All of my experience growing vegetable is in topsoil,
mainly in the topsoil that we have here in the midlands of Ireland.
Apart from the growing medium that I use for seedlings and young transplants, I have not
had any real experience growing something in a carbon based rather than a soil based material.
But this past year we grew a lot of potato plants in large grow bags filled with various forms of
soil-less mixes, as well as in bags of the local topsoil that had been amended in different ways.
It was really interesting to explore the issues of growing in these predominately
carbon based materials, and to see how they compare to growing in the topsoil.
I found that growing potatoes in a lot of these carbon based material
was generally more successful than growing in the mineral based soil,
but it really depended on how developed or decomposed that material was.
And in the process I gained a better understanding of how these various forms
of carbon that we often add to the soils in the gardens can help to grow an abundant crop, or not.
There were two different big trials that we did this year
involving growing potatoes in large grow bags.
One involved growing an early variety of potatoes directly into the municipal compost
that I buy bulk quantities of, exploring different options for amending this lower quality compost.
The other large trial involved growing a main crop variety of potato plants in another set of these
grow bags, with some of them filled with topsoils with different forms of amendments mixed in.
And the others were filled with different carbon based materials, which allowed me to try out some
conventional and unusual growing medium. After harvesting all 90 bags, with five
bags of each soil or material type, and each growing 2 potato plants of the same variety,
the results were quite interesting, with some general trends and fairly extreme differences.
But there was also some outlier or unusual results where some plants did much better than others,
likely because their roots had been able to grow out of the bottom of the bag into the soil below.
Rather than looking at the average of all five bags, including the unusually bad or good results,
I found focusing on the median, or the middle bag, gave a more realistic indication of how well
the different soil or carbon based options were able to support a crop of potatoes.
Overall I was a bit disappointed with what we were able to harvest from these grow bags.
I would generally hope that each plant of a main crop variety would produce about 2kg of potatoes,
or 4kg from the two plants in each bag, but only 7 of the 90 bags produced that much, or more.
Although it is possible to get a higher yield, in this trial it seems that 3kg of
potatoes per bag is a more realistic target, at least for this season.
The median or middle bag of three of the 10 soil based options met that target, each one
with a substantial amount of fertility added. But most of the carbon based options produced
3kg or more, and if I exclude the failed experiments of trying to grow potatoes in
hay or straw, only one of the soil-less bags produced less than 3kg of potatoes.
Simplifying the comparison a bit more, the median of all of the carbon based
grow bags produced in the region of 20% more than the bags of amended soil.
I think there are a few reasons why the carbon based material was able to grow better plants.
The organic material can likely hold onto a lot more nutrients that the the topsoil that we have,
that is predominately made up of silt and sand. And some of the organic material contains a
lot of nutrients within its structure, and in some cases the continuing decomposition
of this material would have released even more nutrients for the plants.
And this seems to have been the case, especially earlier on in the season,
as the plants were significantly bigger. I had measured the approximate length of the
longer haulms of the potato plants in each bag at the beginning of August, ten weeks after planting,
and the carbon based bags were supporting plants significantly larger than the soil based bags.
This indicates there was significantly more nutrients available for the plants
earlier in the season, especially nitrogen.
The carbon based materials would likely be able to hold onto more water which is
useful in the smaller volume of the grow bags. But this became an issue during a warm and dry
period in the summer, as lot more moisture was pulled out of the bags by the larger plants,
causing them to dry out faster than the bags supporting smaller plants.
And I didn’t keep up with the watering, or I wasn’t able to fully wet the material in
the bag once it dried out, and some of the plants definitely suffered.
This water stress would have slowed down the growth of the plants,
and it also would have disrupted the soil organisms that were involved in
the continuing decomposition of the organic material in some of the bags.
Which likely reduced the availability of additional nutrients for the plants,
and these plants ended up producing less than I would have expected.
Comparing the size of the plants in the summer and the amount of potatoes harvested at the end of the
season, these seems to be a general trend. And a lot of the bigger plants, seemed to
fall short of what I would have expected if that trend or relationship had continued.
