Influence of incorporated magnesium (Mg) into synthetic calcite and dissolved organic carbon (DOC) on P sorption by synthetic calcite (SC), synthetic Mg-calcite (SMC), and some selected calcareous soil samples were examined in different laboratory experiments. The sorption of P onto SC or SMC was time dependent particularly by SMC. After 6 h of reaction, 81% of P was removed from solution by SC which represented 9 folds of that removed by SMC. The sorption progress is characterized by C shape for SC and S shape for SMC, indicating retardation effect of Mg. Sorption of P throughout 24 h reaction period was significantly positive correlated with the equilibrium solution pH (r = 0.90**). Mg incorporated into SC reduced the adsorption of P onto calcite and/or hindered nucleation and crystal growth of the precipitated P. Soils were selected based on Mg/Ca ratio of their carbonate fraction. After 24 h of the sorption, removal of P by soil samples ranged between 22 to 50% of the added P that accompanied with slight increase of pH ranged from 0.07 to 0.31 units. The multivariable regression analysis showed that not only calcium carbonate is the sole factor controlling P sorption in calcareous soils, but also soil texture, ionic strength and organic matter are important factors. Applied dissolved organic carbon (DOC) of poultry manure (PM), wheat straw (WS), and faba bean straw (FBS) inhibited P sorption by SMC and soils containing 49.8 and 40% total carbonate while, had no effect on P sorption by SC and soils containing 30 and 19% carbonate.  It seems that incorporation of Mg into calcite and addition of DOC can enhance P availability through decreasing P adsorption and/or precipitation and can also add a value to the effect of DOC.