Capturing carbon in soil is often described as a simple fix for climate change and poor soils, but it is not that simple. What matters most is not total soil carbon, but how much of that carbon becomes stable humus. Unstable carbon breaks down quickly and returns to the atmosphere as carbon dioxide or methane. Humus, by contrast, can persist for long periods and can transform difficult clay into productive soil. This article explains why humus matters, how it forms, and why wicking beds may help.
Why Total Soil Carbon Is Only Part of the Story
There is a widespread view that simply capturing carbon in the soil will both improve soil quality and mitigate climate change. There are truths in these views, but the situation is more complex than “more carbon equals better soil”. Total soil carbon content is only part of the story.
What really matters is the form of the carbon in the soil. If the carbon is in an unstable form, meaning it is readily digested by bacteria, then any benefits for climate change and food production are limited. If it is in a stable form, what we commonly call humus, it can deliver long-lasting benefits for both climate change and food production.
The critical factor is therefore not the total amount of carbon in the soil, but the amount of stable carbon, or humus, held in the soil.
Background — Why This Became a Personal Research Problem
In the mid seventies, Australia suffered severe dust storms, losing millions of tonnes of topsoil. I started a research project on how to regenerate topsoil using a block in which the topsoil had been lost by excessive grazing by goats, leaving the underlying clay base.
The characteristics of the clay were painfully clear. When dry it was like concrete, almost impossible to work. When wet it became little more than an unworkable, gluey mess. Plant growth was extremely poor.
The carbon, or organic content, was very low. However, simply adding organic material, or even the so-called clay breakers, made little difference. It remained essentially a mix of organic material and clay with the same negative characteristics.
Yet, using an appropriate process, the characteristics could be completely transformed. The result was a soil that was easy to work wet or dry, with reduced density and significant elasticity (a kind of springiness). Above all, it became highly productive. Plant growth was used as the measure of whether regeneration had been effective.
The Simple Process, and the Less Obvious Mechanics
The process that produced this transformation is simple to describe: the soil must be maintained moist and plants must be continuously grown. The mechanics behind the transformation are not obvious and are only partially understood.
We do know some basic things. Organic material on the surface will be decomposed by the combination of UV light and oxygen. In aerobic conditions, bacteria will rapidly attack the softer organic material and release carbon dioxide. Under anaerobic conditions, bacteria will release methane. Fungi are more adept at attacking harder components, particularly lignin.
Of course, bacteria and fungi attack organic material because it is a source of energy. This is exactly the same reason every living creature depends on organic material. But organic material that is simply devoured for energy provides very little benefit for soil structure or for averting climate change, because it mostly ends up releasing carbon dioxide and methane back to the atmosphere.
This is why measuring soil carbon can be misleading. It may indicate a high level of carbon, but much of this carbon may be temporary and unstable.
Humus — The Carbon That Matters
However, a certain proportion of organic material is converted into a substance generally referred to by its common name: humus. The critical issue is how much carbon becomes locked into the soil as stable humus.
Despite the lack of a widely used scientific name, and despite not being fully understood even after extensive research, humus is vital for life on earth. It is one of the great “quiet foundations” of soil fertility and stability.
Why Humus Matters: Two Immense Properties
Humus has two properties of immense importance. Firstly, it is stable and can resist decomposition for long periods of time. Some people claim for hundreds of years. This stability makes humus valuable as a means of averting climate change.
Secondly, humus has a remarkable effect on soil quality. It can transform virtually unworkable clay into top-grade soil. This occurs through a process of aggregation, in which fine particles are held together in small clumps.
How Aggregation Might Work
There are different views on the mechanism. Some people argue it is physical: long-chain molecules entwine around soil particles and bind them into aggregates. Others suggest it is the result of Van der Waals forces, the secondary forces that occur when molecules are in close contact. Either way, the practical result is clear: aggregation improves structure, aeration, water handling, and root penetration.
Humification: Important, Powerful, and Not Fully Understood
The process of forming humus (humification) is therefore of great importance. At a macro level, it is clearly the result of microbiological action. Creating beneficial conditions for the appropriate microbiological activity has a major influence on the proportion of humus that is formed.
At a micro level, it has been suggested that enzymes which fungi release from their hyphae to dissolve mineral particles may be involved in the formation of humus. We do not need perfect scientific certainty to use what works, but understanding is still valuable.
A Window into the Science
To indicate our current scientific understanding, I will quote James Amonette (Pacific Northwest National Laboratory, Richland, WA.):
While incompletely understood, the humification process by which soil C is stabilized is believed to involve several parallel pathways. Of these, the polyphenol formation pathway generally dominates. The rate limiting step for this pathway is believed to be the oxidation of polyphenols to polyquinones, which then polymerize with amino acids to form humic material. This oxidative polymerization reaction is catalyzed by polyphenol oxidase enzymes such as tyrosinase, but soil minerals such as allophane, Fe and Mn oxides, and smectites also promote the reaction.
Using the Process Even Without Full Understanding
While science may not fully understand humification, this does not prevent us from taking advantage of the process to resolve major challenges. In practice, this can be relatively simple and can be backed by simple experiments.
Why Wicking Beds Are Effective for Humus Formation
The wicking bed process, with its moist conditions and recycling of air and water, can be observed to be highly effective in creating humus. The stable moisture environment supports the biological processes that favour long-term carbon retention rather than rapid oxidation.
This effect can be accelerated through the use of fungal initiators or inoculants introduced into the water stream. The intention is to encourage the forms of microbiology that favour stable humus formation, rather than rapid “burning off” of organic matter as short-lived carbon dioxide or methane.
The Adoption Barrier: Measurement and Payment Timing
One of the major hurdles to adoption of any carbon capture method is measurement. It can take time for organic material to convert into stable humus. A common approach is therefore to measure carbon content over time, which can require farmers to wait months or years to receive payment from carbon trading. This removes a major incentive for adoption, especially when farming already carries high risk.
Why Wicking Beds Improve Measurement and Carbon Trading
Wicking beds use organic waste from external sources, so it is relatively easy to measure the quantity added. Only a small proportion of this organic waste is converted into stable humus. However, the ratio of organic input to humus formed can be measured in a separate, controlled trial by measuring the amount of humus generated from a given quantity of organic waste.
Once this ratio is established, it can be used to predict the amount of humus likely to be created in real systems. This provides a simple basis for carbon trading, because the carbon benefit can be estimated from the measured organic inputs rather than waiting years for slow soil testing.
