Among the desired nutrient effect of fertilizer, the shelf-life of a high quality fertilizer is also an important property. In terms of the shelf-life of the fertilizer the water adsorption is a key factor. During the production process of the fertilizer the additive Al-2(SO4)(3) center dot 14H(2)O was added to improve the product quality. This results in different aluminum modifications. In this study the mechanism of water adsorption of the fertilizer composed of urea and ammonium sulfate is described using aluminum hydroxide Al(OH)(3) and its polymorphs bayerite, gibbsite and nordstrandite and aluminum oxide hydroxide AlO(OH) (boehmite) for an explanation.