black medic, Medicago lupulina

Black medic is an annual or short-lived perennial in grasslands and other open areas.  They do love our lawns.  We often say that they and other legumes fix nitrogen.  Nor entirely true.  It wasn't really broken, but in its atmospheric form nitrogen is chemically unable to react with other elements to form organic compounds we cannot live without.  Black medic and its relatives are hosts to micro-organisms that convert atmospheric oxygen to compounds we can use like ammonia.  Plants in some other families can also do this.  So those black medic and clovers in our lawns?  Good for them!  Black medic grows in every state and province except LB and YT, and on GL and SPM.  Hillsborough Co FL, 1/24/13.  Bean family, Fabaceae.