Subproject 1
Nitrogen use efficiency of vegetables

Participants
Head of research unit Kristian Thorup-Kristensen and researcher Hanne L. Kristensen, Dept. of Horticulture, Danish Institute of Agricultural Sciences, Årslev.

Emails: ktk@agrsci.dk; Hanne.Kristensen@agrsci.dk;

Background

Nitrogen is often limiting yields in organic vegetable production. A successfull production therefore depends on the N use efficiency of the crops, that is how efficiently the N left in the soil last year can be used. This means that growing a crop successfully not only means achieving a good yield but also a good N effect for the crop next year. In this project we have worked with the use of "spring green manure" to improve the N supply the next season. In addition we have investigated the properties of 6 different vegetables, properties that are important to known to be able to optimise N supply and uptake by the placement of the vegetables in crop rotation.

Activities

Experiments have been performed with white clover and medic as green manures. The green manures were established in a grain crop in the autumn and left growing after harvest. The next spring the green manures were incorporated into the soil at three dates: before start of growth and after spring growth periods of different lengths. The effects of the different incorporation times were measured in the following crop of white cabbage.

Yield, N uptake, root growth and other measures were obtained from 6 vegetable crops: beetroot, celleriac, sweet corn, potatoe, chinese cabbage and squash. The results improves the ability to place the crops in crop rotations with the aim to increase the N use efficiency of the rotations by a better use of N left behind by last years crops as well as of N left after harvest of the vegetables under study.

Results

There was no effect of spring incorporation time for the green manures. They all had the same good effect on the N supply for white cabbage. This was despite the fact that this kind of prolonged growth periods in the spring has shown good results in USA. The lack of effects was probably due to the fact that the green manures grew well and had a good effect already in the autumn. In cases where green manures are established "a bit to late" it is likely that there will be a good effect of leaving them growing for some extra weeks in the spring.

The 6 vegetable crops showed big differences in root depth and N use. Celleriac had a shallow root system of only 0.4 m depth, potatoes reached 0.6-1 m depth whereas squash and beetroot had deep root systems (>1.5 m). Sweet corn and chinese cabbage showed results in between these extremes. None of the crops depleted N efficiently to more than 1.25 m depth, thus being far less efficient in N uptake with depth as for example white cabbage. The root depths tell us where to place the crops in crop rotation to achieve the optimal N use efficiency. Substantial amounts of N will often have leached to deeper soil layers when N has been left behind by the previous crop. This N can only be used by deep rooted crops whereas crops with shallow root systems cannot. Shallow rooted crops should therefore be grown where only small amounts of N has been left by the previous crop, and then be fertilized with animal manure, or grown after spring incorporated green manure or catch crops which have gathered the available N in the shallow soil layers.

Figure 1 Root densities of 3 vegetables at harvest. There was good agreement between measurements of root depths and the ability for depletion of available N in the deeper soil layers.

Scientific publications

Kristensen, H.L. and Thorup-Kristensen, K. 2004. Uptake of 15N labeled nitrate of sweet corn, carrot and white cabbage from 0.2 to 2.5 meters depth. Plant and Soil (in press).

Thorup-Kristensen, K. (2002) Utilising differences in rooting depth to design vegetable crop rotations with high nitrogen use efficiency (NUE). Acta Horticulturae 571: 249-254

Danish publications

Christiansen, J.S., Kristensen, H.L. og Thorup-Kristensen, K.(2004) Rodsystemer hos rødbede, sukkermajs og knoldselleri efter grøngødning. Forskningsnytt 4: 20-21.

Thorup-Kristensen, K. (2002) Viden om rodvækst er nyttig. Økologisk Jordbrug 22(277), 14

Kristensen, H.L. and Thorup-Kristensen, K. (2003). Geotekstiler som rodspærre. Miljøprojekt nr. 773, 2003. Rapport fra Miljøstyrelsen, 31 pp.

Kristensen, H.L. and Thorup-Kristensen, K. (2003) Geotekstiler kan forhindre forurening gennem rødderne. Ny Viden 4: 36-39.