Soil Carbon: Some Frequently Asked Questions
Q: Is it fair to say that carbon is the key ingredient that makes the difference between barren, unhealthy soil and fertile, healthy soil?
A: Yes. The glues and gums that hold soil particles together in the form of aggregates (small lumps) are made from carbon. Aggregation is what gives soil its tilth – making it well-structured, porous and friable. Porosity, in turn, improves the infiltration and storage of water. Carbon derived from plants also provides the energy for the soil biota essential to nutrient availability. Plants give to the soil, rather than take from it, as many people believe. The greatest threat to soil health is bare ground. When soil lies in bare fallow for long periods (eg. between crops) – or if it is over-grazed and has bare patches – it will deteriorate. Around 80-90% of plant nutrient acquisition is microbially mediated. If there’s no energy for the microbes (in the form of carbon) there will be very little nutrient – particularly vital trace elements – for plants. These deficiencies are passed along the food chain to animals and people.
Q: When did humans begin to release carbon into the air through disruption of the soil? Can you explain the process of how this happens?
A: As soon as living plant cover is removed, soil begins to deteriorate. Plant litter (mulch) can maintain the condition of the upper layers of soil (topsoil) but cannot maintain the deeper layers (subsoil). There must be active root growth in subsoil in order for it to remain alive and functioning effectively. Grasslands build more soil – and deeper soil – than forests. Due to these deep, carbon-rich soils – and low numbers of trees – grasslands were traditionally the areas first selected for cropping. However, the removal of the actively photosynthesizing year-round groundcover caused immense damage – not only to subsoils – but also to topsoil through wind and water erosion. Fortunately, this damage can be repaired by restoring the flow of liquid carbon via year-round living cover – such as obtained with the use of multi-species cover crops. Best results are seen when cover crops contain at least 20 different varieties of plants and are strategically grazed.
Q: Do you know when scientists first became aware of carbon loss from soil, and of the possible relationship to climate?
A: In ancient times (as far back as the Roman Empire, and possibly before), the word ‘humus’ was used to refer to what we would call ‘soil carbon’ today. Although the actual composition of humus was not known, it was well recognized as the main determinant of soil fertility (soil ‘fatness’ was the general term used for soil health back then). The first explanations of what humus might actually be composed of began appearing in the scientific literature in the 1600s and 1700s. One hundred years previous, Flemish physician Jan van Helmont (1579-1644) discovered that the growth of plants did not reduce the weight of the soil. He was puzzled as to the source of the plant material. If not from the soil, how did plants grow?
Another two hundred years passed before scientists discovered the miracle of photosynthesis (carbon fixing by living plants). Plants are autotrophic, that is, they build themselves from light and CO2. We now understand that this carbon-fixing process – and the translocation of a portion of this fixed carbon to soil microbes in liquid form – is the driver of soil health. It was in the early 1900s that most of the significant research on humus occurred. A lot of the information was discarded, however, with the dawning of the ‘chemical age’ in the 1940s. The destruction to soils (and the deterioration in food quality) caused by synthetic fertilizers was recognized even as early as 1948.
I don’t know when scientists first discovered the relationship between soil carbon and climate change. Professor Rattan Lal was certainly one of the pioneers in this field (Lal, R., Kimble, J.M., Follett, R.F., Cole, C.V., 1998. The Potential of U.S. Croplands to Sequester Carbon and Mitigate the Greenhouse Effect. Ann Arbor Press, Ann Arbor, MI, 128 pp.). I was the first scientist to hold a conference specifically on the subject of the relationship between soil carbon and climate change in the Southern (and possibly also the Northern) Hemisphere.
Q: Do you endorse Allan Savory’s methods? What are other effective methods for restoring carbon to soil?
A: Any method that increases photosynthetic capacity and photosynthetic rate will restore carbon to soil – provided the flow of carbon is not inhibited by chemicals such as synthetic nitrogen, water soluble phosphorus – and of course pesticides and fungicides. Holistic Planned Grazing is an effective way to increase photosynthetic capacity provided the land manager can accurately determine plant recovery and adjust stock movements to match carrying capacity. In farmed lands where livestock may not be an option, multi-species cover cropping is effective – again, provided the inputs of synthetic chemicals are reduced.
Q: What has happened so far in Australia, in terms of individuals and groups taking action, as well as government activity? Is Australia a pioneer in this realm? What is happening in other countries?
A: Any attempt to verify soil as a carbon sink has been blocked by the chemically-funded scientific organizations in Australia. There are countless documents from departments of agriculture and universities which ‘prove’ that it is not possible to increase the level of carbon in agricultural soils. Despite the volume of expert opinion to the contrary, however, I could take you to many farms where soil carbon has been significantly increased.
