I have been successfully using green manures on my plots for several years. Our soil is fertile, but is hungry for humus to prevent caking and to retain moisture. My favourites are field beans over winter and Phacelia in summer. I sow field beans as I do my winter digging, as the ground is cleared any time from October to January. After each spit is turned, I scatter a few beans along the top of the freshly turned soil, then turn the next spit on top no fancy rows or precise spacing. Phacelia just gets scattered across any spare ground and simply raked over, in spring, summer, or September is particularly good, after the potatoes are out. It is excellent for ground you have recently reclaimed from nature, but know you will still need to dig out the last few perennial weeds that always return.
As a bee-keeper, I prefer to let the beans flower before they are hoed off at ground level. I leave them as a mulch, especially on the brassicas patch, which like a firm bed, so I plant sprouts, etc. through the mulch of beans just where they fell no more digging or carting to the compost heap.
Of course, the other advantage of green manure is that, apart from any initial investment) it costs nothing provided you make provision for collecting seed every two or three years. With the beans, just leave a few plants to mature in a back corner, or the ends of the rows. When the leaves have fallen, pull the whole plants, bundle into a sheaf and store somewhere dry (and rodent-proof!). When completely dry, the beans are easy to harvest. I throw the sheaf onto an old plastic groundsheet and jump on it! The beans fall out onto the sheet, sieve off the worst of the debris and store in a tin, they will last five to seven years.
- Phacelia: The phacelia flowers are a gorgeous blue-mauve colour, and the insects love them. The seeds ripen over a long period, so I just keep a paper bag in the shed, and collect a few heads every time I think of it. There is no need to clean the seed, just scatter the heads next season, and rake in. If you do not get round to digging the plants in, the first frost will lay them. I also use winter tares, same cultivation and seed harvesting as field beans.
- Grazing rye: You have to dig Grazing Rye in at just the right time, and not plant seeds for several weeks afterwards.
- Buckwheat: The same is true of using buckwheat as a green manure.
- Clover: I have found Clover hard to establish, and then it comes up everywhere for years and sadly it comes up exactly where you don't want it.
- Mustard: I have used mustard in the past, but find it hard to get the seed these days, and it needs specialist seed harvesting equipment.
- Oilseed Rape: Maybe oilseed rape would be useful if you can get seed, but it would need digging in before flowering as then the plants get woody and tough.