It seems a lot more of these bags could have produced over 4kg of potatoes,
if I had been able to maintain consistent moisture in the growing medium or soil.
The bags filled with the municipal compost that I get delivered had
plants that grew differently than with most of the other carbon based materials.
Even though I had added a fair amount of general purpose organic fertility to
these bags, the plants were significantly delayed,
as if there was something in the material that was inhibiting the initial growth.
And the growth remained slower than the others, but then they grew better in the second half
of the season and continued to grow for longer than a lot of the rest of them.
It seems that earlier in the season, there was not enough fertility being released by this compost,
and it may have even absorbed a lot of the fertility that we did mix in.
The greater proportion of high carbon woody material that this type of compost
was originally made from needed more time, and probably more nitrogen to continue to decompose.
But after a while, and with the warmth of the summer, it seems that the biological activity and
the decomposition had increased enough to be able to support the continued growth of the plants.
These plants didn’t catch up to the yield of the others, but they ended up producing
more than I had initially though that they would. I had noticed this lack of fertility in the other
large trial based on this same municipal compost. Because the early potatoes in that trial had been
started significantly earlier in the season, they didn’t seem to benefit as much from any increase
in nutrient availability later in the summer. And I had added some of this same compost to
most of the soil based grow bags, thinking that it would add organic matter in addition to any
other concentrated amendments that we added. And this might be one of the key reasons that
the plants growing in the soil based bags produced so poorly as the compost
that I had added may have locked up a lot of the nitrogen, keeping it away from the plant roots.
Another set of thee bags was filled with a very different form of compost, from the
community composting facility that I manage. Although this compost was also only partially
decomposed, with lots of the material still identifiable, and a lot of visible biological
activity, it was made from a wide mix of material that had a much higher proportion of nitrogen.
Lots of nitrogen was obviously available to these plants, even
without any additional concentrated fertility, as the plants grew very large very quickly.
And I think the plants in these bags could have produced the largest amount
of potatoes of any of the bags, producing a crop as the material continued to decompose.
Or they could have if I have been able to supply enough water.
And I think that this option of growing potatoes in bags filled with partially
decomposed but quite rich compost like this, is a really interesting idea.
The plants seemed to be able to take advantage of nutrients that were being released as the
decomposition continued, something that the other municipal compost wasn’t able to supply.
And at the end of the season this compost was much more mature and broken down, though the potato
plants would have extracted a fair amount of nitrogen and another nutrients from this resource.
Another set of bags was filled with spent mushroom compost,
a different form of carbon based material that is still decomposing quite a bit
after the mushroom that had grown in it had partially decomposed the original material.
I had mixed a similar quarter litre portion of the same general fertiliser
into the mushroom compost, as I felt that it probably didn’t have all the nutrients
that the potato plants would really needed. These plants also grew a bit slowly at first,
and then had stronger and longer growth later in the season, eventually becoming quit big plants.
I was surprised that they ended up producing quite a good crop, as there seems to have been
a gradual release of additional nutrients over the season, enough to keep the plants happy.
I filled another set of 5 bags with decomposed ramial woodchip, made from
a lot of twigs and smaller diameter wood, that a tree surgeon delivered a couple of winters ago.
I had left it in a pile for fungi to decompose for a few years, and was quite broken down when
I dug it out to fill the bags, thinking it might serve as a decent growing medium.
I decided to add twice as much of the general fertiliser mix, as I figured that this was
quite low in a lot of the nutrients that potatoes would need, especially nitrogen.
The plants grew reasonably well, though not as large as the others,
with a slightly lower yield of potatoes.
I suspect that adding even more nitrogen before planting or topping up on the
fertility during the growing season would have really helped, as I imagine a fair
amount of it was absorbed in the continuing decomposition of the remains of the woodchip.
But this well decomposed ramial woodchip worked much better as a carbon rich growing
medium than I had thought it would, and so did the spent mushroom compost,
and with a bit of extra fertility they could have produced a much bigger crop.
These 4 types of materials are all in various stages of decomposition,
with different source material in the mix, and the success of the
potato plants seemed to have a lot to do with how advanced that decomposition was.
