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).
