The small amount of magnesia contained in tricalcium phosphate replacing lime stabilizes β modification of the phosphate. However, a new compound Ca3Mg3(PO4)4 is formed below 1175°C when more than 0.6 mol. of MgO to 1 mol. of P2O5 is introduced into the phosphate. This compound is soluble in citric acid but less soluble in neutral ammonium citrate similarly as is serpentine-fused phosphate fertilizer. In Ca3(PO4)2-CaNaPO4 system, a crystalline solution with α-rhenanite structure is easily formed within the range of 2.6 CaO· 0.4 Na2O· P2O5—2.4 CaO·0.6 Na2O·P2O5, and is furthermore stabilized by the admixture of calcium orthosilicate. This crystalline solution is highly soluble in both citric acid and citrate, composing a principal part of low-alkali calcined phosphate fertilizers.