Yellow Sweet-clover, /Melilotus officinalis/ (L.) Lam. in the Pea family, Fabaceae. A non-native species, this covers vast acreages in the Rocky Mountains, I believe because it has been included in agencies' restoration plantings following fires. I suppose it might be improving the soil because its roots anchor nitrogen, but it crowds out native species and nobody I know out there seems to eat it.
--Ruth Moorhead