How allotment gardening can save the planet.

Charlie Heasman; 6th Nov 2019




to start off I should apologise for the blatant click bait that is the heading above; allotment gardening cannot save the planet, but I hope to use it as an example to show what can.  I’ve gained your attention, hopefully I can hold it and you will read on.  You may be surprised….

…When my wife and I took on an allotment some years ago I was interested in vegetables; she was interested in vegetables and flowers.  Neither of us were over concerned about soil.  This soon changed.

It turned out that our soil was not particularly good; in fact it was mostly pretty bad.  This was surprising in its own way because only a few years previous it had been agricultural land, to whit: a field.  One would have expected it to be better.

Meanwhile Marion decided that we were going to grow prize winning carrots.  Carrots like fine soil with no stones to encourage forking, so she set to work on a small bed.  She dug down to a depth of 18″ and put every last ounce of it through a fine sieve, discarding stones as she went.

This took a considerable amount of time and effort but eventually she had a fine tilth that even the most dedicated gardener would be proud of.  I remember remarking that if she added some milk and eggs she could probably put it in the oven and bake a cake.  We sowed our carrots instead.

They germinated and grew.  A bit.

All through the summer we kept them watered but they refused to grow to any decent size.  We found out why when we lifted them.

Despite the soil being a fine powder when they went in, and nothing heavier than a blackbird had walked on it since, it had set like concrete.  When it dried out we literally needed a lump hammer to break it up.  We’d just had our first experience of degraded agricultural soil.

In fact we’d already started improving the earth in the allotment generally, buying in compost and both horse and cow manure, gathering seaweed and getting our compost system up and running.  Then in 2018 disaster struck when we bought in a load of contaminated cow manure.  Which I’ve written about before.

At this point Marion declared that we weren’t going to risk buying in anything else, so we’ve stepped up our composting system further and are now ‘closed loop’.

IMG_3402 (1)

3 Bin ‘conveyor’. Far right is a newly emptied bin ready for the next batch; middle, freshly turned and half way there; left, ready to use.

Anyway, the point is that we’ve been continually building the organic content of our soil to improve fertility and soil structure.  In so doing, and completely by accident, we’ve also done something else which is rather surprising.  So hold that thought and let’s talk about saving the planet.


Most of us, unless we’re some class of idiot in the White House, are agreed that we are facing very urgent Global challenges.  Paramount for the majority, and rightly so, is Global Warming.

But there are a whole host of other problems.  Some, like plastic pollution, well known; others like freshwater shortages or soil degradation not really on people’s radar as yet.  In fact soil degradation and global warming are inextricably linked, as we shall see shortly.

Meanwhile we face problems when talking about climate change.  The first is that it is a complicated business requiring joined up thinking and our brains prefer simple linear thinking: methane is a greenhouse gas and cows burb methane = cows are bad = get rid of cows = problem solved.

Cows are ruminants, ruminants have been wandering the Earth burping methane for the last 20 million years; it recycles.  In fact ruminants play a vital role in soil ecology and health and cows, believe it or not, can be used to do so.  Too many cows might not be a good thing but that is a far cry from the simplistic mantra of cows – methane – bad.

Greenhouse gasses are another example.  Methane is a greenhouse gas, so is carbon dioxide, therefore methane is bad, carbon dioxide is bad.  They’re not.  In excess they are, but so is too much of anything.  Pure oxygen will kill you in the most spectacularly painful way if breathed under pressure.  It’s all about balance.

We also tend to put our faith in technology to sort things out.  This is hardly surprising, we’ve come a long way with technology; we’ve used it to shape the world and overcome nature (or so we believe), why stop trusting it now?

This reliance on technology leads some to believe that the answer to Global Warming is to pump CO2 into the ground.  It’s called Geological Sequestration and it’s incredibly expensive and energy inefficient.  In fact it’s so inefficient that in order to mitigate the CO2 output of three coal fired power stations you’d have to build a fourth to do the job.  Clearly ludicrous.

