Earlier this spring I collected soil samples from 7 of the growing spaces we manage,
and sent them off to a lab for detailed analysis, which is something I have done
periodically over the past 12 years. The test results came back with a lot
of interesting details about the availability of a wide range of major and minor plant nutrients.
In addition the tests also indicated the % organic matter found within each soil sample, which is a
key indicator of how successful the different management practices in the gardens have been.
Over the years, the amount of organic matter in some of the gardens has gradually increased,
stayed relatively the same or even decreased in other gardens, and even
changed quite dramatically in one of them. Where the organic matter has increased in the
soil, this indicates that additional carbon dioxide has been drawn out of
the atmosphere where it is increasingly casing problems, and sequestered as the
main component of the organic matter, that is helping to build soil in the process.
But this sequestration involves more than just carbon, and I am beginning
to see how this might be limiting soil development in some of the gardens.
Based on these soil test results, the percentage of organic matter in the seven gardens currently
ranges from just under 9% to over 20%. However I suspect that 20% is not
really representative of the actual soil in that polytunnel.
I think that the soil sample that I took may have included too much of the desiccated compost on
the surface of the beds in this protected growing space, leading to a result that is way higher than
anything I have seen in any of the other gardens. Excluding that potential outlier result,
the No-Dig Garden had the highest amount of organic matter at almost 16%.
The soil within the smaller polytunnel contains about 13.5% organic matter,
and 12.5% was found in the Polyculture Garden, and both of these seem reasonable.
An unexpected result is that the soil in the Intensive Garden only has about 11%
organic matter, especially compared to the almost 10% in the Extensive Garden,
despite significant differences in the amount of compost added each year to these gardens.
The soil in the Simple Garden seems to be degrading, as it only has about 8.5%
organic matter, which is one of the lowest results I have ever seen for the gardens.
The soil in this garden is definitely lighter in colour than the soil in some of the other gardens.
Comparing the dried soil samples side by side, the difference is obvious,
with the soil becoming much darker as the proportions of soil organic matter increases.
I began testing the soil in some of the gardens over a decade ago,
with the others being established since then. And it is interesting to observe how the amount
of organic matter in the soil has changed over time, or not.
Back in 2012 the five gardens that I tested all had below 10% organic matter,
and since then the levels of organic matter in the Extensive Garden have remained relatively
consistent, ranging from 9-11%. In this garden we feed the plants
with concentrated fertility, and seem to be adding just enough compost to maintain the soil quality.
The levels of organic matter in the soils of the Intensive Garden have only slightly increased,
despite adding at least 25mm of compost each year onto the surface of the beds.
In contrast, the amount of organic matter in the No-Dig Garden has increased dramatically to about
16% after I started using the Dowding method of adding substantial layers of compost each year.
The levels in the Polyculture Garden have gradual increased with a steady application of compost and
biochar, both of which I think will be included in % organic matter, but biochar doesn’t decompose.
Unfortunately the soil of the Simple Garden appears to be degrading, at least based on the
two tests I have done, but I only tested the soil in the one bed that had been covered with a huge
amount of organic matter two years earlier. In the older and smaller Polytunnel Garden,
the amount of organic matter in the soil has been gradually increasing at a fairly steady rate.
Ignoring the unusually high results this year from the larger polytunnel in the Black Plot,
it's interesting to note that this soil initially had low levels of organic matter
in the beginning of 2018 before I grew a big green manure crop, which was mulched on
the surface of the beds for the 2019 season underneath a crop of tomatoes and peppers.
However, the next year, in the spring of 2020, the organic matter had dropped further to 7.5%,
raising questions about the extent to which green manures can actually build soil,
at least based on these very limited results.
There are potential issues with this type of soil testing, which can produce misleading results,
as I think happened with the larger polytunnel. I was also unsure how much of the layers of
compost to scrape aside when collecting the soil sample from the No-Dig Garden,
but as the plant roots seem to be able to colonise a lot of the compost it is probably
better to include most of the layer. In the other smaller polytunnel,
the results have been showing a gradual increase in the amount of soil organic matter in over
12 years, which I think I can trust. The test results went from about 9.8%
to 13.4% organic matter, which is an increase in 3.6 percentage points over a 12 year period,
or it takes about three years to increase by one percent.
But this means that there is about 36% more organic matter in this
soil compared to the initial baseline when I started growing in this garden.
And the Total Exchange Capacity, which is a measure of the amount of nutrients that the
soil can hold, has increased from 12 to over 17 in the same time period, and is likely due
to the increase in soil organic matter. This is essentially a process of soil
building that should result in the soil being able to support more crops.
All of this organic matter is carbon-based, including the soil organisms, the decaying
parts of plants, and the residues from the compost and mulch that I add to the gardens.
By making some basic assumptions and using typical figures, I can calculate out how
much carbon there is in this topsoil. These soil tests are conducted on the
top 15cm of soil, which weighs about 220kg per square meter or approximately
22 tonnes for the entire 100m2 polytunnel. So the total weight of organic matter has
increased by almost 800kg over the 12 years, averaging about 66kg per year.
Soil organic matter apparently contains about 58% elemental carbon, so this means that there
has been an increase of almost 460kg of carbon, or just under 40kg of carbon sequestered each year.
