Vegetables

Summary

The project has aimed to improve the possibilities for organic vegetable production in Denmark. This has been done through four sub-projects dealing with different topics.

a) Green manure and catch crops
Lack N and other plant nutrients seems to be one of the most important factors limiting the development of organic vegetable production and organic crop production in general. We have worked with catch crops and autumn grown green manures as methods to reduce N losses and improve the N supply for the main crops. We have found that green manures established in the late summer can supply substantial amounts of N to the crops, and improve the depth distribution of N in the soil. The improved depth distribution is especially valuable before shallow rooted vegetable crops as onion or lettuce. Measurements of root growth among five vegetable crops showed large differences in root growth rates, and the final rooting depths varied only 25 cm for onion to 100 cm for early cabbage. These differences are important for the optimal placement of the vegetable crops within a crop rotation.

Also catch crops were found to have very different root growth. The most deep-rooted species, which were found among the crucifers, were able to exploit deep soil layers below 100 cm, layers not reached by the other catch crops or by many main crops. This will have important consequences for the effect of different catch crop species on nitrogen leaching losses.

b) Green peas
Green peas is an important vegetable crop in Denmark. In organic farming weed control is a main problem in this crop, but the weed control methods used in other vegetables cannot be used in green peas. We have studied the possibilities for increasing the competitive ability of the pea crop by using competitive cultivars and increasing the seeding rate. Increasing the seeding rate clearly improved the competitive ability of the pea crop, and there were significant differences among the cultivars. Cultivars with a fast early growth had the best competitive ability, and in practical terms this means that large seeded genotypes with normal leaf type were found to be optimal for organic production.

c) Weed control
Weed control by hand weeding is expensive for organic vegetable producers. The problems are especially severe in directly sown crops with slow establishment and low competitive ability. Therefore we have studied methods for weed control in sown crops of onion and leek. The aim was to combine methods for physical weed control to leave as few weed plants in the crop rows as possible. Methods as seed priming or placement of manure aimed at improving the competitive ability of the crop did not improve weed control. Optimal combinations of flaming, brush weeding, hoeing and weed harrowing reduced the number of weed plants left in the crop rows with between 40% and 90%, and reduced the time needed for hand weeding similarly.

d) Soil borne diseases
Legumes are important crops in organic production, as they are essential for the N supply of the farming system. One of the factors which limit the use of legume crops is their succeptebility to soil borne diseases. We have studied the possibility to reduce problems with soil borne diseases by growing catch crops. Generally, catch crops reduced the incidence of soil borne diseases on the root system of peas, and the colonisation of the root system by Aphanomyces euteiches, but they increased the colonisation of the root system by mycorrhiza.

The results confirm that catch crops in general do reduce problems with soil borne diseases. Still, the results also show that we need to know more about the mechanisms behind the disease suppressive effects before we can predict the effects of catch crop species and management on the soil borne diseases. Therefore, at present we are not yet able to give advise about how to optimize the disease suppressive effect of catch crops.