Even more cringeworthy is to somehow fire CO2 into space.  Yes, incredible as it seems this has been postulated.  Whoever did so seems to have overlooked the fact that for every carbon atom you get rid of you’re also losing two oxygen atoms!  If only there was an easier way.  There is, and we’re coming to it.

But before that, one further problem.

When talking climate change the numbers are simply too big for the human mind.  This means two things: 1) the problem can seem so huge that we despair and give up even thinking about it and 2) even if we do still want to think about it our eyes and brains simply skid over the noughts.  If I were to write something like “2,108,000,000,000 tonnes of CO2” the chances are that (just as if you did it to me) you wouldn’t even bother to work out what it meant.  So I won’t.

The answer is 2.1 trillion tonnes by the way, and it really does represent the sort of numbers we are talking about.  It is the amount of CO2 that some authorities say we have to remove from the atmosphere.  That looks so depressing.  Clearly we need to speak in more human terms.  Let’s start by dividing by four.

CO2 is the chemical formula for carbon dioxide.  This means it has one carbon atom and two oxygen atoms.  Furthermore carbon has an atomic weight of 12; oxygen 16.  12+16+16=44, this gives a conversion factor of 3.67.  In other words the carbon component of CO2 is a whisker over one quarter.  To be precise, 2,108,000,000,000 tonnes of CO2 becomes 575,000,000,000 tonnes of carbon.

Now, that’s still a huge figure but we’re going in the right direction.  Next all we need is a way of splitting 2.1 trillion tonnes of CO2 and somewhere to put the carbon.

The answers to both questions are staring us in the face.  Let’s deal with where to put it first.

A few paragraphs ago I said that soil degradation and global warming are inextricably linked,  Let us consider that.

We are accustomed to thinking that the woes of the world all stem from the Industrial Revolution and more specifically our own generation.  It might ease our consciences to realise that for once we’re not entirely to blame.  Soil degradation began the moment the Mesopotamians learned how to plough the land 12 thousand years ago and has continued ever since.

Archaeologists can trace the rise and fall of civilisations and, while of course there were all sorts of other circumstances such as war and conquest, in virtually every case they can find a correlation between a fall in soil fertility and population decline.  The Romans did it, the Greeks before them (as Plato himself observed), and the peoples of the Euphrates and Tigris valleys before that.  The only civilisations that escaped the effect were those on river deltas that had the benefit of seasonal floods which fertilised the land such as the Nile or Yangtze.

Image result for plato"

Plato: The rich, soft soil has all run away leaving the land nothing but skin and bone. But in those days the damage had not taken place, the hills had high crests, the rocky plain of Phelleus was covered with rich soil, and the mountains were covered with thick woods, of which there are some traces today.


Of course, we have considerably speeded up the process in the last century or two, so we can by no means be completely exonerated.  But the end result is that it is now calculated that there are some 5 billion hectares of degraded soil around the world, mostly in the Northern Hemisphere.

And what is soil degradation?  Quite simply it is the process whereby soil loses some or all of its organic content, in other words: carbon.  If we want to sequester carbon there are 5 billion hectares that could not only absorb it but benefit from doing so.

Of course, if we want to store all this carbon in this way we have first to devise an efficient way of getting it out of the atmosphere and splitting the CO2 molecule.


But wait!  What about photosynthesis?  It’s a proven method; it’s low tech, non tech in fact; it’s completely free.

We all know the basics: plants use sunlight to convert CO2 and water into sugars and give off oxygen in so doing.  The process is usually illustrated thus:


Which, of course, is a gross oversimplification.

What the formula does not show is that much of the carbon is retained by the plant and used to build itself.  Plant cell walls are made of the stuff,  These sequoia are towering pillars of captured carbon.


Sequoiafarm Sequoiadendron giganteum.jpg

There have long been misconceptions here.

It used to be thought that plants drew all that carbon out of the ground.  We now know that they don’t; they get it from the atmosphere.  It’s still true that when a tree sheds leaves, branches or falls altogether the material is broken down and returned to the soil, but we have only recently learned something else: in the meantime plants deliberately transfer some (up to 30-40%) of the carbon they have captured down through their roots and into the soil.

