Phacelia tanacetifolia, a  'green manure' that's good to look at and attractive to beneficial insects.

Phacelia tanacetifolia, a ‘green manure’ that’s good to look at and attractive to beneficial insects.

I’ve just been reading about green manures, from a small prize I won at my induction training as a ‘Master Composter’. The prize is a slim booklet produced by Garden Organic and it focuses on the use of green or ‘living’ manure in the garden. So, what is a green manure?

‘a plant which is grown to benefit the soil, not, as some might suppose, under-ripe animal dung!’

I’ve had a couple of tries with green manure (mainly because I like the flowers of Phacelia), but have not been totally convinced of its value – it’s hard to check what benefits it brings unless you conduct some sort of rigourous trial, of course. Anyway, this booklet is giving me the confidence to do more and so I thought I’d share its contents with you in a short series of ‘bite sized’ articles over the next few weeks.

Apparently green manures have been used by farmers for centuries to improve their land and gardeners have begun to realise their value too. Seed companies have begun to stock green manure seeds in packets sized for the average garden. They are most often used in the vegetable plot, but can also be used in other areas. In later articles I’ll cover where and when to use them; some of the plants and their benefits; how to choose and grow the right plants and what to do when you’re ready to use them. Today I’m focusing on seven reasons why to use green manure.

1.To feed the soil – green manure crops ‘mop up’ and hold onto soil nutrients and some deep-rooted types can actually gather nutrients from depths that other plants cannot access. By absorbing nutrients the roots prevent it being washed down into sub soil. Once green manures are turned into the soil the nutrients are ready to be taken up by the next crop.

2. To protect and improve soil structure – green manures help to protect the soil surface from the effects of heavy rain (mainly soil compaction and surface ‘panning’). This is a benefit for both clay and sand – dominated soils where organic matter reduces compaction in the former and helps water and nutrient retention in the latter.

3. To stimulate soil micro organisms – when dug in green manures feed and stimulate microscopic creatures that in the process of decomposing this organic matter boost soil health, which in turn helps to develop strong plants.

4. To prevent weed invasion – nature abhors a vacuum/ bare soil – as soon as plants are removed new ones will try to move in and these can often be weeds. Green manures tend to germinate quickly so can be a quick way of covering bare soil and smothering young weed seedlings, also eliminating the need for constant hoeing to remove the weeds.

5. To control pests – some beneficial crittters (like frogs and beetles) love the shady, damp ground under a green manure. Some green manures can be planted to distract flying insects away from crops you want to protect; e.g. underplanting Brassicas with Trefoil disguises the outline of the crop and seems to deter cabbage root fly. Likewise a small patch of Phacelia tanacetifolia or Clover, if allowed to flower, will attract insects that prey on many garden pests.

6. To improve the look of the garden – a green manure or ‘cover crop’ will not ony help to prevent weeds but can look attractive of itself. Some also help to fix nitrogen in the soil which will help plant growth.

7. To ‘rest’ your soil – after a period of intensive cultivation, soil can benefit from lying fallow for a season. Most usual in the vegetable garden, it’s a technique that can be useful in the ornamental garden especially where a new border is to be planted up.

Crimson Clover - another green manure that looks good and helps to 'fix' ntrogen in the soil

Crimson Clover – another green manure that looks good and helps to ‘fix’ ntrogen in the soil

So, on paper the case for using green manures is a strong one. My kitchen garden is currently straining under the weight of the many different crops I have growing in every available patch of soil (and some containers too). But in a month or two, once some crops have been harvested, and where I haven’t planned for any new crops, I’m going to put in a green manure. In next week’s article I’ll cover just where and when to use these valuable plants.

Source: ‘Green Manures’- Garden Organic Guide. September 2010

Old School Gardener

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