These figures are approximate of course and only account for the top layer of soil,
but that’s where most of the organic matter is found
The amount of organic matter in the full depth of soil would likely be slightly larger.
460kg of carbon translates to 1.7 tons of carbon dioxide being pulled out of the air by plants and
sequestered or temporarily stored in the soil in this one garden, or about 140kg of CO2 each year.
Doing similar calculations for the other gardens, some are
losing carbon and others are gaining carbon. But excluding the possible faulty data from
the other polytunnel, in total the 6 gardens are apparently pulling an average of almost 500kg of
CO2 out of the atmosphere each year and storing it in the topsoil, which doesn’t seem like a lot.
Or, in reality we are adding a huge amount of compost and mulch to these 6 gardens each year,
and a portion of the carbon that is in the compost has remained in the topsoil, in addition
to some of the carbon in the remains of the vegetable plants and green manures that we grow.
If management practices change, or I decide to use less compost each year, the amount of soil organic
matter may actually decrease, releasing some of the carbon back into the atmosphere again as CO2.
I doubt that this rate of increase of soil organic matter and carbon sequestration
can continue indefinitely, and it also requires substantial amounts of resources and apparently
requires the availability of other key nutrients. From what I understand, in order for carbon to
remain in a very stable forms in the soil, where it can provide all those
great benefits to the soil structure, soil life and plant growth, the carbon needs to
be combined with nitrogen, phosphorus, and sulphur in fairly consistent ratios.
Apparently these three nutrients act as stabilising elements for the more stable forms of
carbon in the soil, and if these three elements aren’t available in the right proportions,
then the carbon won’t stay in the soil. I think this is really interesting, and
this could be a key factor in why some gardens see an increase in soil organic matter while others
don’t, that it is not just about how much carbon that is added, or if the soil is tilled or not.
According to this research, every kilogram of carbon in these stable forms in the soil
requires about 80g of nitrogen, 20g of phosphorous and 14g of sulphur.
If I assume that the yearly increase in organic matter in the polytunnel is
in this stable form of carbon, and I'm not entirely sure that it is,
then the 40kg of carbon sequestered each year would require 3.2kg of nitrogen,
700g of phosphorus, and 560g of sulphur. That is a lot more than I though it would be,
especially if it happens every year, and this amount needs to be in addition to the
amounts of these key nutrients needed by the plants and the soil biology.
There may be other factors that I don't understand, but this seems to be a
significant cost in building soil and sequestering carbon, that I had not really considered before.
I think this is quite a different way of looking at the processes of building soil in the gardens.
It is not just about the carbon, the compost, cover crops and the diversity of soil organisms,
but also having enough spare nitrogen, phosphorous and sulphur to be sequestered with that carbon.
Nitrogen is of course one of the key nutrients in a vegetable garden,
and some of the vegetable plants I grew use to struggle because of a lack of nitrogen,
until I started using better quality compost, and concentrated forms of nitrogen to supplement
the lower quality compost we were importing. The tests that I did 12 years ago revealed
lower levels of phosphorous in the soil when I started the gardens,
but these levels have gradually increased over the years, though not as much in the Simple Garden.
The tests also revealed a low baseline of available sulphur, which is apparently
easily locked up in the abundant calcium in the calcareous soil that we have.
Despite the addition of compost, general amendments, and occasionally concentrated
sulphur to the various outside gardens, the level of available sulphur has not
changed significantly over the past decade. The Simple Garden's available sulphur, are
particularly low, well below the desired levels. I suspect that it is this lack of sulphur,
possibly in combination with lower levels of phosphorous,
that is preventing the more stable forms of carbon from being established in this garden.
If the soil biology and plant roots find it easier to obtain the sulphur that they
need from within the stable organic matter, rather than trying to extract the sulphur that
is locked up in the calcareous soils, then I can see how the organic matter levels in this garden
could be declining, or at least not increasing. The lack of soil building in this garden could
partially be because of a lack of sulphur, not necessarily because of a lack of carbon.
Continuing to increase the organic matter content in the
soil of all the gardens would be beneficial. More organic matter in various forms would
provide more food for soil organisms, and improve the soil's ability to hold water and nutrients
all of which can be beneficial for the plants. And, building up organic matter, particularly in
the more stable forms, mans that we’re drawing more carbon dioxide out of the atmosphere and
sequestering it as carbon in the soils. But if specific proportions of nitrogen,
phosphorus, and sulphur are needed to retain carbon in those valuable stable forms,
the nutrient with the lowest availability could become the limiting factor.
In some gardens or ecosystems where nitrogen, phosphorus, and sulphur are all available in
sufficient amounts, it might be the amount of carbon that is the limiting factor.
In other soils, limited amounts of nitrogen or phosphorus that might be causing issues.
But in all of the gardens that we manage, it seems that not enough available sulphur could
be limiting the buildup of soil organic matter, especially in the Simple Garden.
If this is what has been happening, and if we can easily correct this by regularly adding
gypsum or some other form of sulphur, then we might be able to improve the soil some more,
so that it can better support the plants we grow, and we could harvest even more food.
It would also make better use of the carbon rich biomass that we collect
from the surrounding landscape for use in this Simple Garden,
and make better use of all of the compost that we make and import for use in the other gardens.
To keep hold of as much of this carbon as possible, to help develop the soil
for future seasons, while also feeding the plants this year in order to feed ourselves.