Green manure suppresses weeds, protects soil from erosion and improves structure as well. Legumes (beans) absorb nitrogen from the atmosphere and ‘fix' it into the soil through root nodules making it easily available for brassicas planted there afterwards.
Sow green manure seeds whenever the ground is to be left bare for more than six weeks. Dig them in while they’re fresh and green and allow to to three weeks before re-planting to begin the breakdown of plant material into humus, releasing nutrients slowly over time.
Managing Green Manure
You can sow green manure crops either by scattering seeds over the vacant site (broadcasting) or sowing the seeds of green manure crops separately.
You’ll want to dig the crop into the soil when it’s relatively young. This is for ease of working it into the soil. If you are away and not able to dig the crop in until it is woodier and more established, you could trim the crop off with a string trimmer, let it break down and then dig in.
Leave the crop to decompose into the soil before digging over; three weeks in warm periods and up to five in colder periods.
Types of green manure
Fast-growing leafy crops like Phacelia and rape seed will release lots of nitrogen in the soil when dug in. Effective but risky to use rape and mustard in soil that has been affected by clubroot.
Legumes like peas and beans fix nitrogen in the soil using nodules on their roots. They also release nitrogen from their leaves as they break down. Check which ones would be suited to your climate as some may not thrive in cold weather. Tares tend to do OK in less than clement conditions, while fenugreek is better in warmer climates.
Fibrous root crops like grazing rye have fibrous root systems that really improve the texture of the soil and introduce some good, effective organic matter.