Last year we put up a new large polytunnel on a rough site, with lots of perennial weeds,
rocks and debris in the soil, plastic from a previous polytunnel still buried in the ground,
and topsoil that was not in great shape. We took the time over the winter and into
the early spring to clear the ground, dig out the plastic and other debris, and sieve a lot of the
soil to ensure that we were able to remove most of the roots of the pernicious weeds.
With a lot of work, especially by Kris, we ended up with much cleaner soil,
but in the process a lot of biomass that had been in the established plants was removed and
placed into the compost pile, taking with it much of the soil organic matter and fertility with it.
So we invested the time and resources to amend the soil with concentrated fertility and a lot
of compost, to hopefully ensure that we could grow a decent crop of tomatoes, aubergines
and peppers in the space this past year. And we added enough compost to really front-load
the fertility in this high value growing space, enough for a couple of years at least,
but unfortunately it didn’t help as much as I had hoped with the crop we grew this season.
Having thoroughly dug over the soil, and cleaned it up as much as we could,
and removed most of the weed roots, I wanted to transition to a no-dig
or minimum till method for managing the soil in this polytunnel into the future.
I wanted to use the method of adding a thick layer of compost covering all the beds, and to
add successive layers of compost on top of this each season, starting with a really deep layer.
But I also wanted to add some concentrated amendments to start to boost the fertility
of the soil underneath, and to add some nutrients that I figured were in short supply.
I had not done a soil test on this part of the site, but a few years ago I had got a detailed
profile of the availability of the major and minor nutrients in another location close by,
and figured it would be close enough. This test indicated that this high pH
calcarious soil would benefit from more phosphorous, potassium and sulphur,
as well as a bit more sodium and a range of trace elements, and of course more nitrogen.
I decided to spread about 20kg of chicken manure pellets, which has
been my main source of concentrated fertility over the last few years,
to boost the levels of the main nutrients of nitrogen, phosphorous and potassium.
I added a fair amount of seaweed to help provide a wide range of trace elements.
I used a pelleted soft rock phosphate to help further increase the levels of phosphorous,
and I used a concentrated form of sulphur, to add a fair amount of this
element which is typically quite low in this soil. These amendment would have provided a fair amount
of nitrogen and phosphorous, a moderate amounts of potassium, as well as sulphur and sodium,
which I thought would help to shift the nutrient balance in the soil, in addition to the compost.
I decided to add half of these amendments to the surface of the soil, and to spread
the other half over the surface of a thick layer of municipal compost that
I was planning to cover all of the beds with. I was thinking that dividing the concentrated
fertility like this would help improve the nutritional quality of this compost, which I
have found to be not very rich, but this ended up being a mistake, at least for this first season.
I spread half of the concentrated amendments on the surface of the soil,
and then covered the entire 110m2 space of the polytunnel with a layer of cardboard to
suppress the regrowth of any weeds roots that were left in the soil.
I set up lines of twine to mark the edges of the beds, and covered the beds with a lot of
municipal compost that the we had bought in, which is quite clean and easy to work with,
without any weed seeds, or disease organisms, or any plastic or other unwanted material.
And we ended up using almost 7m3 of this compost to cover the five beds, enough to provide a
80-90mm thick layer on the growing beds, which brought in a lot of carbon, organic matter and
potential fertility for this growing space. Looking at the typical analysis from the
compost supplier, this amount of compost would have brought in almost 25 times the
amount of nitrogen, 3 times the phosphorous and almost 20 times the potassium that we
had already added with the concentrated amendments, plus a lot of organic matter.
And this was calculated using the lowest range indicated for all of these nutrients,
so there could be a lot more held in that abundant amount of compost.
But the vast majority of it will be locked up in the complex organic matter of the compost itself,
tied up in the continuing process of decomposition.
This compost was likely made with a lot of woody material, with a lot of small pieces
still identifiable in the compost, and this would take some time to fully break down.
But I figured that leaving it on the surface of the soil this decomposition
would continue without locking up more of the nitrogen from within the soil itself,
and gradually release fertility for the plants. But this gradual release of fertility didn’t
happen as much as I had hoped, at least for the first season.
The growth in this polytunnel was really good at first, with the tomato, aubergine or eggplants
and pepper plants growing really strongly, at least for the first half of the season.
There was obviously fertility available in the soil itself,
and with the amendments that were added to the surface of the soil, and some of the plant roots
were colonising the thick layer of compost. But later in the summer the plants started
to look like they were running out of fertility, with the leaves becoming pale,
and over time the deficiencies in the leaves became even more pronounced.
I decided to let this run, without adding any additional fertility, mainly because I
was distracted by other things at a busy time of year, and I also wanted to see what would happen.
These plants ended up slowing down growth significantly earlier in the autumn that
the tomato plants we were growing in the other polytunnel, and although they produced a lot
over the season, they definitely looked like they had just run out of food.
And I think there are a bunch of reasons for all of this,
starting with the original soil not being very fertile.
The amount of concentrated fertility that we had added might have been enough if I
had spread it all over the surface of the soil, but I now realise that the
half of it that I mixed into the surface of the compost was mostly unavailable to the plants.
And the compost didn’t seem to release much additional nitrogen
to supplement the supplies, which I had expected to happen later in the season.
