Soil Carbon 101

Basic information about soil carbon and organic matter

The quiet carbon revolution in Australia

A short article from Australia on the work of Christine Jones:

"Thousands of farmers are joining a voluntary soil carbon movement adopting specialised cropping and pasture practices to improve yields and income, while measuring loads of carbon storage on their farms."

"But the Senate inquiry, looking into the impacts of climate change on agriculture, also heard the results have been largely shunned by the science fraternity because the carbon storage data does not fit into existing carbon models."

The soil organic carbon story

A good introduction to agricultural soil carbon -- what it is, what it does, how it forms -- from Saskatchewan, Canada.

Know-that and know-how

Will reducing carbon dioxide emissions slow down global warming? Hardly, according to the Intergovernmental Panel on Climate Change. "Complete elimination of CO2 emissions is estimated to lead to a slow decrease in atmospheric CO2 of about 40 ppm over the 21st century" (IPCC Fourth Assessment FAQ, section 10.3).

Carbon dioxide is a stable gas, requiring energy to break its molecular bonds. With 100 percent reductions in 2007, we will maintain what NASA climate scientist James Hansen calls dangerous levels—not just for a few years, but for generations. The area under the Keeling curve will remain huge, and that translates into ocean heating.

What this means is that our current widespread advocacy of CO2 emissions reduction has little leverage on what most scientists regard as the cause of global warming—the highest atmospheric CO2 levels in hundreds of thousands of years. The assumption that CO2 emissions reductions will do the trick has become popular groupthink, not subject to scrutiny because it's what we all know, and may seem like the only available option. Once again, we are goading ourselves into a gallant cavalry charge into the barbed wire.

What's needed is to reverse the Keeling curve, to quickly and significantly reduce existing atmospheric concentrations of CO2. According to NASA's figures on the global carbon cycle, fossil fuel burning represents less than 3 percent of the annual flux of CO2 into the atmosphere (3.4% according Rattan Lal's figures). The rest is biology, which also responds to human management.

Hoosier Chapter, Soil and Water Conservation Society, position statement

The Hoosier chapter of the Soil and Water Conservation Society has a well-rounded position statement on the soil carbon sequestration opportunity. Of particular interest is the effect of soil organic matter on agricultural land value in Illinois.

Soil Science Society statement

In 2001, the Soil Science Society of America drafted a position paper that included this statement:

The soil carbon opportunity

Cutting fossil fuel emissions is not enough to avoid dangerous climate change. Too much carbon is already in the atmosphere, and the oceans are continuing to heat. The Intergovernmental Panel on Climate Change stated in 2007 that "Complete elimination of CO2 emissions is estimated to lead to a slow decrease in atmospheric CO2 of about 40 ppm over the 21st century" (about 9% to 1985 levels; see IPCC's 7 Mb pdf file here, section 10.3).

In other words, the strategy of reducing carbon dioxide emissions, by itself, has little effect or leverage on the atmospheric concentrations. By treating global heating as a problem of energy, emissions, or technology alone, we only get to decide whether to wreck our climate slightly faster or slightly slower.

There is a biological side to global warming and the carbon cycle. Carbon is a main ingredient of all life, and of its remains. While planting trees is rightly discounted as a way to reduce atmospheric carbon, we could increase soil organic matter (58% carbon by dry weight) rapidly and cheaply. This will pull excess carbon out of the atmosphere while also enhancing soil fertility, water quality, food quality and human health, and also reducing floods, droughts, and agriculture's dependence on fossil fuels and chemicals.

How it works

Biological processes, such as photosynthesis and respiration, drive 99% of the carbon cycle. There is more carbon in soils than in vegetation and the atmosphere combined. Soil carbon can be more stable than plant carbon (less subject to oxidation or burning).

Soil carbon FAQ (frequently asked questions)

What is soil carbon?

Living organisms contain a fair proportion of the element carbon. So do the remains of living organisms. Some of these remains end up in the soil, processed and decomposed in various ways by fungi, microorganisms, insects, and worms. This soil organic matter can be 50 to 58% carbon by dry weight, and some of it can remain stable in the soil for generations or centuries. The vast majority of carbon in the top layers of soil is in soil organic matter. Darwin called it vegetable mould (though he recognized the important role of animals such as earthworms in its formation), and it is also called humus.

Some soil carbon is inorganic, such as calcium carbonate or caliche. Carbonates are typically more prevalent in arid environments, where soil pH is above 7.5. They do not have the water-holding properties of organic soil carbon, but are a significant sink for atmospheric carbon.

What is the difference between soil organic matter and biochar, which helps form terra preta?

Biochar is a product of fire. It is plant matter that has been burned in a low-oxygen environment. Another word for it is charcoal. It is mostly carbon, and fairly resistant to decay and oxidation, although there are some losses through leaching into water.

Soil organic matter, by contrast, is the product of biological decay processes. These processes are often slow, and require the participation of millions of self-motivated microorganisms.

What removes soil carbon from the soil?

Microorganisms can combine the carbon in soil organic matter with oxygen, creating carbon dioxide. In the soil, oxygen is often limited, especially deep down. When soil is plowed or turned over and exposed to air, these microbes can turn much of the carbon into atmospheric carbon dioxide.

The other side of global warming

by Peter Donovan

Almost all of our proposals and policies for global warming are aimed at reducing fossil fuel use. Yet even if we stopped burning fossil fuels tomorrow, global warming will continue for decades. Why is our approach so one-sided?

The dominant view of global warming is that it's a technical problem. The burning of fossil fuels---often regarded as the lifeblood of modern economies---puts greenhouse gases into the air, mainly carbon dioxide. These trap more solar energy, which makes things hotter and alters weather patterns.

We used to have three greenhouse blankets, and now we have four or five. The solution is defined as reducing greenhouse gas emissions (pollution). The political, social, and moral campaign is directed at technological change, and at using our technology less. Stopping at five or six blankets is better than going to seven or eight, but it won't restore our climate.

Our technology cannot economically remove carbon or other excess greenhouse gases from the atmosphere. Limiting ourselves to technology-focused solutions doesn't give us the leverage we need to actually fix the problem. The best we can do is wreck the world slower.

There is another side to global warming, one that existing scientific panels are ill-equipped to recognize, and that existing political institutions are ill-equipped to act on. Global warming is not just an atmospheric pollution problem caused by fossil fuel burning. Like the ancient problem of desertification, it is also a systemic problem---the result of changes in basic biospheric processes. Let's look at some examples.


Carbon emissions from fossil fuel burning represent less than 3% of the net annual flow of carbon into the atmosphere. The other 97% also results from combustion reactions---respiration, decay of organic compounds, and burning of biomass. These reactions emit carbon and yield energy.

Content type: 


Subscribe to RSS - Soil Carbon 101