Soil, the never-ending no-dig, no compost, green manure experiment. 14/11/2021

I have been an adherent of a no-dig philosophy on my allotment for around six years now and have written the occasional piece about my experiences. I’ve also written several other pieces that are still in the drafts folder on WordPress, if they even made it that far.

Much of the time it all feels like an ongoing experiment, with different crops, different mulches, some supplementary fertilisation, more or less irrigation and so on. As is being realised and embraced to some degree by academic researchers, there are so many variables in a soil system that experiments are very hard to conduct and even harder to interpret when the results come in.

At times it seems that what I do with my 200m2 allotment is profoundly unimportant, at other times it feels close to being a way to save the planet. Sometimes it seems supremely simple, almost a case of the less I do the better; at other times I am almost overwhelmed by the complexity of the soil ecosphere, the extraordinary way that nature has arrived at ways to sustain for millennia, rainforests and prairies and the Australian outback and every other environment without ploughing or fertilisation or selectively killing things.

The allotment, at least in Cornwall’s relatively benign climate, is a continuous process with no clear cut off points in the year. There is some growth even in the depths of winter and some crops standing too. In summer it speeds up greatly, but there is no definable beginning or end to the growing season. If there were it would be easier to write about. With the ground frozen or snow covered, nothing is going to happen for a while and you can reflect on what happened in the season just gone and what will happen in the season to come. For me there never seems to be a right time to marshal my thoughts.

This year I scrapped my compost heaps and have simply shredded and spread all the material that would have gone onto them. My thinking is simple enough. What essentially I am trying to do is to capture as much of the sun’s energy as I can. That is done by the process of photosynthesis, combining atmospheric carbon dioxide with water to produce sugars and myriad other carbon compounds within the plant. The plant then utilises those same compounds to release the stored energy in order to fuel all its biological processes, essentially growing and reproducing.

Organisms that don’t possess the ability to photosynthesize are dependent on taking some of the carbon compounds that the plant has manufactured and releasing the energy contained within for their biological processes, again growing and reproducing, plus moving about. That includes both the soil fauna and fungi as well as all the other creatures that move upon the earth, like us.

Energy containing plant produced carbon compounds are available to the above ground animals as leaves, stems, flowers, fruits and seeds and in a well balanced natural environment the animals need harvest only a modest fraction of what the plant has produced in order to get by. A modest amount of harvesting, grazing for example, be it by slugs or zebras, probably stimulates the plants to grow more. Heavy harvesting reduces the plant’s capacity to photosynthesize, so the capture of sunlight is reduced and the total amount of biomass will go down as less photosynthesis will support less heterotrophic organisms, the ones that can’t photosynthesize.

Below ground, there are living roots that offer a food source for some, but much of what the fauna get from the plants is in the form of root exudates that the plants produce expressly for the purpose. Plants cannot survive on carbon dioxide and water, they also need a raft of other elements, nitrogen, potassium, phosphate, zinc, iron, magnesium and so on. Fungi can to an extent release some of these elements from the minerals of the soil but a large proportion are in the living and dead organic matter in the soil. So effectively does soil biology hold onto these elements that lush natural environments can be supported without additional elements indefinitely. The conspicuous exception is nitrogen, the compounds of which tend to be very soluble and readily leached out of soil by rainfall. Fortunately there are plenty of soil organisms that can pull nitrogen directly from the air and turn it into compounds that plants and the rest can use.

I did away with my compost heaps because I could not for the life of me see any advantage to allowing the energy accumulated by the plant material on them to be released away from the soil on which I am growing my crops. I never have enough material to generate enough heat to kill weed seeds or pathogens. If I didn’t have a shredder it would be messy and inconvenient to spread plant material onto the ground. I do have a shredder and I use it during the day when it is powered by my solar panels and I spread the resultant material both on bare ground and around growing crops when they are big enough not to be smothered by it.

On a compost heap the volume of material shrinks rapidly, especially if there is not a lot of hard woody material. A key function of mulching is to protect the soil surface from physical damage and my way I get far greater volume and for longer, it works far harder for me. The woody material is still the long lasting fraction and stays on the surface for a year or two.

