This investigation was carried out to produce a high quality magnesium compounded fertilizer by initially dissolving calcined dolomite in sea water to form the intermediate Mg（OH）₂ precipitate, and reacting this intermediate with phosphoric acid and ammonia thereafter.
The experimental results were obtained as follows ;
1） The optimum calcination temperature of dolomite was about 950 ℃ for 30 minutes operation and in the displacement of the calcined dolomite in sea water, the optimum mole ratio of calcium hydroxide to magnesium chloride is 1 : 1.2 for 4 hours of the dissolution time.
2） The high yield of the magnesium compounded fertilizer was obtained, when the precipitate was reacted with 20 % phosphoric acid and neutralized with 28 % ammonium hydroxide thereby obtaining the alkalinity of the solution to pH 9.
3） By the results of X-ray diffraction and chemical analysis, the major component of the product was magnesium ammonium phosphate at the above condition and the fertilizing effect was also good.