Green manure
From Allotments4All
Most green manures are sown in spring to be dug in during the autumn, or planted in the autumn to over winter. Also you need to keep in mind that after it has been dug in, the ground needs to be left for usually a minimum of a couple of weeks before planting. Of course, it depends how much later in the year you're thinking of growing your veg
Buckwheat can be grown towards the end of the summer, Alfalfa is quite a quick growing green manure and Mustard is terrifcally fast, 2-8 weeks
Mustard ?
This website is quite useful in sowing times: http://www.btinternet.com/~bury_rd/green.htm and scroll down to the table. It's helpful, but not extensive, there are many more green manures, have a google for it. They also do different things, and that site doesn't say which ones fix nitrogen.
