Green manure – dig or no dig?

[Ken AdamsParticipant]

I was interested in Charles’s comments in the Veg Journal book about incorporating a green manure crop. He seemed to imply that digging it in was the only alternative. But I’m not sure that I understand why a green manure crop should not be treated in the same way as a grass or weed covered bed – ie by covering it with mulch and allowing the crop to incorporate itself (obviously, the crop would first be cut down and the cuttings utilised elsewhere).

I have planted a mixture of Hungarian grazing rye and winter tares, which is highly recommended as a good over-winter green manure – the rye’s tap roots are said to go down to places which other roots never reach and lift nitrogen which would otherwise not be found. At the same time, the roots are said to work wonders with the water retention and drainage characteristics of the soil. The vetch, on the other hand, is a nitrogen fixer and adds nitrogen from the atmosphere (so long as the temperature is approaching double figures).

The biggest problem which this manure presents is that it is hard work to dig in. GardenOrganic recommends a system of growing potatoes under straw on the mulched surface of a bed of grazing rye – I am assuming that by the time the potatoes have been harvested, the grazing rye roots will have decomposed. This technique seems to me to be not dissimilar to the standard no-dig methods and could be applied to any of the green manure crops. An important consideration is to cut the crop down before it produces seeds.

[JohnParticipant]

Hi Ken

Over the last twelve months I have grown 7m x * 1.2m strips of several green manure crops, namely field beans, phacelia and buckwheat. In each case I have scythed down the top growth and added it to the compost heap. I have then planted modules, such as brassicas after the field beans, although not immediately. I have not done any digging.

I have a few photos on my blog – here and here.

I also have a couple of beds of grazing rye for over the winter. I haven’t decided what to do yet but my thought is to cut down the top growth in Feb/March and then cover with mypex until the strips are needed for planting modules. My neighbour tells me that the main benefit of the rye is the dense fibrous root system which I guess is best left where it is.

I also have a couple of strips of comfrey. It’s amazing how well they have grown from 25mm root cuttings and I will start cropping this next year adding it to my compost heap.

I have found that more or less equal parts of veg waste/green manure, old small wood chippings and farmyard manure give excellent compost after sixth months or so.

It’s good to hear other people’s thoughts and experiences with green manure.

[charlesModerator]

Ken you are right, I play down green manures partly as counter-balance to the massive over-simplification that is out there.
They are often sown without realisation of how long it takes to kill them in spring, by mulching as John says.
This increases slug numbers.
However for those with insufficient compost, and plenty of space, they are an option.
Though I reckon it’s cheaper + quicker to buy more compost and increase the veg output proportionately.

[Ken AdamsParticipant]

Thanks for the replies – I think Charles is definitely correct to say that the idea of using green manure is very often made to sound like a simple no-brainer. I am a complete novice to all of this, but my wife was a keen allotmenter for many years and so I’m hoping I gleaned a few glimmers of understanding.

One question which I haven’t yet resolved in my mind is how to make best use of a green manure harvest. If you simply transfer the cuttings to a compost heap, there is a danger that you are simply removing nutrients from the bed. That’s one of the things which I liked about the rye/vetch mix – it is claimed that by lifting and fixing additional nitrogen, it will increase the total amount available. My plan is to use the bed for surface grown potatoes and nitrogen hungry module grown brassicas. Decaying grazing rye is apparently ‘allelopathic’ for several weeks which means that seeds (including weed seeds) cannot germinate.

It’s probably true to say that I am more interested in experimenting with techniques than with maximising my output!

[StringfellowParticipant]

Green manures are an interesting aspect of organic gardening. Pete Budd on here has posted some great info regarding their use in his allotment near York; try a search and they should come up. I’ve been led to believe that green manures, like other plants, absorb energy from the sun and, in turn, this energy is eventually returned to the soil. Also, the deeper rooting green manures draw minerals from subsoil and, once harvested, leave a legacy of organic matter in the soil which help future growth. So, it seems unlikely that green manures are simply transferring fertility from one site to another?

[charlesModerator]

Good points Tris and Pete Budd is indeed the man.
Last night I met a successful farmer who builds soil with green manures only, but he uses a rotovator set shallow, on wheels, to chop them up and roots too, at a depth of max. 2in (5cm).
The trickiest part is always killing them to prepare ground for sowing/planting.
This man finds slugs are not a problem, I am intrigued by that, and he is biodynamic which perhaps helps.

[JohnParticipant]

I mentioned above that I followed winter field beans, sown in October/November, with brassicas which is almost the same as brassicas following broad beans; tops are cleared to the compost heap and then brassica modules are planted within the left-in-the-ground roots. No need to kill off and no digging.

This sequence fits in very well with the no-dig system and I wonder if more experienced green manurers can suggest other sequences, particularly involving over-wintering green manures, which have worked for them.

[StringfellowParticipant]

Good job with those plots John. Did you have any trouble with the phacelia self-seeding or did you remove it all before then? Pete is fond of Hungarian grazing rye and alfalfa for overwintering, both of which are a bigger task than field beans to remove before the area is ready for sowing and planting. A long-term bed of alfalfa is something I have considered but held back through fear of creating a breeding ground for slugs.

