The formation and maintenance of the skeleton, bones and teeth largely depends on phosphorus: 80 % of body‘s phosphorus is located in the bones and teeth, co-precipitated with calcium in the hydroxy-apatite complex. The remaining 20 % is contained in nucleotides such as ATP, nucleic acids, in phospholipids and other phosphorylated compounds. The outstanding importance of phosphorus is therefore also reflected in its metabolic functions: regeneration of energy supply, transfer of fatty acid, formation of amino-acids, cell membrane bilayer and acid-base buffer. Phosphorus is also important in fertility and reproduction as well as in milk secretion, eggshell formation and muscle tissue formation.

Main sources

Dicalcium phosphate dihydrate

Dicalcium phosphate anhydrous
Monodicalcium phosphate

Monocalcium phosphate
Phosphate containing calcium carbonate Magnesium phosphate

Calcium-magnesium phosphate
Sodium-calcium-magnesium phosphate Monoammonium phosphate

Monosodium phosphate

Quality and selection parameters

Depending on the chemical form, the chemical purity and the production process it has been demonstrated by “in vivo” experiments (bone mineralisation) that the relative biological availability (RBV) of feed phosphates can vary between 80% and 100% (available phosphorus). Other “in vivo” experiments determine the phosphorus absorbed in the intestinal tract (digestible phosphorus) mainly for pigs. “In vitro“ methods (solubility) are less accurate but can give an approximation (soluble phosphorus in ammonium citrate or in 2 % citric acid).
Relevant criteria for the selection of phosphorous sources are total phosphorus content, available/digestible phosphorus content, calcium/phosphorus ratio, solubility, pH, level of undesirable substances, physical form (powder / granulated) and apparent density.

Analytical methods

Total phosphorous quinoline, Dir. 77/535/EEC
Phosphomolybdate  gravimetric
Soluble phosphorus   in 2 % citric acid :
in alkaline ammonium citrate (Petermann)
in water 
Calcium  atomic absorption spectrometry
X-ray fluorescence 
Sodium  atomic absorption spectrometry
X-ray fluorescence  
Magnesium  spectra photometric atomic absorption
X-ray fluorescence  

- dicalcium phosphate dihydrate
  - others  

acetone wash method oven drying method

Grade of hydration X-ray diffraction
thermo-gravimetric method

Conversion factors 

P x 2.291 = P2O5  Ca x 1.39 = CaO   Mg x1.66 = MgO  
P2O5 x 0.437 =P  CaO x 0.715 = Ca  MgO x 0.6 = Mg