Soil, Water and Carbon Connection
by David Montgomery
David Montgomery, a MacArthur Fellow, is a Professor of Earth and Space Sciences at the University of Washington and author of Dirt: The Erosion of Civilizations, and Growing a Revolution: Bringing Our Soil Back to Life. He co-authored with his wife, biologist Anne Biklé, The Hidden Half of Nature. This is an edited transcript of a presentation at a Bioneers produced event.
Soil is where the two worlds of geology and biology intersect. Soil supports our ecosystems and it makes the terrestrial environments on Earth habitable. Soil health, or lack of it, has a huge impact on human societies. My book Dirt: The Erosion of Civilizations tells the stories of how societies were undermined by soil erosion that degraded fertility. There are places where soil erosion from thousands of years ago still has an impact today. Syria and Libya are just two examples of where there are Roman tax records of large harvests of wheat, but today barely anything will grow because of topsoil loss. The same lesson has been learned in Asia, Africa, the Middle East, Europe, and the American Southeast.
Farmers converted forest soils to modern agriculture across North America and around the world. Intensive tillage and the use of nitrogen fertilizers have degraded the amount of organic matter in the soil, which dramatically changes how water functions in the soil. And how water functions in the soil is an integral part of soil health.
Studies show that North America has lost about half of the soil organic matter since forest and prairie soils were converted to agriculture. In just a century or two, modern agricultural management has degraded soil fertility with practices that reduce soil organic matter. I view organic matter, which is approximately 50 % carbon, as the battery that powers the life in the soil; it drives the nutrient cycling that make soils fertile.
20th Century agriculture focuses on chemistry and physics in soil, but the biology, which feeds on the organic matter, is ignored. My wife, Anne Biklé, is a biologist and a major-league gardener. She has a self-admitted serious case of plant lust. When we bought a house in North Seattle back in 1997, it came with soil that was in really bad shape; we lovingly call it Seattle glacial till. She restored it and we wrote about that in The Hidden Half of Nature. 15,000 years ago, the glaciers scraped off the topsoil leaving a pile of sand and clay. A mile-high wall of ice moved over the landscape and compressed the soil into concrete-like material. Glacial till is not terribly fertile.
Anne decided to add organic matter because that’s the key to rebuilding the soil and increasing its water-holding capacity. She started intensive composting and mulching. She added wood chips and compost, and sea shells as a source of calcium. She even applied zoo do – composted herbivore turds from the Seattle Zoo, an incredible nitrogen-rich fertilizer.
We also used our neighbor’s grass and leaves. Neighbors in a city or town are usually quite happy to have you rake up their leaves from deciduous trees and take them to your yard. What they don’t realize is that you’re taking the nutrients that their trees are dropping and recycling to their own roots.
The soil we started with had about 1% carbon. The soil we have today is pushing 10%. On our small lot in the middle of an urban environment, we’ve managed to store literally tons of carbon in the soil over the course of a decade. That may not sound like a big deal, but just imagine if that was done on a global scale.
The increase in carbon in our garden resulted in an explosion of plant life aboveground. That explosion of life aboveground was being fueled by an explosion of life below ground. The bacteria and fungi were doing the heavy lifting of turning the compost and mulch into the nutrients that would feed the next generation of plants by building the soil organic matter. Recently published studies have documented that in many cases soil organic matter is primarily composed of the bodies of dead microbes.
Symbiosis in the Rhizosphere
The zone around the roots of a plant is the rhizosphere. The rhizosphere is rich with microbial life. Some of the most densely populated regions of the planet are around the roots of living plants.
In the process of photosynthesis, plants capture carbon dioxide and use the hydrogen from the water molecule to make carbohydrates to feed the plant and release oxygen into the atmosphere. Another important part of the process, which Anne and I never learned in soil science classes, is that plant roots exude into the soil up to 40 percent of the material that they photosynthesize. It’s not because they are inefficient. Nature is brutally efficient. They do it to feed microbes around their root system that provide a benefit to the plant.
Relationships in the rhizosphere are every bit as intimately evolved and important as relationships aboveground.
Plants have colonized the continents for some 450 million years. In the earliest land fossils, there are mycorrhizal fungi wrapped around the roots of plants. Plants are pushing their exudates into the rhizosphere, a zone that’s about a millimeter to a centimeter thick– the thickness or the length of my thumbnail. The exudates, when dripped out into that environment around every root and root hair, are kind of like putting out pizza and beer on a college campus. What do you think happens? People show up and consume it. That is what’s happening in the rhizosphere. Those plants set out a banquet that Anne and I refer to as a biological bazaar. Microbes show up, consume those exudates, and they don’t make it out of the rhizosphere. They’re consumed near the roots of a plant.
Any living thing that eats something turns it into something else. That process of metabolism produces metabolites, the waste products of that organism. We found studies that documented that microbes in the soil were basically consuming exudates that plants produced and metabolizing them into growth hormones that fed the growth of that plant, an example of a symbiosis.
Symbiosis is the mutually beneficial interactions of life forms. This kind of stuff has been going on out of sight and out of mind belowground for a long time. We didn’t even know that microbes existed until the 17th century, and we didn’t know they did anything useful until we figured out that they made wine and beer, and that wasn’t until in the early 19th century.
