Evaluering af overvintrende grønsager i økologisk jordbrug

Summary

Abstract
A rising demand for organic and local commodities is occurring. Meanwhile the production of vegetables is attached with great risk of nitrate leaching, since the nitrogen use efficiency is low. A field experiment in 2017-2018 was conducted to clarify whether overwintering vegetables 1) can be implemented in organic vegetable cropping systems and 2) are able to reduce nitrogen levels in the soil equally with winter rye (Secale cereale). The project is a screening experiment without repetitions and the results must be interpreted as tendencies rather than significant differences. Five different vegetables are analyzed, with one to three different cultivars, one to two sowing or planting dates and one to two sowing densities. It was found that spinach (Spinacia oleracea), garlic (Allium sativum) and onion (Allium cepa) survived winter only depending on species. Pointed cabbage (Brassica oleracea var. capitata), carrot (Daucus carota var. sativus) and turnip (Brassica rapa var. rapa) did not survive. Despite indications of high frost tolerance from a rising chlorophyll content of the pointed cabbage winterkill occurred. Furthermore, it was found that sowing spinach and winter rye October 18 and 1 respectively, increased yields and decreased nitrogen levels in the soil in April when comparing to sowing November 7 and 1. The overwintering vegetables resulted in higher levels of nitrogen in the soil in April than the winter rye. It is estimated that the overwintering vegetables did not lower nitrate leaching due to high levels of nitrogen in deep soil layers at establishment, shallow roots and late establishment.