It is a double salt of ferrous sulphate and magnesium sulphate. Normal ferrous sulphate is a nutrient which will get oxidized to ferric sulphate. Ferrous sulphate is soluble in water while ferric sulphate is insoluble in water and hence the plant do not absorb the iron. To avoid this loss of iron, ferrous sulphate is stabilized by the addition of magnesium sulphate. The presence of magnesium sulphate prevents the oxidation of ferrous ion to ferric ion and prevents its loss.