Nitrogen mineralization and uptake in leek after incorporation of red clover strips at different times during the growing period.

Green manure crops are an important N-source in organic farming systems with field vegetables. Difficulties in controlling N-mineralization from green manure crops and differences in N-uptake pattern of succeeding crops result in large variations in crop N-uptake and increase the potential risk of N-losses. In this work, time of incorporation of red clover was examined as a means to improve the availability of mineralized N to reek. Strips of red clover, left between the rows of leek. were incorporated at planting, or 2 or 4 weeks after planting. The difference in N-uptake was small between treatments with incorporation of red clover strips between rows at planting and 2 weeks after planting. With the latest time of incorporation the N-uptake was lower. The inter-row clover strips assimilated and transferred inherent soil-N as well as N mineralized from red clover incorporated in the leek rows. This kept mineral-N values at a lour level until time of incorporation. The effect was especially pronounced and prolonged in treatments with the latest time of incorporation and contributed to the low growth rate in this treatment. Low growth rate made it difficult for the leek crop to exploit N mineralized between rows and thereby decreased N-uptake and increased the leaching risk after harvest. Results from this experiment show that intercropping, beyond the crop establishment phase, can not be recommended. However, if other benefits such as decreased pressure from weeds and pests. and improved soil structure are taken into account, intercropping may prove to be economically justifiable.