Sodium chloride normally called common salt is mainly produced by solar evaporation of sea brine, subsoil brine and backwater brine. After the separation of NaCl in the salt pans, the left out mother liquor is called bittern. Bittern being an unavoidable, toxic and abundantly available by-product in salt-pans, which can be converted into fertilizers to get maximum growth and yield in plants. Bittern is having enriched levels of magnesium and so it is possible to convert in to magnesium content fertilizer of magnesium ammonium orthophosphate. This paper explain the synthesis of magnesium fertilizer from bittern and its application to the magnesium demanding crop onion (Allium cepa).