P, increased with increasing amounts of P added to the soils. The recoveries of sorbed P by the two test were similarly affected by the P sorption capacities of the soils at a solution P concentration of 9.7 x 10(-6) M or at a constant P addition of 20 mmol kg-1, which explains the close correlation between the two tests (R2 = 0.85, P < 0.001). The Fe oxide strip-extractable P reflected more appropriately an intensity index, rather than a quantity index, in the characterization of soil P availability. Decreased P extraction by the Fe oxide strips at high ionic strengths indicated that desorption of sorbed P from soil limited P extraction by the Fe oxide strips and that maintaining similar ionic strength is important in assessing P availability among diverse soils. Both the Fe oxide strip- and NaHCO3-P tests correlated well with corn yields, tissue P concentration, and P uptake. The Fe oxide strip P test did not seem to have much advantage over the NaHCO3-P test in describing corn response to increased P availability in the soils; however, the Fe oxide strips can be stored easily for later analysis or sent to centralized locations for P determination, if necessary.