3. Lastly are inorganic low solubility compounds that include metal
ammonium phosphates, e.g. KNH4PO4 and MgNH4PO4, and partially
acidulated phosphate rock (PAPR).
A classification of CRFs can also be based on the mode of control
release, i.e. diffusion, erosion or chemical reaction, swelling and osmosis.
Blaylock [11] however, classified CRFs as only two major types: those
coated with low solubility compounds and those coated with water
soluble materials.