Q: Can the earth’s soil sequester just the carbon it previously lost, or can it also actually absorb additional carbon from fossil fuel emissions? If the latter, how does that work? Is the soil’s capacity to absorb carbon limitless?
A: The soil’s capacity to sequester carbon is determined by plant root depth and plant photosynthetic rate. We know a lot more about those things now than we did in the past. Photosynthesis accounts for only a tiny fraction of the light energy that comes to earth each day. If we could figure out how to increase photosynthetic potential and channel more of that energy to soil (as liquid carbon), we could turn every farm into a net carbon sink rather than a net carbon source. That would be great for farmers and unbelievably good for the planet!!
Soil building takes place downwards into the soil profile. That is, as soil health is restored, the soil becomes deeper and deeper. We are finding plant roots at incredible depths on carbon-friendly farms, so I certainly believe we can put all the carbon back that has been lost. Whether we can sequester sufficient carbon to mitigate the ongoing emissions of fossil fuel remains to be seen. Biology-friendly farm practices require far less fossil fuel – and also result in fewer emissions – hence the changed land management of itself will reduce the problem to some extent.
Q: Do you see this movement to focus on soil growing? What has surprised you the most in your work on this issue?
A: From what I’ve seen in my travels around the world, the impetus for soil building is coming from farmers, ranchers and their advisors, rather than from government. Change is being driven by the fact that many farmers are no longer making a profit. Once soil carbon stocks have been depleted, no amount of synthetic fertilizer can maintain yield. There is also an increasing desire on the part of many farmers to reconnect with their land. This vital connection has been lost in the industrial farming model. I don’t believe there’s a farmer on the planet who wants to use more chemicals or destroy soil or make people sick. But sadly, that’s what many are being driven to do. It concerns me that there is neither support nor direction from government to help turn this situation around. There are so many good reasons for improving soils. Applying simple techniques to increase the level of soil carbon will make farming infinitely more enjoyable as well as more productive – and vastly improve the quality of our food. If governments provided financial incentive for soil restoration, they would not need to invest anywhere near as much in hospitals.
Q: Is stable soil carbon the same thing as soil organic matter (SOM)? If not, could you elaborate? Is stable soil carbon inorganic? Does SOM contain carbon? Can both help mitigate climate change? Can SOM eventually turn into stable soil carbon?
A: SOM is derived from plant material, animal remains, compost, manure etc. – i.e. stuff you can see. It is made from carbon and other materials and eventually decomposes to become CO2. This is a catabolic (breaking down) process. During decomposition, the presence of SOM in soil has beneficial effects for the soil food-web, the buffering of soil temperatures and in reducing evaporation. However, SOM is very short-lived (days to months). This is what we call ‘labile carbon’. Great stuff – but not what’s needed to remove CO2 from the atmosphere and lock it away for long periods.
Humus, on the other hand, is a high molecular weight carbon polymer, formed by soil microbes within soil aggregates, from sugars channeled to soil via the hyphae of mycorrhizal fungi living in association with actively growing green plants. The formation of humus is an anabolic (building up) process. Humus is an organo-mineral complex requiring carbon, nitrogen, phosphorus, sulphur and several catalysts, including iron and aluminum. The carbon derives directly from photosynthesis, the nitrogen is fixed by a group of free-living nitrogen fixing bacteria called ‘associative diazotrophs’, while the phosphorus is solubilized by bacteria from either calcium phosphate, iron phosphate or aluminum phosphate (depending on soil mineralogy and pH).
The chemical reactions involved in making humus occur inside soil aggregates. The transformations require microbial activity and the microbes require energy – which is produced by the green plants. That’s why humus cannot form to any significant extent when there are no green plants. Once formed, humus is an inseparable part of the soil matrix and can be very long-lived (as in hundreds of years). Hence it fulfils the requirements for safely removing excess CO2 from the atmosphere and storing it in soil. Humus is called non-labile or resistant carbon. You cannot see humus – you can only see what it does.
The main blockage to humus formation in chemical ag is the use of synthetic fertilizers (N and P) which inhibit carbon flow to soil. Even though green plants are often present at high densities in cash crops, they are not forming relationships with mycorrhizal fungi and associative diazotrophs and hence not building soil. This is a huge waste of photosynthetic capacity! Simple changes to fertilizer management alone could improve soil carbon content (and farmers’ bottom lines). Farmers are being encouraged to use products they do not need by companies interested only in profit.