Time-Dependent Phosphorus Extractability from Soils Treated with Different Fertilizer Phosphorus Sources

Effect of application of different fertilizer phosphorus (P) sources (diammonium phosphate—DAP, single super phosphate—SSP, phosphate rock—PR, partially acidulated phosphate rock—PAPR, basic slag phosphate—BSP, and humo phosphate—HP), and time (up to three years of soil-fertilizer P equilibration) on soil extractable P was studied on two representative Italian soils, principally differing in pH values and the capacity to retain P. Phosphorus extractability was measured by seven chemical and two nonconventional soil P test methods based on the use of anion exchange resin membranes and iron oxide coated paper strips. The increases in the amount of extracted P following soil P fertilization were practically in the same order for both Ravenna and Paliano soils: DAP > SSP > HP > BSP > PAPR > PR; that order reflecting the corresponding scale of P water solubility. More P tended, however, to be dissolved from PR treatment in more acidic soil. On an average, the effectiveness of the different soil P test procedures in extracting P from two soils was in the order: Egner-P > Bray2-P > Olsen-P > Mehlich3-P > resin-P > iron oxide-strip-P > Bray1-P > water-P > CaCl2-P. The Egner soil P test method appeared to overestimate P bioavailability of all P treatments in the calcareous soil, while both the Egner and Bray2 soil tests overestimated the amount of extractable P of the PR treatment in the other soil. Data of P extracted by the Olsen and Mehlich3 procedures resulted the most closely correlated with the corresponding data obtained with the nonconventional soil P test procedures, currently considered with potential for use in widely ranging soils fertilized with water soluble as well as water insoluble P fertilizers.