CALCIUM METAPHOSPHATE

Calcium metaphosphate [Ca(PO₃)₂] is available as vitreous flakes. It is formed by treating phosphorus rock with gaseous phosphorus pentoxide (P₂O₅). Calcium metaphosphate is also obtained by burning elemental phosphorus with calcium oxide at high temperatures in the presence of oxygen. The molten product is tapped out of the furnace, solidified on a water-cooled steel drum, crushed and passed through a 10-mesh screen to obtain 1.6 mm size material.
Pure calcium metaphosphate contains about 72% phosphorus pentoxide and is not water soluble. However, it hydrolyzes slowly, forming water-soluble compounds. More than 98% of phosphorus pentoxide (P₂O₅) in calcium metaphosphate is soluble in neutral ammonium citrate.
The major drawbacks of calcium metaphosphate as a fertilizer are its (a) dependence on elemental phosphorus, and (b) agronomic effect, caused by the rate of its hydrolysis in the soil, which is too slow for short duration crops ( < 90 days).