For general use in the fertilizer
<http://en.wikipedia.org/wiki/Fertilizer>industry, phosphate rock or
its concentrates preferably have levels of 30%
phosphorus pentoxide (P2O5), reasonable amounts of calcium carbonate (5%),
and <4% combined iron <http://en.wikipedia.org/wiki/Iron> and
aluminium<http://en.wikipedia.org/wiki/Aluminium>oxides. Worldwide,
the resources of high-grade ore are declining, and the
beneficiation of lower grade ores by washing, flotation and calcining is
becoming more widespread.