Solid–Liquid Phase Equilibrium of Ammonium Dihydrogen Phosphate and Agricultural Grade Ammonium Polyphosphate (Degree of Polymerization Ranging from 1 to 8) for Mixed Irrigation Strategy

[Image: see text] Water-soluble ammonium polyphosphate (APP) has the advantages of good solubility and slow-release characteristics and has the potential to be used in combination with monoammonium phosphate (MAP) as a high phosphorus content slow-release fertilizer to improve the utilization rate of phosphorus during irrigation. Herein, the effects of the APP1 concentration and temperature (278.2–313.2 K) on the solubility of MAP, solution density, and pH value in the ternary equilibrium system (APP1–MAP–water) were measured. The simplified Apelblat model, two empirical polynomials, and rational two-dimensional functions can describe the experimental solubility data, solution density, and pH value well, respectively, with reliable modeling parameters (R(2) > 0.99). In the OptiMax1001 reactor, the focused beam reflectance measurement (FBRM), the particle-view measurement (PVM), and the ReactIR 15 probes were used to observe and reverse verify that they can be synergistically codissolved to achieve economic efficiency. Basic thermodynamic data and models can guide their collaborative application in irrigation to improve the phosphorus utilization rate.