EFFECTS OF LONG-TERM FERTILIZER N REDUCTION ON WINTER GRAIN AND SUGAR-BEET YIELDS

The results of recent filed experiments concerning the effect of long- term N-reduction on the yield and quality of sugar beet, winter wheat and winter barley on plots which had previously had received ample amo unts of N are studied in this paper. The yield and quality of crops ha rvested on plots where N-dressings had been reduced for 6-8 years were similar to those of crops grown on plots where N-application had been reduced for only 1 year. Grain yield of winter wheat and winter barle y grown without any N-application decreased to about 60% of amounts no rmally harvested under local conditions with recommended N dressing, w hereas the white sugar yield still remained at 90%. The yields decreas ed slightly with an increase in the duration of the experiments. Yield s of both cereals and beets remained constant within each level of fer tilization, even 6 years after inition of trials with 50, 75 and 125% of locally recommended N dressings. On plots that did not receive nitr ogen fertilization, N-contents of grain were between 1.5 and 1.7% for winter wheat and 1.0 and 1.6% N for winter barley. These contents rema ined constant over a trial period of 6 years. The amount of annual exp ort of 55-91 kg N/ha also remained constant. Limited N availability ca uses a decrease in grain protein content rather that in grain yield. C ompared to winter grain species, sugar beet (with 74-117 kg N/ha in th e beet body) could realize the highest annual export of nitrogen for t he plot. Differences in annual N export existing between the various l ocations of the plots cannot be explained by differences in soil quali ty. Continuous high yields that were found even without any N-dressing s may be explained by asymbiotic N-fixation, deposition of atmospheric N and a progressive decrease in soil N with 17-56 kg N/ha removed fro m soil resources annually.