Soil testing for available phosphorus in soils where phosphate rock-based fertilizers are used

Traditional soil tests for phosphorus (P) were developed to arrive at fertilizer recommendations when water-soluble P fertilizers have been used. When slowly water-soluble fertilizers such as phosphate rock (PR) have been used, soil tests using acidic extractants overestimate bioavailability of P, whereas alkaline extractants underestimate it. Therefore, separate calibration curves are needed for soluble and PR-based fertilizers. There are two soil tests that show promise as suitable tests in soils fertilized with soluble as well as PR-based fertilizers. These are the iron oxide impregnated paper (P-i) test and the ion exchange resin paper test. In both cases, the strips act as a sink for P mobilized in a soil solution, and P measured depends only on the concentration of P mobilized in the solution and not on the source of P or properties of the soil. Both tests somewhat simulate the sorption of P by plant roots without disturbing the chemical equilibrium, unlike other tests that extract P by the destructive dissolution of specific soil P compounds. Tn both cases, P measured from soils fertilized with PR-based fertilizers has shown very good correlation with plant response. Field calibration with crops under different pedological and agroecological regimes is needed for using these soil tests in developing fertilizer recommendations.