Last year I decided to stop growing vegetables in my new polytunnel space, in order to deal
with a few problematic issues.
The plants last spring didn’t do very well, and there were some deficiencies apparent
in the crops, and there was signs of a viral disease in the tomato transplants.
I also felt that I didn’t have enough compost to adequately prepare the soil in these garden
beds for the level of production that I wanted.
So I decided to pull the crops, remineralise the soil and sow a green manure or cover crop,
in order to build the soil organic matter, improve the soil biology, as well as improve
soil texture.
I don’t have a lot of experience in growing this types of crop cover in my gardens, but
it's something that I'm really interested in exploring a lot more.
And I’ve never grown a green manure in the protected climate of a polytunnel, especially
over the winter, and it was interesting to see how it all turned out.
My plans were further set back by severe fire damage to the polytunnel, after which the
entire sheet of plastic had to be replaced.
I took the opportunity to level the beds some more and to clear out a lot of the weeds,
and I added a range of concentrated fertility and mineral amendments, including a lot of
chicken manure.
This was all part of the process of starting to fully remineralise and balance the fertility
of the soil, based on detailed soil tests.
It was late autumn by the time I sowed a mix of green manure seed, which was specifically
developed for overwintering in the Irish climate, and included wild rye, black or Japanese oats,
vetch and two types of clover.
Because I had added so much chicken manure to this soil, I was not so concerned about
the nitrogen fixing capacity of this green manure, but some more would definitely be
good.
I was much more interested in growing carbon and building organic matter both above and
below the soil.
I was also interested in growing plants with strong vigorous root systems which would work
to improve the structure of the soil, both within the top soil and deeper in the soil
profile, and a big part of this was to bring life back into soil, to boost the soil ecosystem.
After broadcast sowing the seeds and working them shallowly into the soil, I set up a dripline
watering system back onto beds.
I then I finished off the preparations of the growing space by adding wood chips onto
the paths between the growing beds.
It was really good to see growing space clean and well established, especially after the
problematic events of the season.
The growth of the green manure was slow at first, but the young plants grew steadily
over the winter which was not too cold and the polytunnel offered protection from the
windy weather.
As expected the growth rate accelerated in the early spring as temperatures began to
rise again with the longer days and more sunlight, It because quite an amazing stand of vegetation,
quickly reaching knee high, then waist high with some sections and plants going even higher.
But the growth wasn’t even across the full garden, indicating possible differences in
the underlying soil fertility or soil structure, and a mould developed on the leaves the rye
or oats, especially at one end of polytunnel space.
This indicated to me that something was not quite right, though I’m not sure what it
was, but the humid conditions and lack of strong air movement in the enclosed space
probably contributed to this issue.
There was also lots of other plant species mixed in, including the various types of annual
weeds that are typical in my gardens, as well as a lot of leafy brassica plants, which had
self seeded and cross pollinated when this growing space was abandoned a number of years
ago.
I think that the more diverse range of species can only help, so long as I could get rid
of the plants when I needed to.
One of the big issues with green manures is how to get rid of these typically strong and
vigorous plants so that we can grown other crops.
Green manures like this are traditionally dug or ploughed in, or chopped up with a flail
mower, in order to both kill off the plants and to incorporate the carbon and nutrient
rich biomass into the soil for quick decomposition.
I have wanted to transition this polytunnel into a minimum till, or no-dig, growing space,
with only minor surface cultivation from now on, so digging wasn’t a preferred option,
and I don’t have access to a flail mower.
But I was planning to cover this entire garden space with ground cover fabric, with holes
in it for tomatoes, cucumbers and other crops, and I figured I could simply cut the strong
growth of vegetation, and leave it on the surface of the soil to decompose over time,
underneath the ground cover fabric.
With the this fabric, preventing the regrowth of the plants would be relatively easy, but
I still had to figure out a way to cut or flatten or trample all of the vegetation,
and I figured the best option was to use my scythe to cut it all down.
As I wanted to allow as much biomass to grow as possible, I waited quite late, until a
lot of the plants were starting to flower, and then I used my scythe to slowly and carefully
cut the dense growth of plants.
I was extra careful along the edge of the space to avoid cutting into the plastic, and
I managed to do the full job without a scratch.
But I wasn’t careful enough to avoid the dripline watering pipes, and sliced through
the pipes a few times, but thankfully they're quite easy to repair.
It probably would have been better to take the time to pull out all the lengths of dripline
pipe entirely, or in hindsight I should have installed an overhead sprinkler system instead,
when I originally sowed the green manure, which would have made the whole process easier.
I laid all of the cut material or hay onto the garden beds, and then covered the full
garden with the sheets of ground cover fabric that already had holes in it from a previous
season for the plants that I wanted to grow.
