Green Manures

plants that add to soil fertility and improve soil structure

By Melanie Matthews

What is green manure?

Green manures are plants which help maintain and even improve soil fertility and maintain a healthy soil structure; well worth experimenting with.

When/where do I sow them?

Rather than leaving patches of bare soil (which will fill with weeds) or putting down large sheets of fabric (which will become a slug hotel), in between harvesting one crop and sowing the next, sow a green manure.   Some can also be under sown or fitted in between rows e.g. nitrogen fixing Alfalfa around leafy cabbage.

Don't forget to rotate green manures as you would any other crop. E.G. rotate Vicia, and Alfalfa, with Peas/Beans, Mustard and Fodder Raddish with Brassicas.

Then what do I do?

Green manures can be cut down and dug in, or in a no dig system, cut and left to lie or composted. Cut when young - before they become woody and before flowering. At this stage the nitrogen content is relatively high. Some nitrogen fixers may be left for more than a year because their root nodules add nitrogen and there's improvement over time.

The benefits of Green Manures

  • The shade of dense foliage will reduce weed seed germination**. Mulch also does this but green manure roots may also inhibit the growth of weed competitors.
  • Beneficial soil microorganisms flourish.
  • It protects the soil from winter rains which would damage the structure of the soil
  • Plants absorbs available nutrients that would have been of leached out of the soil. They can now be recycled in organic form to be made available when the plants are dug in or composted.
  • Plants of the Pea/Bean family often have root nodules populated by bacteria that convert nitrogen from air into useful organic nitrogen. Other green manures use an extensive root system to absorb and concentrate nutrients like potassium that may otherwise be unavailable to crops.
  • The roots of green manures can increase the water holding capacity of soil to make it more drought resistant.
  • Green manure roots may improve the stability of soil particles and create useful drainage channels.

** Caution: - some green manures e.g. Secale cereal and Vicia sativa will release germination inhibitors into the soil. That's great for weed prevention but it also means that sowing small seeded crops has to be delayed by about 1 month after digging in.

Be aware

  • Green manure wont produce all the organic matter you need - you'll still need plenty of compost or manur
  • if allowed to become woody they absorb, rather than release  nitrogen, during decomposition and crop growth will be impaired unless fertilisers are added
  • decomposition can adverse effect seed germination. Although it is OK to plant transplants, crops should not be sown directly for 3 to 4 weeks after digging in
  • the green manure can be a host for diseases e.g. chocolate spot of field beans might spread to over wintered broad beans
  • soil covered with a carpet of green manure will be slower to warm in the spring
  • a thick covering of, weed suppressing, green manure can be a slug haven, but, then, ground beetles (major predators of slugs) also seem to like it so it might even out. 
This page was added by Melanie Matthews on 01/11/2009.