Soil testing is a valuable tool for some growers as it helps to identify nutrient deficiencies,
and to optimise yield through more precise fertility applications.
And other growers don’t get their soil tested, either because they feel it’s not necessary,
or is hard to access and understand, or perhaps feel that this kind of chemical
analysis does not account for the more important biological processes in the soil.
Personally, I have mixed opinions about soil testing, and only use them to make make decisions
about fertility applications in a few of the gardens that we manage.
Three years ago I sent samples of the soil from all the main growing spaces to a lab for testing,
and repeated the same testing again this year. It is really interesting to see how some things
have change and other things haven’t, and how the different ways that we manage the gardens
has affected the nutrient balance in the soils. These detailed tests have enabled me to identify
some issues with the management practices in some of the gardens, and potential issues with some
of the resources that we have been relying on, and to better understand our soils in general.
For people who grow in naturally acidic soils, measuring pH can
aid in adjusting the soil by adding lime. However, in areas with calcarious soils,
like the soils we have, measuring the pH is not so useful, except to confirm the presence of a
lot of calcium, which I already knew. Based on this year’s soil tests,
most of the growing spaces in the RED Gardens had a pH of 7.7 or 7.8, which is quite alkaline.
The older polytunnel had a pH of 7.5, and the No-Dig Garden had a pH of 7.2, but these
readings may have been influenced by the remains of the compost layer on top of the growing beds.
I also tested the soil that had been under the composting area, which had a slightly lower pH and
another pile of topsoil that we have been using for the grow bag trials, which had a pH of 7.1.
But that topsoil had been brought in from another part of the site, so it’s not surprising that it
would be slightly different than the soil that is in the area of the RED Gardens.
Apart from the latest No-Dig Garden sample,
the pH of the soil of our gardens has remained in the 7.4-7.9 range for quite a few years.
Even when the pH changed a bit, as it did in the Extensive Garden which dropped to 7.4 after I had
added a lot of gypsum ten years ago, it seems to generally come back to the 7.7 to 7.8 range.
So I have accepted that I can’t significantly change the pH of the calcareous soils that we
have, except perhaps by adding excessive amounts of compost.
Despite this, we can still grow great quality vegetables in these gardens,
and the higher pH can apparently reduce the risk of clubroot in the Brassicas crops.
However, we do have to accept that there will often be a lot of scab on the potato skins,
as the bacteria that causes it apparently thrives in alkaline soils.
I use to think that the pH of a soil was primarily determined by the amount of available calcium,
with higher levels of calcium resulting in a higher pH, and vice versa.
However, comparing the calcium levels to the soil pH,
there does not seem to be a strong correlation. The soil with the highest calcium percentage had
the lowest pH, while another sample with much less calcium still had a relatively high pH.
And two gardens can have significantly different calcium levels,
but still share the same levels of alkalinity. Looking at all of the soil tests that I have
done over the years with the same testing method, there may be a general correlation between the
calcium levels and the soil pH, if I disregard the outliers, but the connection is not very clear.
So obviously there are other things going on besides the levels of calcium, and this is
another reason why I don’t rely on pH when trying to figure out how to improve this calcareous soil.
This type of detailed soil test provides information on the quantity of different
nutrients found in the soil sample, extracted in the lab with a solvent
that apparently mimics what a plant might be able to easily access.
And with the calcareous soils I have been advised to get a soil test that uses an alkaline solvent,
as the weak acid that is normally used in these tests can really distort the results.
These soil tests also calculate the Base Saturation Percentage,
which indicates the proportion of Calcium, Magnesium, Potassium and Sodium that make
up the total amount of cations or positively charged nutrients found in each soil sample.
The tests indicate a recommended proportion of each of these nutrients that should be
available in the soil, in order to provide an appropriate balance for good plant growth,
but these recommendations are based on an acidic soil, including
a significant amount of exchangeable hydrogen. But none of the tests that I have done have found
any exchangeable hydrogen, so the recommended proportions need to shift in order to be useful.
