Phosphorus Availability to Corn as Affected by Granulating Manganese with Ortho‐ and Pyrophosphate Fertilizers

The presence of MnSO 4 at a level of 10% Mn in granular ammonium polyphosphate (APP) and triammonium pyrophosphate (TPP) significantly reduced corn ( Zea mays L.) forage yields and P uptake relative to treatments where P and Mn were applied separately in greenhouse pots. A significant reduction in P uptake resulted when MnSO 4 was combined with monoammonium phosphate (MAP), but no yield reduction occurred. Chemical extraction of the fertilizers indicated that the solubilities of both P and Mn were reduced when MnSO 4 was granulated with APP or TPP. A compound of very low solubility, Mn(NH 4 ) 2 P 2 O 7 · 2H 2 O, was identified as the principal reaction product. Accordingly, movement of P in soil was restricted to a greater extent when MnSO 4 was combined with TPP than with MAP.