Orthophosphate Solubility Transformations and Availability from Dual Applied Nitrogen and Phosphorus

Knife applications of NH⁺₄-N and P fertilizers into common injection zones (dual N and P) have produced higher crop yields and P absorption than other N and P application methods. Enhanced solubility of the P fertilizer reaction products due to the presence of NH⁺₄ could account, in part, for these crop responses. Greenhouse studies were conducted to characterize P reaction product solubility transformations and P availability in dual N and P injection zones. Treatments were ammonium polyphosphate (APP) + NH₄OH (used to simulate anhydrous ammonia), NH₄OH or KNO₃ + nitrapyrin solutions injected simultaneously (dual N and P application) into four different soils (Aridic Argiustolls, Udic Argiustolls, Mollic Albaqualfs, and Typic Argiborolls). Monocalcium phosphate potentials and NaHCO₃ or Bray extractable P were used to characterize P solubility and availability relationships in the injection zones. Dual application of NH₄OH plus APP with and without nitrapyrin resulted in increased NaHCO₃ or Bray-extractable P in three of the four soils. The mechanisms that resulted in higher extractable P are concluded to be associated with the high injection zone pH caused by NH₄OH applications. The postulated mechanisms are: (i) the initial high (alkaline) pH immediately after dual application allows formation of highly available Ca-P reaction products as opposed to less available Al- and Fe-P reaction products that exist in slightly acid soils, (ii) then the production of H⁺ during nitrification increases P solubility as phosphate potentials move up the dicalcium phosphate dihydrate and dicalcium phosphate isotherms, and (iii) this rapidly decreasing pH allows the soluble reaction products to persist which results in higher P availability and solubility for a longer period of time. Inclusion of nitrapyrin (a nitrification inhibitor) prolonged the time of Ca-P reaction product persistence in the injection zone. Dual KNO₃ + APP applications resulted in suppressed P availability, as compared to NH₄OH + APP, apparently due to formation of less soluble reaction products. This suppressed P availability could account for the relative ineffectiveness of NO⁻₃ -N in dual application.