Modified techniques for preparing paper strips for the new P_i soil test for phosphorus

Abstract

The P_i soil test, a new approach to measure plant-available P, uses strips of filter paper impregnated with iron oxide as a collector for P in the soil suspension. A modified method for impregnating the paper strips with iron oxide was developed by exposing the FeCl₃-treated paper to ammonia vapor instead of immersing it in NH₄OH solution. The paper strips prepared by the vapor technique had a more uniform deposit of iron oxide, and the P extracted from four soils ranging in pH from 4.5 to 8.2 and fertilized with two P sources and three rates correlated significantly with dry-matter yield (r = 0.751) of maize.

The vapor technique was effective in preparing strips impregnated with both Fe and Al oxides. The ratio of Fe and Al on the strips was about the same as that in the solutions in which the papers were immersed. Strips containing iron oxide alone extracted more P than did strips containing both Fe and Al oxides. The dry-matter yield and P uptake of maize correlated significantly with P extracted by the strips impregnated with Fe, Al, or (Fe + Al) oxides. Phosphorus extracted by the paper impregnated with Fe oxide, and a mixture of Fe and Al oxide in the ratio of 2:1 correlated best with dry-matter yield (r = 0.751 and r = 0.736) and P uptake (r = 0.776 and r = 0.777).