The layer of cardboard probably created a barrier for part of the season,
at least until it decomposed, and the watering system I used probably exacerbated these issues.
This past year we also did a big trial of growing potato plants in grow bags
of this same compost supply, to explore the possible benefit of amending this
lower quality compost in different ways. And one of the main things we learned from
this trial is that there is not a lot of fertility being released to the plant roots
from this not fully decomposed material. It seems that nitrogen was the key limited
nutrient, as the bags of plants that were fed with different forms of fertility with
a lot more nitrogen in it, did a lot better, producing a lot more potatoes in the end.
So despite having a huge amount of nitrogen and other nutrients in the compost itself,
most of it was not bioavailable, yet, or at least the plant roots could not access enough of it for
strong and healthy growth, unless there was an addition supply of nitrogen regularly added.
It seems that this compost just needs a lot longer to complete the processes of
decomposition before it can be really useful for the crops we want to grow.
I had seen this in the No-Dig garden, where we have been adding a fair amount this same
type of compost on the surface of the soil for a few years, and it is only this season
that the plants are really starting to do well. It seems that the compost we added to this No-Dig
garden several years ago is now releasing the nutrients it contained for the plants,
and the layer we added this past spring might not start feeding the plants for another year or more.
I had decided to use a drip line system in this new polytunnel to water all of the beds,
and we used only two lines on each bed, instead of the four lines that we have
been using in the other two polytunnels. This was more than enough to supply water
to the plants, which were planted deeper in the soil underneath the cardboard and compost,
and to keep the lower levels of the soil moist, but large areas of the compost dried out.
We have had problems with this type of compost in the past, as when it does dry out on the surface,
it can be really hard to re-wet again. The drip line did keep a cone of compost and soil
adequately moist directly underneath each dripper in the pipe, but farther away from these wet
patches the compost dried out to a deeper level. And the surface of the compost over most of the
polytunnel was quite dry, which created a dust mulch that served to greatly reduce the amount
of evaporation from the soil, and it kept the air in the polytunnel a lot drier, which was useful.
But without adequate moisture the biological decomposition of the compost and the amendments
we had added on top, would have slowed down considerably or even stopped, effectively
reducing the volume of compost and the amount of amendments that were available for the season.
Learning from this issues in the other polytunnel, I had stepped on the compost
of these new beds to compress the particles together, in order to increase the potential
for capillary flow of the moisture up into more of the compost, similar to how a sponge gets wet
sitting in a shallow basin of water. And this did help, but not enough.
After clearing all of the plants from this polytunnel at the end of the season,
I carefully dug trenches across the width of the growing beds in a few locations.
I wanted to see what was happening underground,
to better understand what might be happening to this layer of compost.
There was still a significant layer of compost on the surface, about 50mm deep,
but I was surprised how little of it seems to have worked into the soil below,
as there was still a very distinct line separating the dark compost from the paler soil below.
Apart from the one place where the remains of growing medium from a
transplanted tomato plat was buried deeper. In the outside No-Dig garden this shift
from dark compost on top to and lighter soil below has become very gradual over the years,
and I guess it takes a lot more time for this merging to take place.
And I think the flow of water through the soil might help with movement of dissolved organic
compounds into the soil, which would happen with the heavy rains in the outside gardens.
But inside the polytunnel, this is less likely, unless I use the sprinklers to
water the full volume of the soil and compost a lot more, and to keep it moist.
And this would also help a lot more of the worms and other soil organisms to
do the task of decomposing the compost and distributing the fertility into the soil.
I had assumed that the amount of fertility that we had added at the beginning of the year would
be enough to feed these hungry plants fro the full season, but they obviously needed more.
The easiest way to have changed this would have been to have spread all of
the concentrated amendments on the surface of the soil, as this would have possibly doubled
the amount of fertility available to the plants, and adding even more probably would have helped.
The amendments that I had added to the top of the compost are still there, and will likely be
of benefit at some point in the future, but they were a waste of resources this season.
The easiest way I could have corrected this mistake midway through season would have been
to add a liquid feed in with the water in the drip lines, as this would have brought the fertility
directly to where the plant roots were. And I recently bought a Venturi syphon
adaptor that would have made this a lot easier, but regular hand watering with
liquid feed would have also helped, though this would have taken more time and effort.
There is still a huge amount of fertility left in this compost, and I suspect more of
it will be released in the next few seasons, but regular watering will definitely help,
and I suspect that a biologically active compost tea will help to bring in more of
the biology to help speed up this process. I am not sure that making the effort to keep
the full volume of the compost moist for the full growing season next year is achievable
with the taller plants getting in the way of the sprinkler systems.
And I am not sure that this would be overly desirable, as a dry layer on
the surface of the compost really helps to reduce the evaporation and to keep
the humidity levels down in the polytunnel. So, I think I will rely on liquid feeding
through the drip line system to supplement the feeding for the plants when I think they need it,
using bought in fertility, urine, compost teas, or even nettle and comfrey teas.
And I plan to add another layer of this municipal compost over the winter or in
the spring, though probably less than what I added the first time,
continuing the process of front-loading the fertility in this valuable growing space,
even though the benefits might not be realised for quite a few seasons.