Farmers can now better predict how different nitrogen sources affect crop yield in organic farming. By analyzing data from long-term experiments in Denmark, researchers found that nitrogen in soil organic matter, organic matter applied to soils, catch crops, applied manure, and weed pressure all impact the yield of winter wheat and spring barley. This discovery allows for the development of a simple tool to plan crop rotations in organic farming, considering factors like soil history and nitrogen supply through manure or weed control measures.