Estimating nitrogen supply and cereal crop yield in organic crop production

Hitherto it has been difficult for farmers to predict the effects of various sources of nitrogen (N) supply in organic farming, since these vary greatly in quality and thus in effect on crop yield. This has limited the planning of organic farming crop rotations and the application of manure and other mobile N sources to the crops, since the effect on crop yield would not easily be estimated. Here we used data from long-term organic crop rotation experiments in Denmark to determine the contribution of various N sources to yield of winter wheat and spring barley. The results showed consistent effects of N in soil organic matter, in organic matter applied to soils, in catch crops, in applied manure and of weed pressure. This opens the possibility for building a simple tool for crop rotation planning that will allow the effects of such crop rotations to be assessed in terms of cereal crop yields, and likely also yield of other non-legume cash crops. Such a tool can be used for strategic planning of crop rotations in organic arable farming taking into account the prehistory of the site as well as the possibilities of N supply through manure or measures to control weeds.