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Crops in organic farming can enhance their nitrogen fixation fix

Grass-clover and nitrogen-fixing cover crops can contribute substantially to crop nitrogen supply in organic crop production.

[Translate to English:] Kløvergræs er en god kilde til kvælstof i økologiske planteavlssystemer. Foto: Janne Hansen

Crops need nitrogen to grow and thrive. In conventional agriculture nitrogen is applied via fertilisers and animal manure. Organic farming does not have the same options since inorganic fertilisers are not permitted and there are limited quantities of organic manure available. There are, moreover, plans to phase out the option of importing conventional manure for organic farming in Denmark. Organic farmers therefore seek other options. Conventional farming can likewise benefit from having other nitrogen sources besides inorganic fertilisers.


Scientists from Aarhus University have investigated whether nitrogen-fixing crops in the rotation can help supply this vital nutrient to crops. The studies included both organic and conventionally farmed fields. Professor Jørgen E. Olesen, Aarhus University, will be presenting the results of these studies (in Danish) at the 2014 Plant Congress to be held in Herning, Denmark from 14-15 January 2014.


Plant N-sources

- There are three main sources of nitrogen in crop production. The smallest input comes from nitrogen deposition in rainfall and air, and from nitrogen added with the seeds. These sources together contribute about 20 kg N/ha, explains Jørgen E. Olesen and further adds:


- Organic matter in soil is a significant nitrogen resource, the size of which depends in part on how the soil has been cultivated historically and especially on whether the land has been under pasture or has received animal manure. The quantities of nitrogen applied in the rotation over the past 5-10 years also make a difference. The vast majority of nitrogen in organic systems originates from the biological fixation of nitrogen in plants that live in symbiosis with nitrogen-fixing bacteria in root nodules. This fixation takes place in grass-clover or alfalfa, in pulses and in nitrogen-fixing cover crops.


The experimental results showed that there was a big difference in how much nitrogen the individual nitrogen-fixing plants contributed. The contributions were not only dependent on the type of plant cultivated, but also on the plant’s living conditions, for example on conditions during establishment and on soil type and fertilisation. In general, the fixation was highest in clover and lowest in cover crops, although for cover crops fixation could be improved by early establishment.


Clover and the right cover crops

Based on the measured yields and plant biomass in crops and cover crops scientists could calculate the total nitrogen balance. An increasing nitrogen supply was generally associated with higher yields, although this depended on local soil conditions. Soil with a high content of organic matter was basically more fertile and had a higher N supply than sandy soils.


- In organic cropping systems, a large proportion of the nitrogen derives from nitrogen fixation. This supply can be increased by growing clover or nitrogen-fixing cover crops. This involves, however, a trade-off between the cultivation of cash crops and the control of weeds. It is also feasible to increase the utilisation of the available nitrogen if it can be converted into mineral nitrogen in manures that can be applied at the right time and with the right method in the spring. Here, the digestion of grass-clover and cover crops for biogas would be ideal.


For further information please contact: Professor Jørgen E. Olesen, Department of Agroecology, e -mail: JorgenE.Olesen@agrsci.dk, telephone: +45 8715 7778, mobile: +45 4082 1659