Most of what I understand about soil testing I have learned from Steve Solomon and Erica
Reinheimer, and thankfully they have worked out recommended proportions
that are suitable for calcareous soils. Comparing the RED Gardens’ soil samples
to both versions of preferred or target proportions the thing I notice the most
is the excessive amount of potassium, and the relative proportion of potassium to
magnesium indicates that there may be too much potassium or not enough magnesium in the soils.
Looking at all the tests in this way, there is quite a difference
in the proportions between the different gardens,
which can be attributed to how we have been managing their fertility over the years.
The Extensive and Simple Gardens have lower potassium levels, ranging from 3 to 5%, but
we don’t add a lot of compost to these gardens. We do use a lot more compost in the Intensive,
No-Dig and Polyculture Gardens, typically the municipal compost that is made with a
lot of woody material, which probably led to the higher levels of 8 to 9% potassium.
And in the two polytunnels there is 11 to 13% potassium, which is much higher than
the recommendations, which is partially due to the use of the municipal compost.
These covered growing spaces are also a lot drier, with less irrigation to wash excess potassium and
other nutrients deeper into the soil, so perhaps the levels of potassium can build up a lot faster
than in the outside gardens where the heavy rains we get can cause lot more leaching.
The tests in did in all these gardens 3 years ago show mostly
2-4% potassium which is close to the general recommended ranges,
but this has increased in all the gardens, except for the Simple Garden, which is probably because
we have used a lot of municipal compost in the other gardens in the last three years.
Looking at the changing levels of calcium over the same period, the reduction in the
amount of calcium generally reflects the increase in potassium, so the potassium is
replacing the excess calcium in these soils, and in some cases it may have gone too far.
However, it is also possible the soil samples that I took in some of the gardens contained a
fair amount of the compost that we have been adding to the surface the beds.
I tried to scrape away a lot of this compost layer that remained from the previous year
when collecting the soil sample, but if enough of it was included in the sample,
it could distort the lab results quite a bit. This isn’t such an issue in the outside gardens
where the plant roots can generally occupy a lot of this surface layer, but in the
drier conditions in the polytunnel, this surface compost has a tendency to dry out, delaying the
decomposition or incorporation into the soil. So the levels of potassium might not be so
high deeper in the soil, where the plant roots mostly colonise.
There is a lot to work through with the relative proportions of these main
cations found in the different samples, and I can easily get caught up in the details.
However, these tests also convert all these details into an overall
measurement called the Total Exchange Capacity,
which as I understand it is an indication of the total amount of these positively charged cation
nutrients that the soil sample can loosely hold onto and make available to the plants.
This capacity is influenced but the quality and quantity of the clay particles in the soil itself,
which I don’t think I can’t really change, as well as the presence of various forms
of organic matter in the soil, which can change depending on how the soil is managed.
Put simply, a higher Total Exchange Capacity should indicate a better soil
quality and greater nutrient holding capacity, which should help us grow more plants, but a
lot of that depends on the proportions of the different nutrients that are actually contained.
When I first started these gardens the Total Cation Exchange Capacity was around 12, which
apparently if fairly good, and it is interesting to see that this has increased in some of the
gardens, remained relatively stable in others, and possibly even dropped in the Simple Garden.
When comparing the Total Cation Exchange Capacity, to the percent of organic matter
in the soil samples, there is good correlation between them with most of the gardens I tested,
with an increase in organic matter resulting in a higher nutrient-holding capacity.
For the one larger polytunnel, this correlation or relationship seems to
be broken, as the test came back with more than 20% soil organic matter, but the Total
Exchange Capacity remained relatively low. I suspect that this is because too much of the
dried and undecomposed compost from the previous year made it into the soil sample I collected,
which skewed the results, but there could be something else going on.
Last year, I used the soil from beneath our composting area as
part of a potato grow bag experiment, as I expected that some of the compost
and nutrients would have worked its way down into the soil below.
The potato plants thrived without additional amendments, and I was curious to compare this
soil to the soil in the nearby gardens. Surprisingly it had the highest Total
Exchange Capacity, yet its organic matter levels were lower than expected.
This could be due to this soil containing a higher proportion of stable forms of organic matter that
can hold onto more nutrients, and less material such as plant roots that are still decomposing.
