This past year I decided to do a few big trials involving
growing potato plants in large grow bags. One of the trials involved growing bags
filled with the standard bulk municipal compost that I buy in for the gardens, to see if it is
possible to grow potatoes in this compost only. And to test how different amendments added to the
compost might improve how the plants grew and how many more potatoes they might be able to produce.
And it seems that regular feeding of the plants with something that is high in
nitrogen produced the best result in this trial. Which raises the question of what happened to
all of the fertility that I added to the other bags, and why couldn’t the potato
plants get enough nitrogen from these amendments or from the compost itself.
I have been buying in this type of municipal compost for a few years, to supplement the compost
that I make myself for use in all of the different gardens and growing spaces that I am trialing.
This type of municipal compost is generally made from a lot of landscape waste,
which contains a lot of woody material, which means that it is generally not so rich, or able
to provide a lot of nutrients to the plants. But it is high in carbon, which can be a useful
thing to import into the gardens, as well as being convenient, clean and free of weed seeds.
I had a large pile of the compost left over from last year, had invested in a lot of
large grow bags this year, and I thought this was a good opportunity to grow a
load of potatoes this year, as well as trying different options for improving this compost.
We filled a lot of large grow bags with this compost, which were amended with 10
different types of fertility either before or during the growing season,
and planted 50 of the bags with two seed potatoes of the same variety of early potatoes.
This formed the main part of the trial as it included 5 bags of each of the 10
different amendment types, including a control with no amendments added.
I was generally disappointed with the amount of potatoes that was produced,
with very poor harvests out of most of the bags, at least compared to what I have been
able to get from the same potato variety grown in the soil of the outside gardens.
All of the different amendments that I added to the compost at the beginning of the season had
very limited effect, and it was only the plants that I have fed more regularly during the growing
season that produced a significant increase in the weight of the potatoes that I was able to harvest.
So I was able to confirm that this compost is not great as it is for
growing these types of hungry vegetable plants, and that regular feeding helps,
and unpacking some of the reasons for all of this allowed me to learn a fair amount.
I don’t know the specifics of what this compost is typically made from,
but the supplier does have information about typical total nutrient values, including
1 to 2.5% nitrogen, which is a big range. It also gives a carbon to nitrogen ration of less
than 20:1, but I have learned from other sources that a ration of less than 15:1 is preferable,
and closer to 12:1 or even 10:1 is much better if you want to use it to feed vegetable plants.
So this compost is only partially decomposed, which is apparent with the small pieces of
woody material still visible in the mix. The general advice for making a compost pile
is to start with a mix of materials that would provide a carbon to nitrogen ration of about 30:1.
As I understand it, the composting process gradually reduces this ratio towards 10:1,
essentially because all of the organisms that are involved in this decomposition,
or eating all of these carbon rich matter, are continually breathing out or releasing
carbon dioxide into the atmosphere. This results in a reduction of the
total amount of carbon contained in the mix, while hopefully the amount of nitrogen stays
about the same, resulting in a reduction of the carbon to nitrogen ratio in the process.
I had hoped that storing this compost for almost a year would have allowed it to decompose quite
a bit more, to get well under that 15:1 ratio, but a lot of the compost dried out under cover,
which would have really slowed down the decomposition process.
The details from the supplier indicate that at the bulk density of the compost would be
between 350-600 g/L, but I dried a litre of compost and found that it weighed about 400g.
We were able to put about 40L of compost into each grow bag, which would translate into a
weight of about 16 kg if it was fully dried And if it contained between 1 to 2.5 percent
nitrogen, as specified by the typical nutrient values, this would translate
to between 160g and 400g of nitrogen contained in each grow bag of this compost.
Which I think is a surprisingly large amount of nitrogen, but unfortunately the vast majority of
this would not be available to the plants, at least until there is a lot more decomposition.
Based on my fairly basic understanding of all this, it seems that a lot of that nitrogen will
be within the organic matter or the bodies of all the bacteria and fungi, and other organisms, as
well as embedded in the complex clusters of humus and other organic molecules of the compost itself.
