The present investigation was carried out in the period 1977-1981 at the Department of Soil Science and Plant Nutrition of the Agricultural University Wageningen, Netherlands. This university supplied the major financial sup­ port through a research grant, with additional financial support rendered by the Dutch Nitrogen Fertilizer Industry and Unilever Research, Netherlands. This support is gratefully acknowledged. In the present form this report is the author's doctoral dissertation submitted to the Faculty of the Agricul­ tural University. The author hereby wishes to thank prof. dr. ir. A. van Diest for his criti­ cal reading and correction of the English text, and drjr. J. H. G. Slangen for his valuable advices during the investigations and in the course of the preparation of this report. The au thor also acknowledges the valuable support received from several Agricultural Research Institutes and University Depart­ ments which made facilities and manpower available to conduct the ex­ periments and to transform the results obtained into the present report. Contents Preface / v Abstract / 1 List of ab breviations / 2 Introduction / 3 2 Literature / 6 2. 1 Nitrogen transformations in soil / 7 2. 2 Nitrate uptake / 7 Nitrate reduction in plants / 7 2. 3 2. 4 Nitrate contents in plants / 8 2. 5 Cultural measures / 9 2. 5. 1 Nitrogen dressing / 9 2. 5. 1. 1 Nitrogen amount / 9 2. 5 . 1.