For the past few years I have been buying in a lot of bulk compost from a municipal scale
composter for use in some of the gardens. While it is useful stuff, it is not very
rich or well decomposed yet, which limits what I can expect from it in
the vegetable gardens, at least initially. And I have been wondering if it just needs
time to mature some more before using in some situations or if it can be amended so that it
can be a better resource for vegetable growing. So this past year I filled a lot of large grow
bags with the remains of a huge pile of this mediocre compost, added different types of
amendments, and planted seed potatoes that were leftover from the main plantings in the gardens.
I planned this large trial in order to hopefully get a better understanding
about what might be the most useful way to amend or supplement this compost.
Although the amount of potatoes that was produced by the plants in these bags was
less than I had expected, I learned a lot, especially how this low grade
compost could be amended in order to grow a bigger crop of potatoes.
A few years ago I found I needed a lot more compost in the gardens than I could make myself,
especially when I changed to mulching with a finished compost as the main
way of managing the No-Dig Garden. So I started to buy in large bags of
this compost from a company that collects and processes landscape waste at a municipal scale.
This bulk supply was convenient, clean, supposedly free of plant diseases and weed seeds,
and didn’t contain any plastic or other debris, but it was reasonably expensive,
especially with the volume I needed for the developing new growing spaces.
Last year I joined up with the local community farm to buy in a full truck load of this compost,
and ended up with a fair amount of it left over at the end of the season.
The pile of compost had been covered for almost a year,
which had caused it to dry out in the middle, halting a lot of the decomposition process,
but the bottom layer of the compost was quite wet from the rain that hadn’t been able to drain away.
So it wasn’t a very uniform or consistent material, which isn’t
a great start to a big trial like this. But we used up the remains of the pile to
fill a load of large grow bags that I had invested in, with about 35-40
litres of compost going into each bag. We then moved these heavy bags to the
site between the two large polytunnels, and spread them out to hold down the ground cover
fabric that we were using to help clear the rough ground of persistent weeds.
We have used these large grow bags this year for a couple of different explorations,
including filling 90 of them with different amended soil and other possible growing medium.
These were planted with main crop potatoes as another large trial,
but for this compost amendment trial I wanted to use the remains of the bag of early seed potatoes
the I had bought for use in the other gardens. And I wanted to use potatoes, which are generally
a hungry crop, or at least can benefit from the high fertility in whatever they are grown in,
to test out different possible strategies for amending this mediocre quality compost.
I had 50 grow bags of compost set aside for this trial,
with enough seed potatoes to plant 2 in each bag, though in hindsight
I wonder if it would have been better to grow just one potato plant in each bag.
But before planting I divided the bags into 10 amendment types, 5 bags for each set, and
I tried to make sure that the bags from each set they were randomly distributed around the space.
I left one set of 5 bags without any amendment or additions, to serve as a type of control.
Three sets would have different forms of concentrated fertility added at the beginning
of the year, with a focus on nitrogen in case not enough would be available for the plants yet.
In two of the sets I mixed in other amendments that might be missing in the compost material,
without adding nitrogen, focusing more on improving the compost in the longer term.
In another three sets the compost was left unamended at planting, with plans to regularly
liquid or foliar feed the plants as they grew. And with the last set, I decided to try using
conventional soluble fertiliser, a non-organic form of fertility that is widely available in
this part of the world, but something I have never used in the gardens before.
So there was one set of bags with just the compost and nothing else added.
I mixed chicken manure pellets into the top layer of one set of 5 bags,
which has been my general purpose fertiliser in the gardens, and another set had hair meal
pellets mixed in which I have started to use specifically to boost nitrogen.
And to another set I added some Complete Organic Fertiliser or COF,
which is a fertiliser that I mix myself based on seed meals for lots of nitrogen plus and a range
of other concentrated amendments to hopefully supply a complete and balanced mix of nutrients.
I added some soft rock phosphate, some gypsum and some seaweed meal to another set,
thinking that this would supply a range of major and minor nutrients that might
be deficient in the compost, and that are typically low in the soils around here.
