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 been 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 reported. This study was conducted to evaluate the suitability of the Pi test for both PR and TSP. In a greenhouse study, two acid soils - Hartsells silt loam (fine-loamy, siliceous, thermic Typic Hapludult) and Hiwassee clay loam (fine, kaolinitic, thermic Rhodic Kanhapludult) - were mixed with ground (0.15-mm, 100-mesh) North Carolina (NC) PR or commercial-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 three 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 effective as TSP in increasing dry-matter yield and P uptake, respectively, in Hiwassee soil. When P uptake was plotted against soil test values, both the pray I and Pi test with CaCl2 underestimated available P from NC-PR for TSP-treated soil. Available soil P estimated by the Pi test using KCI instead of CaCl2 was more closely related to P uptake with both 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.