Phosphorus fertilizers are processed from rock phosphate mined from apatite mineral deposits around the world. The processing increases the availability of reactive and water-soluble P content. Many products formulations are available. The P content of any fertilizer is listed in the N-P-K ratio on the label as the percent P2O5.
Water solubility is a measure of the fertilizer’s ability to dissolve into the soil solution. Some of the water-insoluble fraction of the fertilizer P can be extracted by citric acid. The remaining P is citric insoluble and remains in the soil until soil processes mineralize the insoluble P. The water soluble fraction and the citric acid soluble fraction comprise the total plant available P. The formulas to convert these factors are based on the molecular weight:
%P = % P2O5 x 0.43
% P2O5 = %P x 2.29
Using a higher solubility fertilizer, while perhaps best for the plant, increases the risk of leaching or runoff contamination. Phosphorus fertilizers are listed in the table below with the corresponding fraction of Total Plant Available P (water soluble and citric soluble fractions).
Phosphate fertilizers (Tisdale, 1993)(Turgeon, 1985)
|Fertilizer||%N||%P2O5||%P||% Total P Available||Cold Water Solubility||Salt Index|
|Monoammonium phosphate (MAP)|
|Diammonium phosphate (DAP)|
|Urea ammonium phosphate|