It has the advantage of being highly water soluble, having a high analysis and often a price advantage. Diammonium phosphate has an acid effect upon the soil similar to anhydrous ammonia. Because of the high ammonia content, this material can cause germination injury if used in direct contact with the seed. Monoammonium phosphate has a lower ammonia content and may be less injurious to germinating seeds than diammonium phosphate. The general agronomic effects of diammonium and monoammonium phosphates are equal for most soils.
Polyphosphates differ slightly from the more common orthophosphate fertilizers.
Nearly all of the liquid fertilizers containing P are of the polyphosphate type. Commercial ammonium polyphosphates are usually a mixture of ortho- and polyphosphate. With prolonged storage, polyphosphates will hydrolyze to orthophosphates. The time required for hydrolysis to occur varies with soil conditions. Under cool, dry conditions, hydrolysis may take longer.