They were thought primarily as germs, pests and pathogens. But they actually do a lot of things in partnership with, not only plants, but with humans as well. Symbiotic relationships turn out to be critical to a whole number of things, not the least of which is plant defense and plant health. When a plant gets nibbled on by an herbivore, some microbes in the rhizosphere can send out certain exudates into the soil that will recruit particular microorganisms that make compounds that help repel that pest, or make compounds that trigger the plant to make other compounds that repel that pest. These kinds of relationships in the rhizosphere are every bit as intimately evolved and important as relationships aboveground; we are just starting to understand some of them.
The Subterranean Food Web
It really matters how we feed our plants. You can grow big plants on a fertilizer diet in soils poor in organic matter, but plants will tend not to invest much in their root systems in those conditions. They are getting as much nitrogen, phosphorus and potassium as they need to grow, so they don’t invest as much in their root development. You can do this experiment on any paired set of corn fields across the American Midwest; pull up a conventional corn stalk by their roots. Ones that are grown in organic matter rich soil tend to invest more in their root system making them harder to pull out of the ground.
What does that mean in terms of plant health? Well, if you don’t have much of a root system, you’re not producing as many exudates, you’re not recruiting as many microbial partners, and you’re going to be missing out on the mineral micronutrients and the beneficial microbial metabolites that those microbes in a healthy, fertile soil help supply to the plants.
It’s not a coincidence that in the decades after the widespread introduction of intensive nitrogen-based fertilizer in the early 20th Century that global sales of pesticides went through the roof. Why? Because we dismantled the natural defensive systems of our crops. There’s a direct connection.
A 2005 Japanese study on onions is just one example of a number of studies that prove that connection. It shows how the phosphorus uptake by onions differs in a control where there’s no inoculation of the plant with arbuscular mycorrhizal fungi compared to plants that were inoculated with those fungi. The inoculated onions took up 10 times the phosphorus from the identical soil. The difference is not in the soil or fertilization. It’s in the transport. It’s in the mining and the delivery of the phosphorus by microbes in the soil. By some estimates there’s 100 years’ supply of phosphorus in the world’s agricultural soils that we’ve already applied as fertilizer that’s now insoluble because phosphorus doesn’t tend to stay soluble for very long in an oxidative environment. This research demonstrates that fungi can unlock it.
These kinds of revelations led me to think about how could we apply the lessons that Anne and I were uncovering in the literature – I’m not saying that we discovered any of this; we discovered it for ourselves; it’s all out there in the scientific literature. If Anne could use these ideas to restore the fertility of our land in an urban environment, I wondered if the same could be done on fully functional farms in different environments around the world.
Ditch the plow, cover the soil with cover crops and grow diversity.
I visited the Brandt Family Farm near Carol, Ohio. David Brandt uses tillage radish to open his soil and feed the microbes. He considers himself a microbe rancher. He has livestock, but they’re invisible; they live belowground. He sells corn, wheat, and soybeans into the North American commodity markets. He also grows diverse mixes of cover crops. Glyphosate-resistant weeds are growing in 25 % of his neighbors’ fields. They get sprayed three times a year, but they cannot be killed. About 25 % of his neighbor’s fields are covered with weeds that don’t produce a marketable crop. They spend a lot of time and money to try to control those weeds, and it doesn’t work.
David is making more money than his neighbors for a very simple reason. He’s not tilling, so he’s saving on diesel; he’s not buying much fertilizer (he is almost down to zero fertilizer), and he’s using a fraction of the glyphosate that his neighbors use. David is proof that restoring fertility to soils on agricultural lands is the key to restoring profitability to small farms. The USDA needs to stop encouraging farmers to get bigger and instead encourage them to be smaller and more profitable.
Gabe Brown is introducing another element into the equation of regenerative agriculture–livestock. I was trained to think of livestock as a detriment to the soil because they caused erosion. I did part of my PhD thesis in the Tennessee Valley. It’s a beautiful place, but there are gullies that run down the middle of it that were carved as a result of dairy operations between 1880 and 1900. So, my framing of what cows do to the landscape is they cause gully erosion.
When I went to visit Gabe’s farm, I realized that the problem is not the cows. The problem is the people that are managing the cows and how they graze them. Gabe has adopted intensive rotational grazing – moving the cattle around frequently on the landscape. He has shown that you can use livestock as a tool for soil regeneration.
He manages his cattle to graze off the remains of the cover crops that were planted in the field where he had his market garden. He used to have to spend money on herbicides and fertilizer, but instead he’s letting the cattle eat off the cover crops and terminate them. They turn the cover crops into bovine exudates, aka manure, which fertilizes the land. Instead of paying for agrochemical inputs, he’s selling beef, chickens, and eggs. This is a way to reconceive the profit stream of a farm.
Gabe started with soil that was below 2 % carbon and now on most of his farm, he’s built the soil up to six percent or more. In certain areas, he’s boosted it higher with additional inputs. He has doubled or tripled the amount of carbon in his soil in the course of decades. It takes nature centuries to build fertile soils. By adjusting farming practices, soil fertility can regenerate remarkably fast.
