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.
