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Book of Abstracts

11th IFOAM
Scientific Conference
11-15 August 1996
Copenhagen, Denmark

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N Effects of Green Manures for Vegetables S20

Thorup-Kristensen, K.

Danish Institute of Plant and Soil Science, Dep. Fruit and Vegetables, DK-5792 Aarslev

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Autumn grown green manure crops can supply substantial amounts of available nitrogen for succeeding vegetable crops. Contrary to many other organic nitrogen sources they can mineralize substantial amounts of nitrogen even in a cold spring soil (Thorup-Kristensen, 1994), and this will be important to many vegetable crops. Further, previous results show that effect of green manures depends strongly on the character of the succeeding crop, especially its effective rooting depth.

The objective of the present study was to examine the effect of green manures on 1) Nitrogen leaching losses, 2) Nitrogen supply for succeeding crops, 3) Yield and nitrogen uptake of two crops with contrasting root development: Onion (Allium cepa L.) and white cabbage (Brassica oleracea L. convar. capitata).

Six green manures were established in the early autumn, and their nitrogen uptake and depletion of available soil nitrogen were measured. In the spring, the plots were split and grown with onion or cabbage. During the season nitrogen mineralization was followed by soil analysis and by growth and nitrogen uptake in the vegetable crops.

The results show that pure legume green manure crops did not prevent nitrogen leaching efficiently, but that mixtures of legumes and non-legumes did. At the time of vegetable establishment, one month after incorporation of the green manures, nitrogen availability had already been increased with up to 40 kg N/ha. All green manure crops depleted subsoil nitrogen, and mineralized their nitrogen in the topsoil. This concentration of nitrogen in the topsoil increased nitrogen uptake of the shallow rooted onions after all green manures. The concentrating effect was weakest after winter killed pure legume green manures, and only these increased nitrogen uptake in the deep-rooted cabbages. In green manures containing also winter persistent non-legumes the depletion of subsoil nitrogen was as important as the nitrogen enrichment of the topsoil.

Thorup-Kristensen, K. (1994): Effect of nitrogen catch crop species on the nitrogen nutrition of succeeding crops, Fertilizer Research, 37, 231-239.