Laboratory preparations of pure ammonium ortho- and pyrophosphates were blended to give mixtures containing 0, 20, 40, 60, 80, and 100% of the total P as pyrophosphate at pH values of 3.6, 4.0, 5.0, 6.0, 7.0, and 8.0. Each ammonium phosphate mixture was blended with ZnSO4 · H2O to yield a product containing 2 or 8% Zn, formed into a pellet, and reacted with soil solution. Residues were recovered and analyzed for chemical and mineralogical composition.
The quantity and species of Zn precipitating in the residues were markedly influenced by the pyrophosphate content and pH of ammonium phosphate mixtures. At pH 3.6 and 4.0, the quantity of Zn in the residue was increased with increasing pyrophosphate content. At pH 5.0 all of the Zn precipitated when the P was 100% orthophosphate but as the pyrophosphate level was increased to 60% more Zn moved out of the pellet. The Zn content of the residues decreased as pH increased (6.0, 7.0, and 8.0) and was lowest when the mixture contained 40, 60, and 80% pyrophosphate, respectively.
Two zinc ammonium orthophosphates, ZnNH4H3(PO4)2 · H2O and ZnNH4PO4, were identified in the residues. Zinc ammonium pyrophosphates identified were Zn(NH4)2H4(P2O7)2 · 2H2O, Zn3(NH4)2(P2O7)2 · 2H2O, and Zn(NH4)2P2O7 · H2O.
Ammonium phosphate mixtures with pH values of 3.6, 6.0, and 8.0 containing 2% Zn as 65ZnSO4 · H2O were spot-placed in pots and evaluated in growth chamber experiments. Plant uptake of Zn was proportional to the quantity of Zn which moved from the placement site.