And some forms of nitrogen will be stuck to the surfaces of these organic molecules and clumps,
which is apparently the form that is most readily available to the plants.
I had hoped that there would be enough of this available or readily useable nitrogen,
and all the other nutrients, for the plants to thrive in this compost.
But it seems that this was not the case, as the two potato plants that I grew in each bag of this
compost without any additional amendments did not do well, producing quite a small crop of potatoes
compared to what I was able to grow in the soil of the gardens with the same potato variety.
I had expected that this might be the case, and I had added a range of
different amendments that contained additional nitrogen to some of the other bags of compost.
I was thinking that this would boost the levels of nitrogen and other nutrients that
would be available to the two potato plants that were growing in each bag,
but the amount harvested from these bags was only marginally better that the control.
Looking back at the amount of the different amendments that I had added,
each with different concentrations of nitrogen, it seems I had added about
5 to 7 grams of nitrogen to each bag before planting the potatoes.
Which is more than the recommended amount of nitrogen for a crop of early potatoes,
if I translate the recommended application rates down to the size of the grow bag of potatoes.
So this amount of nitrogen should have been more than enough to grow a good crop of potatoes.
But this is only a tiny amount of additional nitrogen compared to the 160 to 400g of
nitrogen that should already be in the compost in each bag, so it is not surprising that such
a small increase on what was already there had little effect, but it did improve things a bit.
The amendments that I added at the beginning of the season all gradually
release the nitrogen and other nutrients, and some of this could have been available
to be taken up by the plant roots, especially early on in the season.
But I suspect a lot of it would have been taken up by the decomposing organisms in
the compost, and incorporated into the structure of the compost itself.
It seems that the compost needed more nitrogen, and this took priority over needs of the plants.
With other bags of compost I had mixed in other amendments at the beginning of the season that
didn’t contain nitrogen, but would boost supplies of phosphorous, sulphur and other
trace elements that I thought might be missing or a bit deficient in the original compost.
This was more of a strategy of improving the compost in the longer term, thinking
that these amendments would help to improve the compost once it had fully matured, and settled
into a better carbon to nitrogen ratio. The yield from these plants was not
significantly different from the others, but it is possible that these plants would have produced
potatoes with better nutrient content, thought the plants did not look healthy.
A lot of the leaves started to turn pale and then become yellow, before dying off,
and this was fairly common among the potato plants in all of the bags of amended compost,
as well as with the compost that was not amended. This looked a lot like the signs of nitrogen
deficiency in the leaves, which is quite a bit different than the usual signs of potassium,
phosphorous or other forms of nutrient deficiency that I have seen or researched.
And this deficiency was also significant on the other bags of plants that were regularly
sprayed with a high potassium foliar feed that is often recommended for potato plants.
In hindsight I suspect that here was already plenty of potassium available
in the compost for the plants, and it would have been better to use a foliar
feed that was high in nitrogen, and to spray it more frequently.
There was a fair amount of variation with all of these amendments of course,
likely caused but the variable condition of the original compost that was mixed into the bags.
But the leaves of the potato plants that were being regularly liquid fed a more
consistent and abundant supply of nitrogen did better, especially with the plants that
were being liquid fed with dilute urine. The leaves on most of these plants were
consistently more green, with fewer signs of deficiencies for longer in the growing season, and
ended up producing a heavier yield of potatoes. And the plants that had been side dressed
with a concentrated form of soluble non-organic fertiliser were similar.
So a more regular supply of nitrogen during the growing season definitely improved things.
Each of the bags of potatoes that I had been feeding with a bought in liquid fertility,
would have received a total of about 100mL of this concentrated fertiliser,
which translates to about 5-6 g of nitrogen in total being added to each bag.
With the concentrated soluble fertiliser, I had side dressed each bag with 25g of
pellets on two occasions, which would have added a total of about 9g of nitrogen.
With the urine it is harder to calculate, as the concentration of nitrogen in the
urine varies depending on diet and the amount of water that I drink.