For the beds that I was planning to transplant into right away, I moved a lot of the hay
onto the paths adjacent to the bed, to make it easier for transplanting, and I figured
that the material would be broken up and crushed as I walked back and forth, which I think
would have speeded up the decomposition.
In hindsight, I probably should have done this right away with all of the hay material,
to crush it and mash it underfoot for a while, before putting it back onto the beds, which
I think would have really helped.
I then marked out the location of each hole, pulled back the ground cover fabric, and then
dug over the stations for each plant, chopping up the remains of the green manure stems and
the root systems in the process.
While this was going against my preference for reduced cultivation, it was only a small
portion of the volume of soil, and it made it a lot easier to transplant the tomatos
through the holes in the ground cover fabric once it was put back in place.
When the plants were a little bit more established, I lifted up the edges of the ground cover
fabric, and scraped the crushed remains of green manure material, back onto the beds,
where I figured it would act as a type of side dressing for these heavy feeding plants
during the growing season.
As the plants grew bigger, it was interesting to see the roots growing right into area where
the the decomposing biomass was touching the surface of the soil.
This mulch, under the ground cover fabric, allowed the roots of the tomato plants to
colonise the whole volume of the soil and then some.
Unfortunately, a short time after transplanting, the tomato plants were showing signs of nitrogen
deficiency, indicated by a pale discolouration of the older leaves.
It was only then that I realised that most of the nitrogen that was in the soil had been
incorporated into the roots and leaves of the green manure.
I had thought that a fair amount of the fertility would have remained in the soil, but apparently
most of it was used up by the strong growth of these green manure crops.
And the process of decomposition hadn’t started to release the nitrogen yet, or at
least not fast enough for what these young tomato plants needed.
I fed these plants with a liquid feed of a high nitrogen fertiliser and then foliar fed
them a few times over the next few weeks, and a month later they were growing really
strongly.
In hindsight I should have provided a liquid feeding while transplanting, and it would
have been even better if I had mixed in some compost when I dug the stations for each plant,
and I made sure to take these precautions with the other plants that I transplanted
a few weeks later.
I assume that cutting the green manure material a few weeks earlier would have partially alleviated
this issue, but it would have been even better to have dug the green manure into the soil
in the first place which would have sped up the decomposition, and released the nitrogen
and other nutrients closer to the roots of the tomato plants.
Of course digging it in would have further delayed getting the possible benefits of a
no-dig soil ecosystem in these beds, but even partially digging it in would have helped
to speed up the transition from green manure to crop.
No doubt this blanket of biomass will continue to feed the soil biology and plants with a
gradual release of nitrogen, carbon and other nutrients over the long growing season, rather
than a quick flush of fertility at the beginning of the season.
I suspect that this slow and steady decomposition is probably a good thing, but it is something
that I definitely want to keep eye on as the growing season progresses.
Overall I still think it was beneficial to take the time to grow a green manure to help
to improve the quality of the soil in this valuable growing space.
Button course it would have been much better if I had established the beds and then grown
a green manure for at least a year before putting up the polytunnel in the first place.
But growing in the shelter of the polytunnel did allow a lot more biomass to be produced
in a cold part of the year, and I could have grown an even wider range of species.
Using a sprinkler watering system instead of drip line would have made everything easier
and would have prevented the hay from drying out, which would have helped to speed up the
decomposition.
If I was to do it again I would consider using a mulcher or a lawnmower to cut the green
manure a few times while it was growing, or even scything it a few times.
A more gradual buildup of organic matter on the surface of the soil, in layers over time,
might have worked better, and would have avoided the huge amount of biomass at end.
Or I could have cut the green manure a few weeks or even a month earlier to allow the
decomposition and soil to settle before transplanting.
But this would have significantly reduced the amount of organic matter that this crop
of green manure would have reenable to produce, which was one of the main reasons for growing
it in the first place.
No doubt there is a balance there somewhere, or a more suitable method, in trying to grow
as much organic material as possible, without negatively impacting the following crop, and
I’m not sure that I found it on my first try.
This video was all about growing a green manure in a polytunnel, and then getting rid of the
green manure so that I could grow another crop.
I’m planning to make another video in the near future, where I’m going to explore
the question about whether it was worth growing the green manure in the first place, compared
to other options that I might have had.
If you’re interested in this kind of thing, keep an eye out for that video, and perhaps
subscribe if you haven’t already.
This process of trying things out, and then questioning and wondering whether it was worth
it, or how to be able to do it better, is a big part of this RED Gardens Project.
If you value this approach, and want to support me in this endeavour, I’ve identified a
few options in the description below.
Any financial support that you can give would be greatly appreciated.
But the biggest support that you can give is of course continuing to watch my videos,
and thank you for making it all the way to the end of this one.