We use a lot of municipal compost in this RED Gardens project, as we just can't produce
enough compost of our own to be able to satisfy the needs of all the spaces that we grow in.
And while it is very convenient to be able to buy in large bags and even truckloads of
this clean source of organic matter I found it is not very nutritious or very fertile.
So last year we conducted a big experiment where we filled a lot of large grow bags
with this municipal compost, amended it in different ways to try to improve its quality,
and then grew potato plants in it. Overall I was really disappointed with the results
of this experiment as most of the potato plants produced very low yield, though some did produce
considerably more. And I suspect that this compost just has not had a chance to mature
enough to be able to release enough nitrogen and other nutrients for the growing plants.
So we decided to repeat the experiment again this year to see if this same batch of compost would
improve with age, and even before harvesting any of the plants it's obvious that it hasn't improved
enough to grow a good crop of potatoes, at least without additional amendments
The purpose of this experiment is not to figure out the best way to grow a
large crop of potatoes in grow bags like this, but rather to use potato
plants to see how different amendments can improve the compost that we are using.
And the main issue is probably a lack of nitrogen. Or there's probably a lot of nitrogen in this
compost but because it was made with a lot of woody material, any available nitrogen is likely
still locked up in the compost itself; that the carbon to nitrogen ratio is still too high. And
in order for this compost to be a lot more useful for growing vegetables we would either need to add
a lot more nitrogen and perhaps other nutrients or to allow this compost to mature a lot longer.
The batch of compost that we used in this experiment was originally delivered more
than two years ago and spent the first year underneath the tarp where it had dried out a
bit and then another year in grow bags growing the potato plants and then other crops before
overwintering into this year.I expected this compost would have matured quite a bit more
by now though it did dry out a few times during the growing season last year which would have
interrupted the decomposition process. And perhaps it just wasn't biologically diverse or active
enough to begin with which is something that I wanted to address at the beginning of this season
There was obviously some decomposition as the volume of the compost in each of the
bag shrunk. Some of this would have been compaction but a lot of it was a continual
decomposition of the organic matter or the carbon sources within the compost releasing
CO2 gas into the atmosphere and hopefully making nutrients available to the potato plants. So the
first task of this season was to consolidate the bags to use the compost in one bag to top up the
other five bags of the same amendment type. While doing this I came across quite a few
worms which was a good sign that there was some life and some decomposition within the compost.
So we started this season with nine sets of five bags each one filled with compost that
had been amended in different ways last year and repeating the same amendments this year
In three of the sets of bags we didn't add any additional nitrogen, including a control with
just the compost itself. In another set we added some more rock dust to boost the amounts of trace
elements within the compost. And to another set we added a mix of soft rock phosphate,
gypsum and seaweed meal in order to boost the amounts of phosphorus,
sulfur, calcium and trace elements which we figured were missing in the compost.
To another three sets of bags we added a source of fertility that contained a fair amount of nitrogen
including one bag that was amended again with chicken manure pellets. Another bag that had a
fair amount of complete organic fertilizer or cof mixed into it which is a general purpose
fertilizer that I use that contains a wide range of different plant nutrients. And to another set
of bags I added a conventional soluble fertilizer to boost the amount of nitrogen, phosphorus and
potassium. With the remaining three sets of bags we didn't add any additional fertility
at the beginning of the season instead relying on regular feeding as the plants were growing.
We liquid fed one set of plants every few weeks with a form of organic fertilizer that I buy in.
Sprayed the leaves of another set of plants with the same fertilizer as a form of foliar feeding.
And we regularly water the remaining set with dilute urine in order to boost
the amount of nitrogen that would be available to the growing potato plants
I decided to expand the experiment a bit this year, to fill some of the empty grow
bags with a fresh supply of compost we had just had delivered and amended it in three different
ways several weeks before planting the potatoes. I prepared a slurry of a range of different types
of biologically active composts that we had in order to amend or inoculate this municipal
compost with a diversity of biology. In another batch of this fresh municipal compost I added a
lot of urine in order to boost the amount of nitrogen. And we mixed in a lot of amendments
including nitrogen into another botch in order to really boost the fertility of this growing medium
So that gave us 12 sets of five bags, each filled with the same type of municipal compost that had
been amended in 12 different ways. We set out all 60 bags in a long double row over ground
cover fabric, but before planting the potatoes I wanted to make sure that there was enough
biology in each of the bags. So I mixed some of our own biologically active compost, some
decomposed wood chip and some leaf litter in with a lot of water to create a compost slurry that we
watered each of the bags in order to inoculate them with this biology. Then we planted two seed
potatoes into each of the bags at the beginning of April, though I was obviously distracted that
day because I failed to plant potatoes into two of the grow bags. We used an early variety of
potatoes called Vitabella, which we had grown in these same bags last year and have used in some
of the other gardens so we're fairly familiar with how it grows and would have something that
we can directly compare to last year. We then set up a dripper system to make watering all
of these bags a lot easier and to ensure that the compost didn't dry out during the growing season.
