By using soil as substrate, white and yellow lupines (Lupinus albusL.,Lupinus luteusL.) assimilated higher N amounts than under quartz sand conditions. This was caused by spontaneous infection of lupines with wild Rhizobia strains and also by an additional N uptake from the soil. In yellow lupines without inoculation in non-sterile soil, only the additional N uptake played a role. Differences in P and K supply as the cause of different N acquisition from soil and quartz sand could be excluded. As compared with white lupines, yellow lupines inoculated with Rhizobia had a high N2fixation that exceeded the effect of spontaneous infections. This result as well as the positive effect of spontaneous infections with soil-borne Rhizobia on white lupines indicates insufficient effectiveness of the strains used for inoculation on this plant species.