[en] The principles of isotope technique, covering formulations for determining the nitrogen concentration in roots, isotope dilution technique for determining the nitrogen coming from soil and that from fertilizer in plants, as well as formulations for determining the percentage of the fertilizer applied to soil which can be absorbed by plants, are described. The isotope technique for efficient application of fertilizer, especially nitrogen and phosphor for macro nutrient, and zinc (Z) for micro nutrient has been applied. Research on the behaviour and the fate of fertilizer in soil is still to be carried out by using a more sensitive instrument and complicated sampling techniques. (RUW). 7 refs