Greater biological N fixation requires good management

Leguminous green manure, cover crops and grain legumes can provide substantial nitrogen (N) inputs to cropping systems and thus substitute needs for imports in fertilizers or manure. However, climatic and soil conditions seem to be the most determining factors for the nitrogen contribution of legume components.

Legume based cover crops can contribute to N fixation when well established.

Nitrogen is typically the most important yield-limiting factor for crop production. Achieving sufficient N inputs is, therefore, one of the greatest challenges for ecological intensification in organic farming. There is a restriction in input of animal manure from conventional origin in organic farming in Denmark which is expected to be phased out by 2021. This will make nitrogen management in organic farming even more challenging. 

Inclusion of grain legumes and legume-based whole year green manure crops and cover crops can provide a reasonable amount of N through atmospheric N2 fixation and can substitute external N requirement in organic farming.

Large variation in biological N fixation between crops, sites and years

In a long-term crop rotation experiment in three different locations in Denmark, faba bean, grass-clover grown for a whole year and legume-based cover crops showed potentials to substitute external N inputs (i.e. fertilizers or manure) in organic cropping system.

Data were collected during 2005-2008 as part of the CROPSYS project from the experiment that started in 1997. The experiment included two different rotations. The first rotation was organically managed and called “O2” which includes a whole year green manure (grass-clover) in a 4-year rotation. The second rotation included grain legume (faba bean) and has both organically managed (“O4”) and conventionally managed (“C4”, applied with fertilizers and pesticides) treatments in a 4-year rotation (Table 1). Both rotations were tested both with and without the use of cover crops, and these were legume-based in the organic systems.

Table 1. Structure of the crop rotations in the long-term experiments at three sites in Denmark during 2005-2008.

O2 (green manure rotation)

O4/C4 (cash crop rotation)

Spring barley:ley

Spring barley*


Faba bean*



Winter wheat*

Winter wheat*


*: Legume based cover crop in O2 and O4, and non-legume based cover crop in C4

 Figure 1. Biological N fixation of individual crops and cover crops in O2, O4 and C4 at three sites.

The results showed a large variation in biological N fixation (BNF) between crops, sites and years (Figure 1). The grass-clover in O2 fixed about 300 kg N ha-1 (Figure 1), which on average over the crop rotation was about 60-70 kg N ha-1 (Table 2). The N fixation of the grain legumes (mainly faba bean) was lower than for grass-clover and slightly lower in the organic rotation compared with the conventional system. The BNF of the grain legume, as an average over the organic crop rotation, was 25-50 kg N ha-1. The BNF of the legume-based cover crops were considerably lower than of the main crops (Figure 1), in particular at Flakkebjerg where the cover crop was sown after stubble cultivation in autumn, and where the growth duration therefore was short.

Table 2. Average annual biological N fixation (kg N ha-1) over the rotation of the two organic cropping systems during 2005-2008 in treatments with cover crops and use of manure.



Main crop

Cover crop

































The measurements showed that clover in the grass-clover mixture derived 100 % of its N through BNF and the grain legumes derived more than 80 % of its N from the atmospheric N2. Grain legumes showed a greater susceptibility to the weather (in particular spring drought) compared to grass-clover mixture and were also affected more by pests.

In the experiment, legumes fixed a higher amount of N2 at Jyndevad and Foulum compared to Flakkebjerg, which received lower precipitation compared to Jyndevad and Foulum. The amount of N2 fixation by grain legumes had a linear relationship with the accumulated monthly precipitation during the main growing period at Flakkebjerg. Such relation was not examined for Jyndevad and Foulum because irrigation was applied at these locations.

Even though the whole year grass-clover mixture provided high inputs of N from N2 fixation, farmers need to pay the price of excluding a cash crop for the year. Growing a cover crop after the harvest of the cash crop did not allow sufficient time for the cover crops to establish well and to have substantial N inputs, and therefore, it could not fully provide the N supply in the rotations. On the other hand, better pest and weed management strategies are required for grain legumes crop to perform well.

RowCrop develops new strategies for management

New strategies are needed to improve the growth of legume-based cover crops in organic crop rotations. This is being tested in the ongoing RowCrop project under OrganicRDD2. Here the effects of enhancing row spacing of the cereals main crop on establishment and growth and N2 fixation of the legume-based cover crop are being explored.

RowCrop will develop the scientific foundation for improved control of aggressive annual and perennial weeds in a row cropping system and for cultivating more productive Nitrogen (N) fixing catch crops to enhance crop N supply. It will document the effects of the row cropping system on productivity, weed infestation, N cycling, N leaching and soil carbon in a long-term crop rotation experiment representing different organic crop rotation systems and different fertility and weed infestation levels. The expected results are increase of yield in organic cereals, reduced nitrate leaching and enhanced soil carbon storage.


The project is coordinated by ICROFS and part of The Organic Research, Development and Demonstration Programme Organic RDD 2, supported by GUDP under The Danish Ministry of Environment and Food.

Visit the project website of RowCrop