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Plant-Based Manure for 'Greener Crops'

03 February 2012

DENMARK - The manure of the future does not originate from animals but from plants, according to new research. Green manure is the way ahead at a time when the use of non-organic animal manure in organic farming is going to be phased out.

Research states the use of non-organic animal manure in organic farming will soon be a thing of the past. In the near future this will be replaced by green manure – in other words, a plant-based manure. In a new project at Aarhus University scientists will therefore look at how to get the maximum benefit of nitrogen in green manure while optimising the quality of the fertiliser.

"The objective is to optimise the yield of the nitrogen in plant-based fertiliser that is stable and readily available when needed," says senior scientist Jørn Nygaard Sørensen, who is behind the project.

The background for the project is that the current use of non-organic animal manure in organic farming will gradually be phased out from 2015. Organic farms therefore need alternative sources of fertiliser. There is also an ambition to increase the use of green manure to avoid contamination with animal manure, which represents a potential health risk.

Green manure quality

Previous studies of green manure have shown that it is not just a question of producing a high nitrogen yield per hectare, but that the quality of the green manure is also very important. The quality depends on the carbon/nitrogen (C/N) ratio of the manure.

"If the C/N ratio is too high, the nutrients will be released too slowly for the cash crop to get full benefit of them," explains Jørn Nygaard Sørensen.

To achieve the highest yield when used on cash crops it is therefore important that the C/N ratio in the green manure crops is low. A low ratio is achieved by harvesting the green manure crops at an early growth stage, because the later the plants are harvested, the higher the C/N ratio.

A balancing act

The challenge for the scientists is to find not only suitable plants with a potentially high nitrogen content but also the optimum harvesting time for the green manures when the nitrogen content is high and the C/N ratio low.

"When producing mobile green manures the optimum harvesting time will be a balance between the amount of nitrogen produced per hectare and the C/N ratio of the biomass. The nitrogen yield per hectare is also a balance between the quantity of biomass produced and the nitrogen content. In this way the nitrogen yield can be maximised and the quality optimised," concludes Jørn Nygaard Sørensen.

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