Farmers can also use by-products of the meat industry, such as feather and bonemeal as fertilisers, as well as conventional straw for animal bedding. In this way, organic farms may benefit from nutrients sourced from conventional farming.

Now researchers from France have analysed just how much organic farms rely on importing nutrients in this way. Their findings could be key for planners looking to scale up organic farming in the future. Today only 0.7 per cent of the world’s agricultural land is farmed organically; in France 3.5 per cent of farmland is organic.

"Scenarios of 100 per cent organic farming have been designed," Sylvain Pellerin of INRA and the University of Bordeaux told Environmentalresearchweb. "However, none of them have taken nutrient flows from conventional to organic farming into account. Accounting [for] these flows and, therefore, indirect reliance of organic farming on manufactured fertilizer may modify previous assessments."

Plants need a number of elements for healthy growth, including phosphorus, potassium and nitrogen. While some plants can access nitrogen directly from the air, in a process known as nitrogen fixation, other nutrients must be added in the form of fertilizer in order to maintain crop yields.

"Nutrient management is critical for organic farming, especially for elements like phosphorus and potassium, which cannot be supplied by biological fixation," said Dr Pellerin. "There were a few preliminary results in the literature suggesting that some inflows exist from conventional agriculture through material exchanges. We wanted to get an idea of the magnitude of these inflows."

The team looked at organic farming over two years on 63 farms in three districts of France – Lomagne, Pilat and Ribéracois – with different amounts of arable and livestock farming. Lomagne specialises in arable production, chiefly growing cereals, oilseed and protein crops, along with some poultry production. Pilat is a dairy region and has around 1.15 cows per hectare, while Ribéracois is more mixed, with both arable land and animal production at 0.63 cows per hectare. The team reckons that these three districts represent a wide range of agricultural conditions in Europe.

On average, organic farms in the study received 73 per cent of their phosphorus from conventional farming, 53 per cent of their potassium and 23 per cent of their nitrogen. "These inflows were mainly due to fertilizing material importations such as imports of manure from conventional farms," said Dr Pellerin. Feedstuffs, fodders and straws also provided some nutrients.

Farms with no or few livestock tended to rely more on conventional farming for their nutrient needs. In areas with lots of different types of organic farming, such as arable, livestock and mushroom production, farms were able to exchange organic products like manure, feedstuffs and straws, cutting the dependence on conventional farming.

The team believes that this is the first quantification at the farm scale of nutrient inflows from conventional farming to organic farms through material exchanges.

"All nutrient inflows from conventional farming may not come from manufactured fertilizers," said Dr Pellerin. "Some nitrogen may come from biological fixation and some nutrients may come from the natural fertility of the soil. Additional studies are therefore required to discriminate between nutrient inflows from conventional farming and from manufactured fertilizers, which is not exactly the same. We are currently working on this question for the case of phosphorus."