The purpose of this experiment was to try a number of different ways to see how this
low-grade municipal compost can be improved, but of course I also wanted to grow a lot of potatoes.
I was hoping that enough time had passed, allowing this compost to mature enough and we had added
enough of all the different types of amendments so that the potato plants in all of the different
bags would be able to grow really well. But this was not the case, as even by the beginning of June
there was obvious differences in the color of the leaves between the different sets.
The darker green leaves indicated that some of the plants were able to get enough nitrogen and
the paler or more yellow leaves indicated that there probably wasn't enough nitrogen to keep
these potato plants happy. By mid-June there was some significant nutrient deficiencies
visible in some of the plants and by the end of June some of the plants were already starting
to die back but others were still growing really strong with very vibrant leaves.
The plants that are growing in the bags that we didn't add any additional nitrogen to,
are all starting to die back, including the control and the
bag that we added rock dust to, though there is some variability.
The soft rock phosphate gypsum and seaweed meal that we added to other bags seems to be
allowing the plants to grow a little bit better but they're still dying back. The plants that
we had been foliar feeding are also dying back and I still haven't been able to figure out how
to get this foliar feeding to work, perhaps I need to be spraying a lot more frequently.
The plants growing in compost that we had added additional nitrogen to at the beginning of the
season are doing a bit better on average but not great compared to some of the other plants, and
it seems that some of the nitrogen we added had been absorbed into the decomposition process and
locked up into the compost itself which seemed to have been the same thing that happened last year.
The bags that had been amended with conventional soluble fertilizer at the beginning of the season
helped the plants grow a lot better, and we added additional supply to boost the fertility at the
beginning of the June, which seems to be allowing the plants to grow much longer than a lot of the
others. And the plants that were regularly liquid fed with either the organic fertilizer
that I buy in, or with the dilute urine all did better and are still growing strong.
We noticed something really interesting with the three sets of bags that were filled with
the new compost at the beginning of the year, as the potato plants seemed to have been delayed,
that they didn't start to appear until well after all the other bags, even though they were planted
at the same time. I noticed something similar in the bags that we started last year, and in
another trial, and it seems that there's something in the new compost that is delaying growth but I
don't know what it is. But now all of the plants in this new compost are growing really strongly,
even in the bags that didn't have any additional nitrogen added. Because they had later start I
wouldn't be surprised if they did grow for longer and it will be really interesting
to see how they continue to develop and how many potatoes they actually end up producing.
Even before we harvest any of the potatoes from any of these grow bags I've already
learned a lot from this experiment. It seems that the municipal compost that we use just
can't release enough nitrogen or release it fast enough to sustain the growth of potato plants,
even though it has it had all this time to mature. This of course is a controlled experiment,
with a plant only growing in the compost plus whatever amendments we added, and if we
had added the same compost to the soil of the gardens different processes might take place,
but I can't expect this municipal compost to adequately feed the plants in the gardens. And
this is something that I've noticed in a few of the gardens where we have been adding this
municipal compost for several years, that it seems to take quite a bit of time before the plants
start to really grow well. Even adding a wide range of different nutrients doesn't seem to help
that much, and it's only by adding nitrogen and a lot of it and ideally at several times during
the growing season that the plants seem to be able to grow well. It is disappointing that even
after another year this compost has not decomposed enough to be really useful by itself though it is
still a good form of carbon, and can be used as a mulch in the gardens. But I wonder what would have
happened if we had allowed the compost to mature for a full year without growing any plants in it,
as the plants would have absorbed any available nitrogen leaving less for the compost and
the decomposition process, which might have allowed it to actually mature a lot faster.
We’re going to begin to harvest the potato plants soon and it will be really interesting to see how
the yields compare within the different types of amendments used and how they compare to last
year's crop. And I'm going to continue with this experiment, planting another crop in these bags
for the rest of the season and possibly even another batch of potatoes in the same compost
next year. And hopefully, eventually, the compost might mature enough to actually be really useful
without additional amendments, but I don't think I can count on that. But I am tempted to amend
all of the bags with an additional supply of nitrogen before planting anything else,
as it would be really good to get a much better crop on average, rather than knowing that some
of the plants that we plant in some of the bags just aren't going to produce very well.