But if I use estimated concentrations of 7.5g of N in each L of urine, I would have added
about 17g of nitrogen to each bag of two potato plants over the course of the growing season.
I suspect this extra amount of nitrogen was part of the reason why I got a significantly
higher yield out of these plants, as more nitrogen was added in total,
but I think more importantly it was being added on a regular basis,
in a form that could be made more available to the plant roots quite quickly.
The plants could take up some of the nitrogen through their roots before
the rest of it was absorbed into the living ecosystem of the compost itself.
And as a result these plants were able to grow for longer, even after a lot of the other plants
seemed to give up and die back earlier. I think mainly because the other plants
would have ran out of nitrogen, and possibly other nutrients, even though
there would have been an abundance available in the compost, just not plant available yet.
One other factor that probably affected all of this, is the moisture content of the compost,
and the difficulty I had in keeping up with the watering at times.
And this type of compost often seems to get hydrophobic when it dries out, where it seems to
repel water making it difficult to get wet again. I tried to keep up with the watering during the
dry spells that we had, but most of the water that I added would
have flowed down past any dry compost near the surface, flowing deeper into the bag.
Every time I checked there was still plenty of moisture in the compost at
the bottom off each bag, so the plants would have had access to enough water.
But with any compost that dried out near the top of the bag, this would have slowed
down any biological activity and nutrient availability in that part of the compost.
And it was in this top layer of compost that most of the amendments had been added,
so this could definitely contributed to the plants not being able to get
access to the nitrogen and other nutrients in that volume of compost.
Because the liquid feed and dilute urine would have added additional water to the bags,
this may be part of the reason for the better growth and increased yields,
or at least it would have brought the nutrients down to where the roots have been more active
But none of the plants actually wilted, or showed any signs of water stress, they just
weren’t able to get access to enough nitrogen or other nutrients in the slightly reduced
volume of compost at the bottom of the bag. And the bags that had been amended with the
granular soluble fertiliser, did a lot better, even though they would not have been given any
more water than the others, but they still grew better and produced more potatoes.
So I think the moisture levels of the compost would have been a factor, but it doesn’t explain
the significant difference in the yield or the growth of the plants that I observed.
So, it seems that I can get good results from this compost as a grown medium by simply adding regular
liquid feeding with something high in Nitrogen, and probably other nutrients would help as well.
This could be urine, or nettle tea, or some other form of high protein plants that are
soaked in water until the nitrogen and other nutrients are broken down into the liquid itself.
Or I could simply just buy in some liquid fertiliser that is high in nitrogen.
Keeping all the compost moist through the full volume of the soil bags is also important,
and I have invested in a dripper system to help with this next year.
Which would work well with the liquid feeding, with the help of a Venturi
fertiliser adapter or some other method of adding the nutrient dense feed to the irrigating water.
Adding soluble granular fertiliser on a regular basis seems to be an easy way to
really boos the yields, and adding other forms of concentrated amendments and side
dressing with these on a regular basis will probably help as well.
Amending with concentrated fertility at the beginning of the season didn’t show
much promise with this one crop, and I suspect it would take a lot more to really make a difference,
which could start to get expensive, and perhaps a waste of resources.
But most of that fertility and other minerals will still be in the compost, and should be
made available to plants at some point in the future, assuming that it isn’t washed out with
watering or excessive rainfall in the meantime. But it seems that with this compost I should
be trying to progress the process of decomposition before I can expect it
to feed the plants either in these bags or in the gardens that I might add this compost to.
Keeping the compost adequately moist and aerated will help, and I suspect adding some worms or
inoculating it with high quality biologically rich compost or activated compost tea will help.
It will be interesting to see how this compost has developed this past season, and how things
might change when I grow another hungry crop in the grow bags in the same compost next year.
But based on this trial, I think I can use this type of municipal compost as a
growing medium for vegetable plants, even when it isn’t fully decomposed,
so long as I don’t assume that the compost will be able to supply much of the nutrients
for the plants, and everything will need a lot of extra feeding.