If we ditch the plow, cover the soil with cover crops and grow diversity, that’s a recipe for cultivating the beneficial life in the soil that could be put to work for growing the fertility of the land and reversing the kind of erosion problems I wrote about in Dirt: The Erosion of Civilizations. Using less fertilizer, pesticide, and fossil fuel have all kinds of off-farm benefits–reduced off-site pollution, less nitrates in our water supply, and increased carbon and water stored in soils.
Lessons from Regenerative Farmers
I took six months off from my job at the University of Washington to visit farmers around the world who had already restored fertility to their land to find out how they have done it. The farmers that I visited in Equatorial Africa, Central America across North America, had already done the kind of thing to their farms that Anne had done to our yard. They all embraced three simple principles of conservation agriculture: don’t disturb the ground by plowing, known as no till; maintain a permanent ground cover by planting cover crops; and plant a diversity of crops both in the rotation and in the cover crop mix. It turns out this works surprisingly well for a really simple reason: it’s a recipe for feeding the beneficial life in the soil.
How would you feel if somebody came by once a year, took the roof off your house and stirred up all your stuff with a giant spoon? You’d probably move. Think of what tillage does to worms and other soil life. It’s incredibly disruptive. Permanent ground cover is key to the soil life food supply. You’re adding carbon to the landscape, and you’re adding more exudates when you have living roots in the soil. Plant diversity will produce a diversity of exudates which will feed a diversity of microbes. A diverse ecological community is more resilient than a monoculture. There are no monocultures in nature; humans invented them. Resilient communities tend to be diverse.
These three principles are the exact opposite of how farming systems have been operating for the last 100 years which depend on intensive tillage with a lot of agrochemicals and specializing in one or two crops. The three principles define a new philosophy of agriculture that is framed around cultivating the beneficial life in the soil.
The general principles translate to settings around the world, but the practices need to be tailored to a specific place based on the climate, the geology of soils, the kind of crops being grown, what you can sell in the market, the technology you have access to, and the social structure of the world you’re living in as a farmer. These things all will influence the practices that go into regenerating soil fertility, guided by the three overarching principles of minimal disturbance, keeping the ground covered, and growing a diversity of plants.
One of the big downsides of no till is that most no-till farmers, for the last 20 or 30 years, have used a lot of herbicides to manage their weed pressure. When writing Growing a Revolution, I wrestled with the question: Is it necessary to use a lot of herbicides in order to do no-till? It turns out there are other ways to manage weed pressure. Jeff Moyer of the Rodale Institute developed the roller crimper which knocks down and kills the cover crops, turning them into a mulch that can be planted through. So now, no till can be done organically without the use of herbicides.
Soil, Carbon, Water Connection
Most carbon tends to be in the topsoil. The subsoil, just below the topsoil, is the parent material made up of the geological and mineral material that formed the soil. A soil that’s in decent shape is composed of about half mineral material and five percent organic matter. That was the makeup of soils on the Eastern Seaboard when they were first farmed.
A lot of agricultural land today is below two percent organic matter. Good soils also have a fair amount of void space and air. A nice, fluffy soil can be up to half void space, which can be filled with air or water. The soil’s capacity to retain water is important to reduce erosion and to create resilience against drought. For each one percent increase in the amount of organic matter in the soil, you increase its ability to hold water by over three percent. In other words, a little bit of organic matter helps you a lot in terms of keeping water in the soil. What that means is that for each one percent increase in soil organic matter, you can hold an extra 20,000 gallons of water per acre in the soil. That’s water your plants can take up and use, instead of having to irrigate them.
For each one percent increase in soil organic matter, you can hold an extra 20,000 gallons of water per acre in the soil.
When rainfall hits carbon-rich, fertile soils, it sinks in. Well aggregated soil has clumps that stick together with open spaces in between them. Plowing breaks up the soil surface layer turning it into dust. When water hits that dust layer, it crusts and more water runs off than penetrates into the soil. Farmers want the soil to hold water so it can be used by their crops. It’s counterintuitive. We often think that if we plow and break up the soil, that will help water sink into it. I’ve heard many farmers make that argument. But it doesn’t work that way.
Tilled soil, when exposed to rain, falls apart because its structure has been destroyed. Its ability to aggregate has been destroyed. It’s a powder that dissolves and erodes. The no-till soil stays intact.
Water seeps into no-till soil with hardly any runoff. Regenerative farming practices conserve topsoil and organic matter and are a recipe for rebuilding the fertility of the land. Fertile soils are rich in carbon.
Carbon plays a vital role in the soil in terms of water storage, which is increasingly important as the world is subject to more droughts and extreme weather. There is a direct correlation between organic matter (which is made up of about 50 % carbon) and the water-holding capacity of soil. 1% increase of organic matter increases water holding capacity by 3%. By increasing the soil organic matter content of an agricultural soil, more of the water that falls on a field as rain will seep into the ground and be held there and retained as a reservoir to feed the crops rather than running off into a river or stream and bypassing the agricultural system.