Elliott colemans and Iain tolhursts books might provide some ideas, but they are geared towards larger scale growing. Still great reads though.

Interesting about that farmer Charles, although the ‘R’ word puts me off!

[BreretonParticipant]

Dear All,

As a newcomer to this site could I ask if the following regime would work in your opinions?:

We have recently moved into an old house in SE. Cornwall with a large garden. One SW!facing area 200′ x 30′ was formerly cultivated we think (orchard, poultry, bees and vegetables) but was terribly overgrown and neglected. We have cleared all the tall vegetation and redundant trees and won’t want to do anything with it for at least 12 months.

I have bought some ‘soil enhancing green manure’ from GSS seeds ( Altaswede red clover 20%, Jeanne Italian rye grass 60%, Treposno Cocksfoot 20%), and had proposed to sow it in the spring, and then mulch clippings on top till we are ready to think about planting ( with a mix of vegetables, cutting flowers and fruit trees and bushes).

Should I then start with the compost mulch directly on top, or will I need to dig or rotivate the crop in?

Your collective wisdom and Charles’ especially of course will be much appreciated.

[charlesModerator]

Welcome Bremerton, I suggest mulching on top, to preserve the soil structure which will be building next year.
How deep a mulch depends whether there are perennial weeds such as couch grass, growing among your green manure. If there were a lot of couch, you may need to use polythene as well, so check how clean the soil is of perennial weed roots, before sowing the green manure in spring.
Check for bindweed in summer too: if there is a lot, in 2018 you may wish to cover some ground with polythene for the whole season, to clean it, while cropping the rest of your area.

[BreretonParticipant]

My internet connection just crashed jettisoning my reply – apologies if this is a repeat if the earlier message somehow got through!

Thank you very much for your reply Charles, in light of what you say, might it be sensible to start with putting polythene down for a few months, then the green manure for 12+ months, sowing in say late summer, then mulch – though that would extend the time before I can start cropping…?

Also how much compost shall I need for this area 200’x30′)? Cornwall Council’s subcontractor sell it by the ton.

With many thanks to you and your fantastic site.

Best wishes,

[Ken AdamsParticipant]

Hi Brereton – I’m new to all of this, but I’ve been doing some research and I’m interested in your situation.

If your plot is still completely covered with weeds etc, then I would suggest that your first priority would be to cover it completely with plastic or permeable membrane in order to kill the existing growth. I would suggest that the sooner you do it, the better. It is probably too wet, but consider rotavating it before covering it. You will need bare soil to sow your green manure, and it seems an unnecessary expense to buy compost in order to grow green manure.

If you are not planning to use the land this year, then maybe you should leave it covered until late summer before sowing the green manure. I think that the mix you have chosen is recommended for a medium term manure, but I daresay that it would also be useful for a single winter’s growth.

In the spring of 2018, I think it would be appropriate to cut down the growth and cover it for a week or three. You would then be able to choose between sowing seeds into a mulch of compost or planting seedlings into the remains of the green manure.

I am completely open to correction on anything I have said!

[JohnParticipant]

Hi Ken and Brereton

I think there are two things that need addressing, these being to clear the plot of weeds and to improve the soil.

Annual weeds soon die off but some perennials take much longer although you say you have twelve months before thinking of cropping and this should be sufficient to clear almost everything if you cover with black polythene or mypex-type membrane.

Why not start the soil improvement immediately by adding as much compost/manure/green waste as you can get hold of before laying the plastic/membrane? The manure needn’t be fully formed compost as you have twelve months for it to mature during which time the soil fauna will be digging the soil for you. The organic layer will also help with weed suppression but you do need a thick layer to combat many perennial weeds unless plastic/membrane is used on top.

I definitely would NOT rotovate. It is the best way to propagate bindweed and other weeds by root cuttings! Let the organic matter do the work.

I would leave sowing the green manure until you have a weed free plot.

This link might be useful although, since doing it again, I have mucked and covered at the same time.

[charlesModerator]

A fair mix of advice here!
Ken, no need to rotate!
Much depends how Bremerton is for time e.g. mowing the green manure.
The original idea is sound, perennial weeds being the unknown (to us) variable.

[BreretonParticipant]

Thanks so much for all your advice.

When we cleared the ground in the autumn (with machinery that compacted the soil a bit, though left good furrows for planting), I had hoped to be able to seed with green manure to help keep existing weeds at bay as well as build soil structure before weeds took hold once more. I also thought this mix would be attractive to look at while I am deciding how to design and lay out the space. We were too busy with other jobs, clrearing and grass seeding elsewhere, so missed the best time.

Now with our mild climate down here the weeds and grasses are back – perennial ones seem to be dock, thistles, nettles. Couldn’t see any couch and it is too early for bindweed. The land is adjacent to pasture and would have been taken from grazing pasture land. I don’t want the weeds to reclaim their former majestic ground while I am planning !

So I guess I will need to mulch or polythene before sowing the green manure.

Despite the expense of municipal compost, I am guessing that it would be less work to mulch with compost, sow green manure, wait 12 months plus (the seed mix is designed for 12 months minimum), then compost again before sowing eventual crops? Alternatively I would have to buy polythene, lay it, and work the soil before sowing green manure?

Thanks so much all

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