Does it Take 100 Years to Make an Inch of Topsoil?

Slow & Fast Carbon Cycles

It takes 100, 500 or even 1,000 years for one inch of topsoil to form, according to many top universities and global organizations. Combined with current soil erosion rates, one quickly understands why soil is treated as a quasi-non-renewable resource, only able to support a few more harvests.¹ We’ve all seen the headlines. These statistics are enough to make any rational person head for the degraded hills and fear for the future of humanity and planet Earth.

One of the beautiful aspects of regenerative agriculture is the renewed sense of hope that accompanies one’s journey down its path. Yes, topsoil is precious and erosion from agricultural land is occurring at an alarming rate. This cannot and should not be avoided. However, farmers and ranchers can make positive changes nearly overnight. The 4 Ecosystem Processes are sent into productivity hyperdrive through implementation of the 6 Principles of Soil Health and the 3 Rules of Adaptive Stewardship.

To help illustrate why this is true, imagine sitting in the woods on a cool, autumn evening. The stars are coming out of hiding and the air temperature is dropping below comfort level. At this point, most campers stack a few logs, light them and feel the warmth they give off as they burn. We do this intuitively, but where does that warmth come from? This energy is contained between the bonds in the wood. It was there all along! The heat from a spark sets off a chain reaction leading to the breaking of the bonds, converting chemical energy into light and heat energy at an explosive pace. We call this “combustion”.

So, if the energy is already contained in the log, why don’t we just hug a log to feel the same warmth contained inside? The bonds, and the energy contained in the bonds, are held together very tightly in a stable form. Just like it “takes money to make money”, it takes energy to break and release this latent energy. Geologic weathering due to factors such as wetting, drying, freezing, friction and sunlight exposure, eventually break the bonds and release the energy over hundreds to even a thousand years. This is the thinking behind the phrase, “It takes 100, 500 or even 1,000 years to form one inch of topsoil.” Weathered bedrock often takes this long to become topsoil in what is called the “Slow Carbon Cycle”.

What’s important to understand is that there is a “Fast Carbon Cycle” occurring simultaneously that is not always considered by these estimates. Here’s how it works. Plants, and other organisms like plankton, inhale carbon in the form of carbon dioxide and take energy from the sun to build their bodies. This carbon will eventually cycle back into the air as carbon dioxide once again. Oxygen combines with carbon compounds and releases energy, carbon dioxide and water. The process is technically called combustion, whether it’s done by fire, decomposers, herbivores, predators or even humans. This gives new meaning to the phrase “burning calories”!

Living organisms in our soils are constantly consuming organic matter to enzymatically combust and harness its carbon and energy. The microbial processing of soil is happening in the geological blink of an eye. In fact, dead and decaying microbes themselves are considered to be major contributors to soil organic carbon². Given that there are one hundred billion (100,000,000,000) bacteria in a gram of fertile soil³, each completing a life cycle in mere minutes, we now know one reason why many regenerative farmers are growing topsoil and building organic matter levels at rates far quicker than is believed to be possible. They have learned to start the fire and keep it burning by feeding their microbial employees who give their lives to the cause.

Take a look at these images to the fast carbon cycle in action.

Sidewalks and brick walls are good representations of barren landscapes, like bedrock and scraped land after a glacier has receded.

Colonizer organisms like fungi, algae and mosses are well adapted to mine building material and energy from such environments to start the cycling of carbon.

Moss covers the exterior of a carbon-rich “barren landscape” to gain energy and building material from it.

These mosses, along with microbes fed by their exudates and sloughed roots, live and die, leaving behind dead cells rich in carbon that becomes dark, organic material. Mix this with the unique sand, silt and clay mix in your location and Voila! You have dark, carbon-rich soil accumulating.

Top view of a decomposing stump and the black, carbon-rich material left behind, like soot from a campfire.

Zoom in and white-rot fungus can be visually observed. These organisms use “enzymatic combustion” to extract food and energy from nearly impenetrable woody material.

These photos give me hope for the future of agriculture. Farmers and ranchers have the agency to implement practices that mimic these natural processes and speed up fertile, topsoil formation even quicker than our stump examples. Regenerative producers see it over and over again on their own operations and it’s a real possibility on yours as well.

My advice: think small. It’s a microbial world. We’re just livin’ in it.

References

¹https://www.scientificamerican.com/article/only-60-years-of-farming-left-if-soil-degradation-continues/

²Kästner M, Miltner A, Thiele-Bruhn S and Liang C (2021) Microbial Necromass in Soils- Linking Microbes to Soil Processes and Carbon Turnover. Front. Environ. Sci. 9:756378. doi: 10.3389/fenvs.2021.756378

³Borer B, Or D (2022) Bacterial age distribution in soil – Generational gaps in adjacent hot and cold spots. PLoS Comput Biol 18(2): e1009857. doi: 10.1371/journal.pcbi.1009857