Artificial and natural PO(4) and NH(4)-contg. sewage sludges were treated with MgCl(2) at pH 8.5 to produce MgNH(4)PO(4)center dot 6H(2)O crystals. The crystals were sepd. by filtration, studied for size distribution (40 and 80 mu m in size in av.) and used as fertilizers for growing white mustard. The dry plant material contained similar P and N amts. as the plants fertilized with corn. superphosphate.