Last autumn I planted a mixed green manure in my relatively new polytunnel, with the
purpose of improving the soil conditions in this high value growing space.
Earlier this spring I cut down the abundant growth, covered it with a ground cover fabric,
through which I planted tomatoes and other heat loving crops, based on the idea that
the decomposition of this sheet mulch of biomass will feed the soil and the plants throughout
the summer.
I learned a lot in my first attempt at building fertility in this way and at this scale, and
like almost anything else in the garden, there are many refinements that I would make to
the methods that I used.
If I was to do this again I would want to find a better balance between the amount and
quality of organic matter that was produced, the simplicity and ease of managing it, and
maximising the benefit to the soil and the following crop.
But when growing a green manure in this way it means that I can’t grow overwintering
or early spring crops in this valuable space, so there is a cost to this process of improving
the soil, in the form of having less food to eat, share or sell.
This raises the question if this strategy was the best option, when there are other
methods I could use to improve the soil quality and fertility, that would have allowed me
to grow these additional crops as well.
There are many benefits to growing green manures, or other types of cover crops, but how would
I go about answering that question of whether it was worth it in this particular context.
In order to figure out an appropriate answer to this question I need to go through the
possible benefits of a green manure and how they relate to the particular issues of my
situation.
Planting an overwintering green manures or cover crop like this can be useful to prevent
soil erosion, or to reduce the chance of nutrients leaching away in the heavy winter rains.
But those are not relevant in this context as this garden is protected by a sheet of
plastic.
Another benefit to growing specific types of green manures is that they have the capacity
to fix additional nitrogen in the soil, such as with the two types of clover and the vetch
that were part of the mix that I had sown.
This could be beneficial but is not an essential issue in my context as I had already added
a lot of nitrogen to the soil in the form of dried chicken manure.
There are also lots of other ways of adding nitrogen to the soil that are much more convenient
and affordable at this scale than growing a nitrogen fixing green manure.
Beyond the role of nitrogen fixing, I had selected the specific mix of green manure
seeds as it included a range of different plants that had a vigorous root systems, which
would grow deep into the soil, as well as leaving the topsoil in great condition.
The texture of the soil definitely seemed to improve, becoming more friable, and I suspect
that the soil condition was better than it would have been if I had chosen to continue
to grow crops, but I really don’t know, as some of these crops would have also had
aggressive and extensive root systems, which would have done a similar job.
Improving the texture of the soil can be done in a number of different ways, and if I was
going to grow a green manure I would definitely prefer growing something that left the soil
in great condition, but it wasn’t a critical factor here.
The weed suppressing potential of cover crops also wasn’t a significant issue, given the
work I had already done to remove the problematic weeds, and the plans I had for covering the
whole garden with a ground cover fabric in the summer.
Helping to boost soil life was one area that I was interested in, both through the beneficial
secretions from the roots of the plants while they are growing and in the availability of
food for the soil biology in the form of organic matter as the plants are dying back or decomposing.
I really don’t know where to begin to evaluate the possible impact of this, to compare the
possible benefit to the soil life from growing this green manure, compared to managing the
soil in a different way, and continuing to grow a range of edible crops.
A related part of all of this is the desire to increase the amount of soil organic matter,
both in the form of the soil life itself, and in the food for the soil biology, but
also in the form of humus and other long lasting organic compounds which enable the soil to
hold more nutrients and moisture as well as improving the soil structure.
This was one of the main reasons for growing this green manure in the first place.
But the big question for me is how does the amount of biomass that can be produced by
growing these plants, and the subsequent increase in the amount of soil organic matter, compare
to ways of improving or increasing the amount of soil organic matter, such as simply adding
compost or aged manure.
Looking at the impressive growth of this diverse plants that make up this green manure, I wanted
to get a sense of how much organic matter had actually been produced, and I decided
to take a sample to get some approximate figures.
I measured out 1m length of one of the beds that seemed to be typical of the growth in
the polytunnel, and I carefully cut and collected all of the above ground biomass.
As there are 5 beds in this polytunnel, each 20m long, this means that the section of biomass
that I removed represents about 1% of the 100m of the beds of the polytunnel, and therefore
approximately 1% of the total above ground biomass produced.
I was able to compress all of this organic material to fill a 25L (6.6 gallon) bucket,
so I can estimate that there was roughly 2500L or 2.5m3 or 3.25 cubic yards of fresh organic
matter in total, when compressed.
This amount of material also weighed 6.5kg, or 14.3 pounds.
so in total there would have been 650kg or 1430 pounds of wet biomass produced by this
green manure.
So, if I had removed all of the above ground biomass produced by this green manure, rather
than letting it decompose on the soil, it would have produced a compost pile that was
roughly 2.5m3 to start with.