Volcanic rock dust was added to another set to provide a wide range of trace elements,
though the release of these nutrients would take place slowly over time.
One set of five bags was designated for a foliar feeding on a regular basis with a
dilute general liquid fertiliser that I buy in, containing extra potassium
which is apparently good for growing potatoes. Another set would be fed with weekly watering of
same general purpose liquid fertiliser, diluted to about 1 part fertiliser to 10 parts water.
And another set was designated for a weekly liquid feed with dilute urine, an abundant and
nitrogen rich resource that I collect myself. I wanted to use some conventional soluble
fertiliser on the last set, but didn’t get around to buying any
until the plants had grown quite a bit. So I sprinkled some 18:6:12 concentrated
fertiliser on the surface of the compost when the plants were starting to develop,
then added another amount a few weeks later to top up the fertility when the plants were fully grown.
In hindsight, I should have topped up the fertility in a lot of the different
types of amendments, as I wasn’t really impressed with the growth of the plants.
I hadn’t grown potatoes in containers like this before,
and this was my first year using this variety of potatoes,
so I didn’t know what to really expect. But the plants in the grow bags seemed
smaller than the same variety growing in the soil of the family scale gardens.
There was a period of high winds which impacted the plants quite a bit,
as the bags were spaced out and not able to shelter each other from the wind.
And I wonder how the relatively warm weather that we had at several times during the summer
would have affected things, as the compost in the black bags would have warmed up quit a bit,
but I don’t know whether this would have been a positive or negative impact.
I did struggle to keep enough water in the grow bags during the dry weather we had,
and when this compost dries out it can be difficult to wet again.
It was a lot of work to water all of the bags, and I suspect that I wasn’t
providing enough at times, which would have affected the yields in general, but no doubt
some bags were affected more than others. While there were differences in the size of
some of the plants, the first real difference I noticed was deficiencies showing in plant growing
in some of the amendment types, while the leaves of other plants stayed quite vibrant and green.
And as the season progressed, I noticed that the bags that had been amended at the start,
before planting the potatoes, were showing signs of the plants finishing earlier.
But the bags that were receiving a more regular top up of fertility
were continuing to grow for longer. And this first solid indication of
the main things that I learned during this trial, that the fertility in the compost wasn’t enough,
and the amendments that were added at the beginning of the season had ran out.
I started to harvest the bags of potatoes at the end of July when some of the plants were still
growing but others had died back, making sure to harvest one bag from each set of different
amendment types each time I harvested. I built a type of sieve or riddle to help
with the harvest, into which I could dump a lot of the compost from a bag,
including all the hidden tubers. I could then shake the riddle or work
with my hands so that all the compost would fall though the metal screen into a container below,
where it could be added back to the original grow bag, leaving all of the potatoes on the surface.
I really didn’t know what to expect when I first started harvesting, but wasn’t so
hopeful about how many potatoes I would find, and overall I was disappointed with the results,
at least in terms of the amount of potatoes that I harvested compared to the amount of
work that we put in to grow them. From the first few bags that I
harvested it was apparent that some produced a reasonable number of medium to large potatoes,
and others produced mostly small tubers. Some of this difference would have been due
to natural variation, or different condition of the original compost,
or the microclimate and amount of sunlight that that particulate bag got each day.
And I suspect the variable moisture that was available in the compost would have
affected the yield, but all of this would have been randomly distributed between
the different sets of amendment types. And I had to wait until all the 5 bags
of each type was harvested in order to better understand the difference
that each amendment type made in the weight or yield of the potatoes that I harvested.
After all the bags were emptied, I calculated that the 5 bags of plants grown in the compost that
was not amended, produced 1.4kg of potatoes from two plants, or about 700g per plant on average.
This is not a lot for two plants, and a bit disappointing compared
to the yield of 1.2 to 1.7kg that was produced by each potato plant
of the same variety grown in soil of the different family scale gardens this year.
