ploughed field with mountians in the background

Soil organic carbon | Is there more carbon in the soil than in the atmosphere?

Is there more soil organic carbon than carbon in the atmosphere? Simple enough question, but do you know the answer?

Yes, there is more soil organic carbon than carbon in the atmosphere. Roughly three times more with 2,500 Gt carbon in the soil to a depth of 1m, four times more than the carbon stored in plants and animals.

My guess is you didn’t know that fact. There is no reason to.

The impression from all the climate change hype is that the atmosphere and maybe the oceans are the carbon sinks. Humans have burnt the carbon, which goes up as greenhouse gases into the atmosphere to increase the thermal blanket we assume is where there is most carbon. Nope.

No matter. Emissions and extra warming are valid too.

If I ask you about climate, your mind will look up and not down at your feet. It makes logical sense for the warmth and the weather to come from the sky; everyone knows this. 

Why would the soil and the soil organic carbon it contains be of any interest?

Soil organic carbon matters

The soil carbon (C) pool to one-meter depth is estimated at 2,500 Gt C, roughly about 3.2 times the size of the atmospheric pool and 4 times more than the carbon stored in plants and animals.

Around 60% of this carbon comes from the decomposition of plant material and soil organisms and is biologically active and called soil organic carbon—sometimes shortened to SOC. It gives the soil a darker colour, a musty smell and holds onto moisture to help bind the soil together. Gardeners will call it humus and know how good it is for the plants.  

At sustainably FED we love soil carbon because soil without soil organic carbon is dirt—inert and unable to support soil biodiversity or plant growth. 

We also know that soil organic carbon is easy to lose. 

Typically, 50% of the soil’s organic carbon is lost when land is converted to agriculture. When vegetation is cleared or disturbed by livestock, the soil dries out more easily, and carbon oxidises to the atmosphere. Disturbance also disrupts the carbon inputs to the soil from plants and the food web that decomposes and recycles nutrients.

Less soil organic carbon means loss of soil biodiversity because the microbes and soil animals lose their food source and the carbon that used to hold onto the moisture  means the soil is drier changing the conditions for many species.  Most soil degradation is a consequence of soil organic carbon losses. 

One consequence of agricultural practices worldwide is a soil organic carbon debt of 116 Gt SOC

This loss of soil organic carbon in cultivated soils is from a combination of erosion, lower C inputs, reduced stabilisation of SOM due to deteriorated soil aggregation, and mineralisation promoted by increased soil temperature and aeration. 

In short, agriculture messes with soil. 

We owe nature roughly 10% of the soil carbon it once had.

The good news is that what management takes away, alternative management can restore, at least in part. 

Research has shown that one or more management actions restore up to two-thirds of soil organic carbon lost. 

  • retaining vegetation cover on the land
  • conservation tillage 
  • cover cropping 
  • reducing inorganic inputs
  • addition of organic manures 
  • soil amendments (eg biochar)
  • mulching
  • soil fertility management 
  • agroforestry 
  • rotational grazing

And there are many others that, alone or in combination, can enhance soil organic carbon.

The what, how and when of restoration depends on soil type, climate conditions, and local history of land management. Decisions for repair are a science and a skill in itself but nearly always achievable.

What management taketh away, alternative management can give back under the basic rule of thumb that more organic material has to go into the soil than is taken away through agricultural practices

Research suggests that the potential exists to sequester into the soil more than half the lost soil carbon stocks in cultivated soils. Cropped soils should be able to sequester carbon for at least 20 years before arriving at a new soil organic carbon equilibrium.

The consensus is such that there are even policy initiatives that can help make this happen. 
For example, the  4 per 1000 initiative on Soil for Food Security, officially launched by the French Ministry of Agriculture at the UNFCCC Conference of the Parties (UNFCCC COP 21) in Paris, including a target of 3.5 Gt C annually sequestered into soils.

ploughed field with woodland in the background showing how soil organic carbon is exposed to the elements
Photo by Slawek K on Unsplash

Why repay the soil organic carbon debt

The media and political buzz is always about climate change and the need to curb emissions and reach carbon neutrality by some near future date. 

Given the lost tonnage and potential for recovery, it’s no surprise that legislators are finally looking to the soil as one option to reach a stretch target. They have to clutch at all straws to remain credible.

The politicians don’t realise that replacing as much organic carbon into degraded soils as humanly possible is good for climate action but critical to future food security. Fail to achieve food security, and society descends into chaos.

Research over many decades has established that soil fertility, health, and functioning are immediate consequences of the amount of soil organic matter (and hence carbon) a soil contains.

And when we say function, we mean food production.

Research also tells us that an increase in soil organic carbon improves the physical properties of soil and related ecosystem services, such as better water infiltration, water holding capacity, and potentially increasing agricultural productivity and ecological resilience.

This is even more important for degraded soils that have lost carbon over time due to management.

It is tempting to default on debt—but with this one, the soil carbon debt, default means collapse.

arable field with a partial cover crop to help protect soil organic carbon
Photo by Roger Bradshaw on Unsplash

What sustainably FED suggests

We have always been advocates for soil organic carbon. 

It is both the fuel and the substrate that makes plants grow well, and every soil on earth produces more biomass with soil organic carbon at or near local potential.

We even put our healthy scepticism aside to advocate for looking after soils. In our opinion, societal collapse is inevitable unless everyone starts looking after the soil.

But we cannot expect commuters on the 7.43 am train to Central Station to see soil organic carbon from the train window or to see what was on the land of our cities now paved over. In everyday life, many critical resources that make our lives possible are invisible and easy to ignore.

More than this, the way that nature works is hidden too. 

However, the commuters know that the atmosphere has more greenhouse gas than it did 100 years ago because humans have been very busy using energy, making stuff and more humans. As a result, the climate is changing.

Human activity has made soil a source of greenhouse gas for hundreds of years. 

But it is possible to change agriculture to make the soil a carbon sink and pay back some of the carbon debt.

Postscript—View a broader horizon 

As individuals going about our daily lives, it can be hard to see beyond our noses. Each person views only a tiny fraction of the world no matter how well travelled—the most networked come into contact with a minuscule proportion of the people alive.

The risk in a narrowness of being is that the small worldview our daily living offers becomes our global view, and we miss some of the most important happenings of our times. 

Fortunately, there are plenty of ways to widen the horizon.

I always encouraged my undergraduate ecology students to make sure they looked out of the window the next time they took a commercial flight or, better still,  hitch a ride in a helicopter. The perspective is very different from high up. The landscape spreads with many elements interwoven and messed around by humans and their machines.

Below 10,000 feet, where light planes fly, you can see where all our food is grown. 
Thanks to fantastic satellite technology, you can take this journey virtually. Just open Google Earth and take a low-level flight across any landscape. It is incredibly instructive.

Hero image from photo by Roger Bradshaw on Unsplash


Mark is an ecology nerd who was cursed with an entrepreneurial gene and a big picture view making him a rare beast, uncomfortable in the ivory towers and the disconnected silos of the public service. Despite this he has made it through a 40+ year career as a scientist and for some unknown reason still likes to read scientific papers.

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