Winter Tares- good at protecting the soil, smothering weeds and maintaining nitrogen in the soil, once dug in. Picture from Garden Organic
Winter Tares- good at protecting the soil, smothering weeds and maintaining nitrogen in the soil, once dug in. Picture from Garden Organic

At one of my recent ‘Grow Your Own’ classes, one of the participants raised an interesting question about Green Manures (GM’s).

He wondered if it was actually worth growing green manures as additional sources of nutrients. He reasoned that as they use up nutrients from the ground there isn’t any real gain in the nutrients avialable to follow on crops. Like me, he had also heard that legumes, (peas and beans) do fix nitrogen from the air and therefore their roots are a source of additional supplies of this if dug into the soil. And he also mentioned that deeper rooting plants like Comfrey tap into nutrients that wouldn’t otherwise be available to plants with shallower roots, so making these available via their leaves once composted, and also from a  ‘tea’ made from these and applied as a liquid feed. So is this all correct?

I decided to contact my colleagues at Garden Organic and ask for their advice on all  this and got a very interesting reply from Francis, their Horticultural Research Manager:

Green manures and nitrogen

Legumes, when the temperature is warm enough and they have the right bacteria, will fix the nitrogen they need from the atmosphere. It is very true, and seldom appreciated, that if a legume crop (eg beans) is harvested then most of this nitrogen is taken away and not left in the soil. However, if the legume is grown as a green manure and dug in whole (usually in an immature state) rather than being harvested then there will certainly be a net benefit; nitrogen fixation (directly or indirectly via animal manures) is the main source of nitrogen for agriculture and horticulture in the absence of artificial fertilisers.

Non leguminous plants can only take up nitrogen from the soil as inorganic ions (ammonium but mainly nitrate). The latter is very soluble in water and so easily washed out by the rain and so lost from the soil, contaminating drinking water and rivers etc. A lot of work was done (some by Garden Organic) to demonstrate that one of the best ways of preventing this was by growing winter green manures such as rye. When this is dug in the nitrogen they have taken up is mineralised to be made use of by following vegetable crops.

Green manures and other nutrients

Other nutrients (especially the metals such as K, Mg etc) are more tightly held on the surface of the soil particles and so are not easily leached so it is true that green manures are less important for keeping them in the soil. However, some do have specific effects (eg buckwheat can help mobilise phosphorus and chicory is deep rooting and so is a source of trace elements from the subsoil that may have been depleted nearer the surface). All green manures will add organic matter to the soil which helps with structure and also stimulates microbial activity, important for general nutrient cycling.”

So, the net result is that there are several good reasons for using GM’s over the winter, including maintaining nitrogen where this would leach away from unprotected soil, weed reduction, protecting soil structure, and the addition of organic matter to help moisture retention and soil structure. However, the legume contribution to soil fertility (assuming you grow these to produce food), is of questionable value if left in the soil and dug in. Better to add animal manure or your own compost to boost nitrogen levels.

Comfrey- reaches the nutrients other plants cannot reach...and you can out them into your soil via a (smelly) tea made from their leaves
Comfrey- reaches the nutrients other plants cannot reach…and you can out them into your soil via a (smelly) tea made from their leaves

Further information:

Garden Organic and Cotswold Seeds have produced a useful advice booklet on soil improvement. It’s available as a pdf to download for free at  ‘Sort out your Soil’

Linked articles:

Green Gold – 7 reasons for using green manures

Green Gold: Where and when to use Green Manures

Green Gold: Making the most of green manures

Green Gold: 12 plants for soil improvement

Old School Gardener