It also depended on how much of the added nitrogen was absorbed by the carbon rich material itself,
or if nitrogen was released for the plants through the process of decomposition.
This depends on the changing ration between carbon and nitrogen in the material,
but it also depend on how quickly the carbon base of this material can actually decompose.
I had filled another set of bags with professionally produced potting compost
or growing medium that I buy lots of bags of for starting young vegetable plants.
This growing medium is very different than the other materials that are also called compost,
and would typically be based on peat moss, but the material I buy in is peat free.
This stuff is very useful for growing plants as it can hold a lot of moisture and nutrients,
but perhaps more importantly it is very slow to decompose, or doesn’t seem to take up any of
the added nitrogen to speed up the process of decomposition of the material itself.
This growing medium comes already loaded with a fair amount of plant available nutrients,
which I topped this up with a similar amount of the general purpose fertiliser.
And this high carbon growing medium grew really big plants with a lot of potatoes,
and probably would have produced even more if we had added more fertility as the season progressed,
and kept up with the watering. I also used a fair amount of
this bagged growing medium as the base for a lightweight mix of material that I would
use on a rooftop growing system, mixed in with lots of inert perlite and vermiculite.
I had also added a lot of this municipal compost, and extra fertility into the mix,
but this combination seemed to work well, though probably would have been better with
more nitrogen or less of the municipal compost. This rooftop mix produced the highest median
yield of all the bags in the full trial, and there seemed to be a definite benefit using
a high carbon growing medium that is slow to decompose even with lots of fertility available.
The bags filled with straw or hay were very different, as neither had started the processes
of decomposition to be able to release nutrients in time for use for the growing potato plants.
And they also weren’t slow enough in decomposition to be able to serve as a more inert growing medium
that was able to hold on to added nutrients to make them available to the plants.
The hay is normally used as a high protein animal feed,
which would contain a decent amount of nitrogen with at fairly good carbon to nitrogen ratio.
But I didn’t fill the bags and start the decomposition early enough before
planting the potatoes, and these plants really struggled, and a few of them died off completely.
It was a similar issue with the straw, except that the straw contains very little
nitrogen to begin with, and I had regularly watered these bags with a liquid feed of
diluted urine to supply this fertility. These plants also grew really poorly,
partially because the straw hadn’t been given a chance to start to
decompose before the potatoes were planted. And a lot of the fertility I added while
the plants were growing was likely absorbed by the microbes to use for decomposing the straw.
Towards the end of the season, a lot of the hay was decomposed enough to become
a source of fertility, and the straw had absorbed a lot of the nitrogen it needed.
But this was too late for the plants this season, though probably a good base for the next crop.
I learned a lot in all of this, especially that
I can grow potatoes in a wide range of different carbon based materials
But it really depends on what that carbon based material it is made from, how decomposed it is,
and how quickly it will decompose if more nitrogen and other nutrients are added.
Our own compost was fertile enough to support the plants as it was continuing to decompose.
The municipal compost needed a lot more time to decompose first, or regular additions of
a lot more nitrogen and other nutrients to feed the plants, and to amend the compost.
The spent mushroom compost was better or at least faster to respond, and the woodchip
seemed to need an extra dose of concentrated fertility to make up for what it as lacking.
The professionally made compost or growing medium seemed to be very slow to decompose,
but this was a really good thing, or at least it didn’t seem to complete with the plants for
any of the fertility, and was able to supply a lot of what the plant needed.
The experiments with straw and hay weren’t prepared early enough before planting to be
a useful growing medium, and I am not sure what would have happened if I had
given it a chance to decompose some more first. What is obvious in all of this is that not all
things we call compost are the same, and this can be really confusing because we use the same
word for materials that behave so differently. I have known this for a while, but with this
trial of growing potatoes in various forms of high carbon material, I gained a much
deeper understanding of how this actually relates to the materials I have been using.
And I gained a better understanding of how the various forms of compost will impact the soil
biodiversity and crops I am trying to grow, if I add them to the soil in the gardens.
It was also great to be able to get the experience to grow potatoes in a lot of
different high carbon materials like this, and I am really looking forward to seeing
how these materials will change, especially with better care, for growing another crop next year.