The present invention relates to compositions containing at least 80% by weight, especially at least 90% by weight, based on the total weight of the composition, of at least one magnesium sulfate-urea compound selected from the compound of the formula (I) and mixtures of the compound of the formula (I) with the compound of the formula (II): [MgSO4.mCO(NH2)2.nH2O] (I), [MgSO4.xCO(NH2)2.yH2O] (II), in which m and x are each in the range from 0.9 to 1.1, n is in the range from 1.9 to 2.1 and y is in the range from 2.9 to 3.1, where the compositions, based on the total weight of the composition, contain less than 10% by weight of free MgSO4 in the form of the anhydrate or in the form of hydrates of magnesium sulfate and less than 10% by weight of unbound urea. The invention also relates to the production of the compositions and to the use thereof as fertilizers or fertilizer additive.