This might have produced in the region of 1m3 or 1.3 cubic yards of finished compost
after months of decomposition, but it was possibly significantly less given that there
was only a total of 650kg of biomass material to begin with.
But of course, this was just the above ground stuff, and figuring out what the below ground
biomass was in comparison would be a much more difficult process.
It seems that with a lot of different methods of analysis there is often these big unknowns,
and in this case it is the amount of biomass produced under the surface of the soil, both
in the total quantity of the roots produced by the plants, and the amount of the secretions
released into the soil by the roots during the lifetime of the plant.
Even if roughly the same amount of biomass or organic matter was produced under the surface
of the soil, as was grown above the surface of the soil, the total amount of organic matter
produced by this green manure crop, doesn’t seem as impressive as I had thought, even
though it looked like a substantial amount of growth.
If I was to make a really rough estimation, I think that the total organic matter produced
above and below ground by this green manure would be roughly equivalent to between 1 and
1.5m3 of already decomposed compost.
The main reason for growing this fertility building crop was that I felt that I didn’t
have enough compost to adequately prepare the beds in this polytunnel, and I thought
that growing a green manure would make up for this.
When trying to amend the soil in an intensive growing space like this I would ideally want
to add at least enough compost to make a 25mm or 1 inch thick layer on the beds.
With 100m of beds, each 80cm wide, that is about 80m2 of growing beds, not counting the
paths, which would mean that that I would want to spread about 2m3 or 2.6 cubic yards
of compost.
If I could I would prefer to add double that amount, requiring 4m3, or more that 5 yards
of compost to produce a layer 50 mm or 2 inches deep, but I didn’t have anywhere near that
quantity of compost, which is why I grew the green manure in the first place.
But with my estimation the biomass from this green manure would have only produced the
equivalent of about 12mm of compost, or about half an inch, which is only 25% and 50% of
the amount of compost that I would preferred to have added.
In all of this I’m not sure how to account for the possible benefits of having increased
biodiversity and soil activity due to the decomposition of this material on the surface
of the soil or within the soil itself, rather that removing the material and decomposing
it in a compost pile somewhere else.
But in the end the amount of organic matter available to the soil ecosystem because of
this green manure crop was definitely substantial, and much better than doing nothing, but not
as much as I would have preferred or thought would have been produced.
This raises a really interesting issue for me about the amount of land that is needed
to grow the organic material to produce the amount of compost that I would want to add
to an intensive garden.
Having access to manure or lots of household and yard waste that I can turn into compost,
can be a great way to make up this amount of organic matter, which is what I'm doing
to supply a lot of the compost needed in my family scale gardens.
But if I was trying to grow the material specifically to produce enough compost to amend the soil
in this garden, then it seems that I would need an area at least as large as the gardens
itself, if not twice as big.
So, reducing erosion and retaining nutrients as possible benefits of a green manure or
cover crop are not relevant in this context, and fixing extra nitrogen would be beneficial,
but not really a priority in this case.
Improving soil structure, boosting the soil biology and increasing the soil organic matter
are all desirable benefits, but these can be achieved through other means, without giving
up the option of growing additional crops over winter and into the spring.
The amount of biomass produced by this green manure crop and the overall longer term benefit
to the soil was no doubt significant, but I think that I would have had to have grown
it for a lot longer, and cut and mulched the material a number of times to get the levels
of organic matter that I was hoping for.
Looking at this another way, buying in 2m3 of finished compost or well aged manure and
spreading it onto the beds, I think would have made more organic material available
to the soil, and with with a lot less time and effort.
The real benefit of course would depend on the quality of the compost that I was able
to get, or how much money I was willing to spend to get really good compost.
Having tried the approach of using a green manure, I think it would have been better
to have spent the money to buy in the necessary finished compost and to continue to grow the
crops throughout the year.
Or perhaps the best method would be to buy in the compost and to grow a green manure
as well, but only for a shorter time over the winter, and then to dig it in earlier
in the spring, so that I could get a spring crop.
But this depends on me having the time and capacity to manage the crops in this garden
during the spring.
This is where I think another important benefit of green manures comes in, to grow something
that benefits the gardens when we don’t have the time or capacity to grow and manage
another crop.
Because growing something is better than growing nothing, and a lot better than letting the
weeds get control again.
And this was probably the deciding factor for me in this case, as at the time I didn’t
feel that I could manage this additional growing space with so many other things going on.
So in this case it was really useful to be able to leave the space to grow a green manure,
to build the soil and to improve the soil fertility, so long as it wasn't too hard to
get rid of so that I could grow the crops that I wanted to, when I was ready.