An inhibitor composition contains dicyandiamide as a nitrification inhibitor, alkyl thiophosphoric triamide as a urease inhibitor, or a combination thereof, dissolved in a liquid medium comprising an organic solvent selected from, among others, one or more polar aprotic solvents, including one or more organophosphates, amine solvents, heterocyclic alcohol solvents, and mixtures thereof, is useful in making fertilizer compositions and in a method of fertilizing target plants.