The impression I get is that the growing crops get a shot of readily available nitrogen, I may be imagining it but they seem to grow better after a mulch has been applied. It seems to me also that worm activity is strongly encouraged, which makes sense in that worm activity in a compost heap is frenetic in fresh material, reducing as material breaks down. Similarly, bacterial activity in a freshly built heap is frantic at first, generating lots of heat (that’s the energy going to waste!) then fading away after a few weeks.

I want all that activity in my soil. It seems to me it should be happening in my soil. Nature doesn’t build compost heaps. Plant material is deposited and breaks down where the plants are growing. It seems such an obvious thing that I can’t quite get my head around why composting has such an enthusiastic and devoted following. I feel I must be missing something but if I am, I don’t know what it is.

Following on from the logic of maximising energy capture, I want to have plant cover throughout the year to the greatest extent possible. I have always struggled to produce enough compost to mulch all the ground that is bare in winter. In theory early crops such as peas and potatoes should be cleared allowing time to grow a second crop that would stand into winter but I have not done well at making it work. I have experimented with cover crops but under no dig I don’t want to have to dig it in come spring nor do I want to cover it with Mypex two or three months before I need the ground in order to kill it.

This year I’ve done a little better with having crops still in the ground, have more material to put on the surface because I’m not reducing the volume by composting it and I’ve been trying to sort out a workable green manure/cover crop regime. There isn’t much empty ground and what there is has a generous layer of shreddings on it.

Phacelia has been by far the most successful green manure I’ve sown but I would like to get a mixture of things growing as different plants bring different benefits to the soil. The bulk of Phacelia would be usefully supplemented by something that fixed nitrogen and a grass that was good at building soil structure. I’m also thinking that a couple of natives, foxgloves and Verbascum, might be excellent bulky soil protectors that could easily be sliced off at ground level come seed sowing time in spring.

This year I left it a bit late to start experimenting with green manure crops; I got a very patchy germination, but I’ve learnt a bit and can hopefully build on that next year. The plot is a mess but I have greenery over most of it and that pleases me. My soil is being well looked after and I am looking forward to next year’s growing season with some optimism.

Chard, Phacelia and bunching onions. I may grow Chard as a component of a mixed green manure, available to eat if required but benefitting the soil whether cropped or not. It could get overwhelmed by Phacelia though.
I grew two varieties of leaks. ‘Winner’, on the left, I planted too close, thinking to maximise my return from the area. They have rust, made worse by poor air circulation. They have been mulched with fresh shreddings which seemed to produce a growth spurt. On the right is ‘Bulgarian Giant’, from Real Seeds, in soil that was poor at the beginning of the year but looks better now for a couple of top dressings with shreddings. They are rust free and uneven; I plan to grow them again.
I planted cell grown Phacelia among my sweet corn before I harvested the small amount that rats didn’t get. Caper spurge, kale, foxglove and forget me nots are among the self sown items.
I sowed Phacelia, Italian rye and winter tares in two beds, drilled in one, broadcast in the other. Germination was markedly better in the drilled bed, which I attribute to there being more moisture that bit lower down. Phacelia has come to dominate so perhaps its percentage in the mix should be reduced next time.

8 thoughts on “Soil, the never-ending no-dig, no compost, green manure experiment. 14/11/2021

  1. Very interesting. You use Phacelia and my father used mustard instead which gives a very positive green manure too: maybe you have already tried it? I try not to leave a part bare after harvest. When I have finished the peas or the potatoes, I have the leeks and the carrots instead. But right now there are bare parts so I cover them with dry leaves, mowing grass and cardboard

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  2. I like the sound of your thoughtful experiments. It seems quite bold to have verbascum as soil cover, although one plant can have a very broad spread. I am always interested to see how wild plants share the same land and you seem to be aiming for a similar effect, but with deliberately planted ones..

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  3. Hey, it is pretty also! I remember mustard in the orchards, but can not remember the processes. Since most of the orchards were abandoned before I can remember, the mustard was probably not managed at that time anyway, although I do remember that it got cut at times. It was not mixed with anything that I can remember. Of course, the mustard could have dominated and killed off other vegetation if there had been any. In may own garden, I actually use zonal geraniums. I know it sounds silly, and is mostly removed rather than getting tilled in, but it is effective.

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    1. It seems to work with my mix of veg and ornamentals. I don’t believe there’s a one size fits all method, how could there be when people garden in such a wide range of conditions.

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