And the range of harvests form each bag went from 1.4 to 1.6kg so fairly consistently poor,
but at least this sets a decent baseline that the various amendments can be compared to.
The bags that were amended with chicken manure pellets, hair meal pellets and the Complete
Organic Fertiliser mix, all did a bit better, but only marginal, which is disappointing.
The hair meal pellets, which apparently provide a
slower release of nitrogen did a bit better than the other two sources of fertility.
Two of the bags with the added the Complete Organic Fertiliser mix to seem to do better
than the other three, but I suspect this was likely due to variability in the original compost.
With the bags amended without nitrogen, the Soft Rock Phosphate, Gypsum and Seaweed meal
didn’t prompt any increase in the average yield, but the rock dust showed a slight improvement.
I suspect that again, this improvement is because two of the bags had compost in them
that was in better condition for the plants, rather than any effect of the amendment.
But with these two sets, I was looking more at improving the compost over the longer term,
so I should not be surprised about the lack of effect, though perhaps the potatoes could be
more nutritious, which is something that I would not have been able to measure.
The plants that were foliar fed also did not show any significant improvement, which I was
surprised about, and I suspect that spraying the leaves more frequently with a foliar feed
that was higher in nitrogen would have helped. So none of these six types of amendments showed
enough of an impact to really matter, but they could have had other benefits in the longer term.
The plants that were liquid fed with the same bought in fertiliser did
show a significant 34% increase in yield to about 1.9kg of potatoes per bag on average.
This is starting to get into the minimum yield I would hope to get from a crop like this.
But the plants that were regularly fed with dilute urine did significantly better again,
producing over a 50% increase in yield compared to the control, with an average of almost 2.2
kg of larger potatoes harvested from each bag. One bag produced a lot more than the others,
over 2.8kg of potatoes, which matches what we harvested from a lot of the potatoes grown
in the gardens, and I am not really sure why. But even if this one good harvest is ignored,
the urine fed plants did better than any of the other amendments.
The last set of plants which were amended with two side dressings of concentrated fertiliser
did well, and I wonder if they would have done even better if I had purchased this conventional
fertiliser earlier in the season, so that it would have had a chance to have a better effect.
So there is a lot to tease out in all of this, and a lot of it will need to
be explored in another video, but there are a few things that really stand out for me.
The first is that this type of municipal scale compost is really quite poor as a
source of nutrients for the plants and I would be hesitant to dig it into the soil in the gardens,
though using it as a mulch on the surface of the soil is probably fine,
but I should not expect much of a benefit for a while.
Although there is likely a huge amount of nitrogen in the compost, most of it would be locked up in
the continuing process of decomposition, and very little of it was made available
during this trial, or at least not enough of it to keep the hungry potato plants well fed.
Adding additional fertility at the beginning of the season didn’t seem to help much,
and I suspect that a lot of the fertility that was added was simply absorbed by the microbes engaged
in the continuing decomposition process of the compost, and very little was left for the plants.
In order to grow directly in this type of compost a more regular feeding seems to be
much more effective, as the plants seem to be able to get what they need before the rest of
the nutrients are absorbed by the compost. In hindsight this all makes sense,
that the visible particles of partially decomposed material in the compost would
indicate that a lot of the nutrients would be still locked up in the compost itself.
A regular liquid feed with something that is higher in nitrogen seems to work best, and I
was impressed how effective the diluted urine was at boosting the yield, even though I was not so
consistent with keeping up with the feeding. Based on this one crop, a lot of the other
amendments seem to be a waste of money and resources, or at least they should
have been added at several occasions while the plants grew, rather than all at the beginning.
But all these other amendment may have benefits to future plants grown with this same compost,
and I plan to continue with the trial with other crops over the winter and again next year.
But with the success of the urine liquid feed I think that it is would be useful to liquid feed
all the bags of compost on a regular basis with something that is high in nitrogen,
as this seems to really help. But then I get into the issue of
reduced availability, as I can only produce so much urine, even though it is free.
And getting enough urine to regularly liquid feed all of the compost in all
of these grow bags, would require a few more people to get involved.