Which seems a pretty unlikely thing to do.  What’s in it for them?  Why would they bother?

The answer is that they are feeding a complex micro-system of fungi, bacteria, viruses and invertebrates with which they have a symbiotic relationship and without which they would struggle to survive.  This is true for all plants from the mighty sequoia to the humblest of grasses.  This area around the roots is known as the rhizosphere and is too complex to go into further here.  Suffice to say that it is (or should be) there, it’s full of carbon, and it’s essential to healthy plants.

So we all go out and plant trees.  Problem solved.

And this is exactly what many are advocating.  I for one would not disagree.  Up to a point.

We can’t plant trees all over the planet; there are some places where they simply don’t belong.  We also have to feed ourselves and can’t plant up all of our agricultural land.  But do we actually need to?

The idea of carbon sequestration using forests is one that has caught the public imagination, but in soil?  Never heard of it.

Fortunately this is changing, albeit not quickly enough.

One reason this is so is because if one looks at a hectare of rainforest there seems to be an awful lot of plant material there; look at a hectare of grassland and there’s very little to be seen.  Grassland couldn’t possibly hold as much carbon as woodland, could it?  But it can.

The reason for this is that a healthy grassland is a solid mat with far more root material below ground than it has foliage above it.  The soil can be thought of as one solid rhizosphere with no gaps.  This compared with a forest with lots of empty gaps.

If we think back to degraded soil, it has very little carbon and therefore a tremendous potential to absorb more.  Here’s some simple maths:


• One hectare = 10,00 sq metres

• Soil 33.5 cm deep (1 foot approx)

• Bulk density = 1.4 tonnes per cubic metre

• Soil mass per hectare = 4,700 tonnes

• 1% change in soil organic matter = 47 tonnes

• Which gives about 27 tonnes of soil carbon

• This captured 100 tonnes of atmospheric CO2


So if we can change our grassland management in such a way that nature effects a 1% change in soil structure we’ve captured 27 tonnes of carbon or 100 tonnes of atmospheric CO2.  Why 100 tonnes?  Remember the 3.67 carbon/oxygen ratio.

Multiply this by the 5 billion hectares of degraded soils worldwide and you get a very big number indeed: 5,000,000,000,000 tonnes of atmospheric carbon dioxide capture.

Of course this is an absolute upper end figure and unobtainable as such, but it serves as a simple illustration of what could be done.  You might doubt that anything like it can be done at all; let us return to the allotment once more.

Our allotment is 200 square meters.  Subtract the paths, footprint of the shed etc and call it 150.  We did not measure or record as we went, but I’m utterly convinced that we have increased the organic matter in our soil by more than 1%.  If the figures above are to be believed, and they look pretty plausible to me, that means that we have accidentally sequestered somewhere in excess of 1.5 tonnes of CO2. 


We’re not asking for your praise; we’re not trying to look clever; it happened purely by accident, we didn’t know we were doing it.  But now imagine if the principle were to be adopted worldwide.

If nothing else you now understand my clickbait title.


Can this have a Global application?  I believe it can.  There is an increasing amount of research and literature out there that agrees.  For instance,  I mentioned earlier that cattle are not necessarily all bad, here’s a video:  It will take a very worthwhile 12 minutes of your life to watch and might just make you a little more optimistic about our future.  Notice it was filmed in collaboration with the World Bank by the way; bunch of tree hugging hippies!

I set out here to write a blog post; not a novel.  As such there is much that I’ve skimmed over or omitted entirely and I’ve still droned on interminably.  I apologise.  But climate change issues tend to be driven by people, people like you and me; not governments or big business, and information is key.  If you’ve learned something from it or it has given you pause for thought then I’ve succeeded in my quest and I thank you for bearing with me.


3 Comments on “How allotment gardening can save the planet.

  1. Fantastic article. Let’s encourage more people to help save the planet locally

  2. Pingback: Learning from Ireland’s only Ecovillage – Sabine McKenna Online

  3. Well done for that good write up and all that Maths too!! It needs to be read by more allotment holders and general gardeners… Thank you

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