Legume cover crops like hairy vetch and red clover can help organic farmers in the northern United States by providing nitrogen for their crops without synthetic fertilizers. A study in Minnesota found that these cover crops can add 30-75 kg of nitrogen per hectare each year, with most of it coming from the air through biological nitrogen fixation. When the cover crops were removed in the spring, the soil had more available nitrogen. The study also showed that the native soil bacteria were able to help the legumes fix nitrogen effectively.