A method for determining an amount of a nitrogen-stabilizing additive selected from nitrification inhibitors, urease inhibitors and denitrification inhibitors, to be applied jointly or separately with a nitrogen containing fertilizer, comprising determining values of at least two parameters influencing the efficacy of the nitrogen-stabilizing additive, determining an amount of nitrogen containing fertilizer that has been applied or is to be applied, determining the efficacy of the nitrogen-stabilizing additive on the basis of said values of said at least two parameters and calculating the necessary amount of nitrogen-stabilizing additive to be applied on the basis of said efficacy of the nitrogen-stabilizing additive and of said amount of nitrogen-containing fertilizer application. Furthermore, the method relates to an apparatus (1) for determining an amount of a nitrogen-stabilizing additive.