Newsletter from Danish Research Centre for Organic Farming • April 2004 • No. 1

Crop cover improves the nitrogen status in vegetable crop rotations

By Kristian Thorup-Kristensen, Danish Institute of Agricultural Sciences, Department of Horticulture

The DARCOF vegetable crop rotation is a six-year rotation with vegetable crops in three of the years and cereals with undersown legumes in the other three years. During the first six years we used one year for green manure, and thus had only two cereal crops in the rotation. To study how far we can get towards optimal N husbandry, we chose not to apply any manure to the rotation, and thus the N supply is based solely on legume N fixation and measures to reduce N losses.
The vegetable rotation includes cereal crops, as problems with weeds, pests, diseases, and soil structure often become serious in very intensive vegetable rotations.

Crop cover

The main tool for optimal N management in the rotation is to maintain crop cover on the soil during the autumn season as often as possible. After cereals the crop cover is established as grass clover mixes undersown in the cereal; after wining peas the crop cover is established by sowing fodder radish after harvest; and after white cabbage we leave the cabbage stubble to continue growth in the late autumn. Only 25% of the area of the rotation, i.e., the fields grown with onion, leek, and carrot, are left without effective autumn cover, as these vegetable crops are harvested too late to allow establishment of an effective cover crop after harvest. Without cover, N is leached down the soil profile after these vegetables. Therefore, in the next year, we try to grow deep-rooted main crops or cover crops to recover N leached to larger soil depths.

The results have shown autumn soil cover to be an efficient method in N husbandry. On average across years and crops, we found only 37 kg N per ha in the soil in November (measured to 1.5 m depth) where we had a crop cover, but 113 kg N per ha where we did not have crop cover, showing much larger risk of N leaching loss during winter. The various crop covers were also efficient in releasing N for the subsequent crops and in May in the subsequent spring we found on average 127 kg N per ha where an autumn crop cover had been incorporated, but only 95 kg N per ha where there had been no cover. Thus, in the fields without autumn cover a net loss of available N had occurred from November to May, even though some N mineralization must also have occurred, whereas a large increase in available N was observed in the fields grown with some sort of crop cover. The crop cover also affected the depth distribution of the available N, and in fields with autumn cover as much as 69% of the available N was found in the 0-0.5 m soil layer, compared to only 46% in fields without autumn cover. Without an autumn cover 31% of the available N was found in the subsoil between 1.0 and 1.5 m depth, where its availability to many crops is limited.

Deep-rooted crops

The strategy of growing autumn crop covers aim at retaining N within the growing system, and in the topsoil. Where this is not possible, the strategy of growing deep-rooted main crops or cover crops aim at recovering some of the N leached to larger soil depths. Especially the crucifer species of white cabbage and fodder radish have shown very deep rooting, and the results from the rotation show that they can effectively take up N leached to 1.5 m depth. Results from other experiments where we have measured even deeper have shown that they may be effective even to 2.5 m depth.

Generally, the N management strategies have worked well, and we have harvested good vegetable crops during eight years without adding any external N sources to the fields of the rotation. Obviously, the N supply for the crops has not been high, but it has been high enough to obtain good yields of several of the crops. As we have grown the vegetable crops at the best positions within the rotation, it is especially the cereal crops, which have been somewhat N limited. Anyhow, the cereal yields have been acceptable, and the limited N supply has lead to very successful establishment of the undersown legume cover crops, which have then worked well as green manures during the autumn after cereal harvest. The only vegetable crop, which seems to have been significantly N-limited, is white cabbage. We have harvested reasonable yields of about 65 t per ha, but there is no doubt that the crop could have produced more, if N fertilizer had been applied.

Weeds, diseases and pests

The rotation has given many other results and experiences. In the diverse rotation weeds have clearly been less of a problem that anticipated. Also some diseases and pests have given fewer problems than expected. This is especially the case with downy mildew in the onions and with the carrot root fly, which have generally caused fewer problems in the organic grown crops than in similar conventional crops grown nearby. The slower spread of downy mildew may be due to a less dense leaf cover than in conventional crops, but the reason for the limited problems with the carrot root fly is not known. One pest, which has caused more problems than anticipated, is the pea leaf weevil. One reason may be that the fields of the rotation are small compared to farmer’s fields, and this makes it easy for the weevils to move from last years pea field the new pea crop. Whatever the reason, the pea leaf weevil has made the peas the maybe least successful crop, which has been something of a surprise in an unfertilized rotation, where pea as a legume crop was expected to pose few problems compared to other crops.


Based on these results, the main conclusion from the rotation is that the need for import of N containing manures to vegetable crop rotations can be strongly reduced. Many of the techniques used to obtain this result can of course be used to improve N management in other types of organic crop rotations as well.