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....