The share of organic fruits and vegetables bought by Danish consumers are still increasing. The sale of organic products actually increased by 28 percent from 2014 to 2015. However, new initiatives are needed for organic farmers to ensure a sufficient production to meet this increasing demand.

The main challenge is manure. At present, it is difficult for Danish organic farmers to procure sufficient organic animal manure and therefore they are given a dispensation to use a small amount of conventional animal manure in the production. However, the organic farmers prefer to use organically produced manure and hope that the need for a dispensation will soon come to an end.

Therefore, a group of scientists from Aarhus University has worked to develop alternative fertilizing sources for organic farming. A recently finished project demonstrates that mobile green manure (plant-based fertilizer) may be an alternative to animal manure – without affecting the yield.  

Mobile manure

Green manure has been used for several hundred years and is an excellent method of building up soil fertility and recirculating nutrients. Traditionally, it entails that the farmer grows legumes such as e.g. red or white clover in his fields. Project manager and Senior Scientist at Department of Food Science, Aarhus University, Jørn Nygaard Sørensen explains:

- In the fall or spring the legumes are ploughed into the soil in order to function as fertilizer for a subsequent main crop. The news is that we have demonstrated that instead of ploughing the legumes into the soil, we should harvest them and use them in another field with e.g. cauliflower or leek, says Jørn Nygaard Sørensen, and he further compares the method to mowing the lawn and then subsequently apply the cut grass to the vegetable garden.

More harvests and improved fertilizer

The major advantage of mobile green manure is that you can harvest repeatedly and achieve different manure crop qualities. The quality depends on the carbon/nitrogen ratio.  If the crop contains too much carbon in relation to nitrogen the nutrients will be released too slowly and, on the other hand, high nitrogen contents will result in a quick release of nutrients.

 - When you grow vegetables it is of major importance that the fertilizer nutrients are released quickly. Therefore, it makes good sense to harvest green manure crops at an early growth stage with a low C/N ratio. Using mobile green manure means that you can harvest repeatedly and – at the same time – achieve improved fertilizer, says Jørn Nygaard Sørensen, and he states that further research is needed on how to produce green fertilizer most appropriately.

It will probably be advantageous to grow green manure crops in marginal land areas. In addition, it should be examined how organic farmers may be able to replace phosphorus as green manure is not the solution here. 

- The crops need plant nutrients – and only by ensuring that the plants achieve an optimum supply of nutrients during the entire growth season will we be able to optimize crop yield and quality. In a few years’ time it is no longer an option to use conventional animal manure, and therefore it is of vital importance that organic farmers have access to new alternatives, says Jørn Nygaard Sørensen.

Further information

Senior Scientist Jørn Nygaard Sørensen 
Department of Food Science, Aarhus University
E-mail: jornn.sorensen@food.au.dk
Tel.: +45 8715 8348

The project ”Maximum production of organic nitrogen using green manure” is funded by the Green Development and Demonstration Program (GUDP), the Ministry of Environment and Food of Denmark.

The project is accomplished in collaboration between Aarhus University, Økologisk Landsforening (Danish Organic Farming Association), HortiAdvice Scandinavia (GartneriRådgivningen A/S),  Skiftekær Økologi Aps (an organic farm) and Farmergødning I/S (a company selling organic manure).