Six years results from an organic vegetable crop rotation aimed at self-sufficiency in nitrogen

One of the main constraints of organic vegetable production is lack of available N, which causes reduced yields and quality of the to the crops. Therefore, we started an organic vegetable crop rotation in 1996 where we could study the possibilities for improved N utilisation. The N supply to the crops is based on the effects of nitrogen catch crops, green manure crops and crop sequences aimed at a high utilisation of precrop effects, whereas no input of any manures are made. Measurements and experiments have been made within the crop rotation to test the effectivenes of the N strategies used. \nThe results show that it have been possible to maintain good yields of the crops in the rotation. Yields of leek and carrot have been as high as in conventional cropping. Yields of onion, cabbage, green peas, and the two spring barley crops included in the crop rotation have been lower, but still good by organic standards. \nThe measurements of N dynamics show that autumn soil cover is an important part of the N strategies. Whether the autumn soil cover is a green manure, a catch crop, or crop residues left growing after harvest, it makes N available in the spring rather than in the autumn, and the available N is found closer to the soil surface. The results also show, that when it is not possible to establish an autumn soil cover, it is important to grow deep rooted crops in the next year, to recover some of the N leached to greater soil depths. Finally, undersowing of legumes in spring cereals have been found to be an attractive method, allowing both a grain cash crop and an effective green manure to be grown within the same year. \nAll together, the results show that limited N supply do not have to be a main constraint to organic vegetable production, especially as organic farms have the possibility to combine strategies as those used in our crop rotation together with whatever resources of organic manures may be available to them. \n