Manganese Reactions and Availability as Influenced by pH and Pyrophosphate Content of Ammonium Phosphate Fertilizers

Ammonium phosphate, composed of mixtures of ammonium ortho- and pyrophosphate, were tested to determine their effectiveness as carriers of Mn. The mixtures contained 0, 20, 40, 60, 80, or 100% of their total P as pyrophosphate with the remainder being orthophosphate. Each series of mixtures was prepared at saturated pH values of 3.6, 4.0, 5.0, 6.0, 7.0, and 8.0. Residues, formed when the ammonium phosphates were blended with MnSO₄ · H₂O and reacted with the soil, were analyzed to determine their composition. Essentially all of the applied Mn precipitated as manganese ammonium phosphate when the pH of the fertilizer was greater than 6.0. As the pH was decreased from 5.0 to 3.6 there was a decrease in precipitated manganese with decreasing pyro- to orthophosphate content at each pH value. When the fertilizer pH was 3.6 or 4.0 and the P was present as 100% orthophosphate there was no residue formed indicating complete movement of Mn from the site of placement. Four manganese ammonium phosphates were identified in the residue samples. The compound Mn(NH₄)₂H₄(P₂O₇)₂·2H₂O was present at pH 3.6 and 4.0 and a pyrophosphate content of 40% or greater. Mn₃(NH₄)₂(P₂O₇)₂·2H₂O formed at pH values of 5.0 or less and with pyrophosphate contents of 20% or greater. When the fertilizer pH was 5.0 or greater Mn(NH₄)₂P₂O₇·H₂O was the dominant compound identified in the residues. The only manganese orthophosphate identified was MnNH₄PO₄·H₂O which precipitated at pH values above 5.0 when 80% or more of the P was present as orthophosphate. Plant utilization of Mn spot-placed with ortho-pyrophosphate mixtures as measured with ⁵⁴MnSO₄·H₂O was proportional to decreasing precipitation of applied Mn. Maximum uptake of applied Mn occurred at a pH of 3.6 and 100% orthophosphate. Residual uptake of Mn from reaction products blended with the soil was high from all treatments. There was a tendency for a reduction in Mn uptake with increasing pyrophosphate at each level.