The soil sample also contains 27% potassium, which is a huge amount, three times the amount found in
any of the outside gardens, and twice the really high levels found in one of the polytunnels.
This has displaced a lot of calcium, bringing the levels
down to something similar to an acidic soil, but it still has a pH of 7.4.
So something quite different has happened to this soil.
I wonder how much of it is because the soil is often covered by big piles of compost,
perhaps creating anaerobic conditions, which could change the nutrient balance,
and the compost above is often covered so the rain can’t flush any of the excess potassium away.
And without plants regularly growing in the soil to extract the nutrients,
all of this could explain why this soil is so different.
Analysing the results for 11 different nutrients from all of these tests can be overwhelming,
so I find it helpful to calculate targets for each nutrient in each garden based on
recommendations from Steve and Erica. Then, I look at the test results for
each nutrient as a percentage of that target, to identify where there are common
deficiencies or excesses and where there are significant differences between the gardens.
And then taking the median or middle of these percentages, gives a better overview of the
soil profile in all the gardens. Looking at the main cations in
this way I can see that potassium is definitely way above recommended target,
and there is generally plenty of magnesium, but the proportions between the two may not be ideal.
The levels of calcium are generally adequate, but the amount of sodium is significantly below
the target, and adding some salt to the soil of the gardens may actually be beneficial.
The trace elements iron, manganese and zinc are all above targets in this soil,
but boron levels are a bit low, and there is a lot less copper than desired,
at least compared to the recommended targets. I do add small amounts of borax and copper
sulphate in the Extensive Garden to improve these nutrients levels, but I don’t know if
these lower levels in the other gardens are causing possible issues with plant growth.
Looking at the main anions, or negatively charged nutrients, the levels of phosphorous seem fine,
but I am concerned it is out of balance with high potassium levels.
But the levels of sulphur are consistently low, and the calcareous soil we have may be
complicating things with this essential nutrient.
The soil tests show that the sulphur levels are consistently low in the outside gardens,
really high in the smaller polytunnel,
and moderate in the other polytunnel and in the soil under compost.
The typical levels of sulphur have always been low in this soil, around 20 parts per million or less.
And in the outside gardens this hasn’t changed significantly,
even with the Extensive Garden where I have been regularly adding gypsum or agricultural sulphur.
I have also been adding sulphur to the polytunnels, and the moderate levels that are
found there may be due to the lack of rainfall to wash it away, but the exceptionally high levels of
sulphur in the smaller polytunnel are surprising. I would have expected that much available sulphur
to lead to considerably lower levels of calcium and a more acidic pH than was found in this soil.
I suspect the high levels may be due to contamination during soil sample collection,
possibly picking up the remains of sulphur pellets that I had been adding in the past, and another
test would be required to confirm this suspicion. However, with the consistently low levels of
sulphur in the other gardens, I think it may be beneficial to regularly add small amounts of
sulphur or gypsum to these soils, as it appears to be the one nutrient that is consistently lacking.
But I don’t really know about the effects of low sulphur levels on plant growth and
the health of soil organisms, and if it’s even worth worrying about.
These soil tests have helped me to gain a better understand of what is happening with the soil in
the different gardens, how they are changing over the years, and to identify potentially
issues that I might need to address. In many ways, they are similar to
blood tests that we might take to investigate issues with our own health.
But what to do with that information really depends on the different
management strategies of the different gardens. In the Extensive Garden and in the polytunnels,
I plan to use the tests results to amend the soil and to adjust the
types of concentrated fertility I add. With the other gardens, these tests
have helped me to understand one of the key potential drawbacks of relying too heavily
on municipal compost, which seems to have brought in excessive amounts of potassium.
I am not sure how to deal with this, or if any action is actually necessary,
but I plan to at least cut back on the amount of this compost that we use,
and to avoid bringing in additional potassium in some of the other sources of fertility.
I am interested in exploring the effects of adding more phosphorus and magnesium to a few of
the gardens to balance out the excess potassium. I also want to try to figure out how to increase
the amount of soil organic matter and to boost the nutrient holding capacity in the gardens
that have not really improved over the years. As usual, there is a lot more to explore,
and to keep an eye on, and it will be interesting to see the results of next year's soil tests.