MODIFIED IRON-OXIDE-IMPREGNATED PAPER STRIP TEST FOR SOILS TREATED WITH PHOSPHATE FERTILIZERS

The Fe-oxide-impregnated paper strip (Pi) test with 0.01 M CaCl2 has b een proposed as an improved soil test for evaluating available P from soils treated with phosphate rock (PR) and triple superphosphate (TSP) . However, conclusive evidence for this suggestion has not been report ed. This study was conducted to evaluate the suitability of the Pi tes t for both PR and TSP. In a greenhouse study, two acid soils - Hartsel ls silt loam (fine-loamy, siliceous, thermic Typic Hapludult) and Hiwa ssee clay loam (fine, kaolinitic, thermic Rhodic Kanhapludult) - were mixed with ground (0.15-mm, 100-mesh) North Carolina (NC) PR or commer cial-grade granular TSP at 0 to 300 mg P kg (1) soil. The soil samples were preincubated for 2 wk before maize (Zen mays L.) was planted and grown for 5 wk. Prior to planting, soil samples were analyzed by thre e soil tests: Brag I, Pi test with 0.01 M CaCl2 and Pi test with 0.02 M KCI, The results showed that NC-PR and TSP were equally effective in increasing dry-matter yield and P uptake by maize in Hartsells soil. North Carolina phsophate rock was approximately 83 and 91% as effectiv e as TSP in increasing dry-matter yield and P uptake, respectively, in Hiwassee soil. When P uptake was plotted against soil test values, bo th the pray I and Pi test with CaCl2 underestimated available P from N C-PR for TSP-treated soil. Available soil P estimated by the Pi test u sing KCI instead of CaCl2 was more closely related to P uptake with bo th NC-PR and TSP. More P was extracted from NC-PR by the Pi test with KCI than viith CaCl2, whereas no effect was observed for TSP.