Soil organic matter, a.k.a. carbon, is black gold—it makes the garden grow and thrive. Soil with a high percentage of organic matter has a loose, crumbly texture and a dark brown color. When your soil has sufficient organic matter, the plants growing in it:
Tolerate drought better.
Organic matter acts like a sponge, soaking up extra water and releasing it when needed.
Grow larger and more vigorously.
Plants grown in soil with abundant organic matter receive a slow release of nutrients all season.
Resist pests and disease.
Healthy plants are naturally resistant to pests and disease. Research has shown that compost has disease-suppressive qualities. For many areas of the country, the peak breakdown of carbon is May through July. Soils are relatively warm and hold plenty of moisture. This coincides with the time our garden plants need lots of nutrients—when they are growing and setting fruit.
How to "Carbonate" Your Soil
Add more. You can add carbon to your soil by mulching, growing a cover crop, and adding manure or compost. Many areas of the United States (especially areas that are traditionally agricultural) have seen a decline in organic matter and a loss of carbon. Degrading soils is "one of the major problems we face," explains Fred Magdoff, Ph.D., a professor of plant and soil sciences at the University of Vermont. This is why it's vital to add organic matter annually. Plants in warmer and wetter areas of the United States (such as the Southeast) use up organic matter more quickly, so instead of adding carbon to their soils once a year, gardeners in these areas should add it continually.
Aerate, in moderation. Because many microorganisms need oxygen, aerating the soil activates them and jump-starts the breakdown of organic matter. But "every time you disturb the soil, organic matter is lost," Magdoff explains. So till or aerate your soil only when you need to. Use raised beds, and avoid foot traffic through the garden so that you can till sparingly and keep the carbon in the soil.
Make compost. Then make more. By composting garden waste, you're using all the carbon produced in your garden. To be successful, compost must contain a balance of four ingredients: carbon, nitrogen, moisture, and oxygen. The bulky material in your garden (such as leaves, straw, and cornstalks) is high in carbon. Learn how to make compost.
Encourage soil critters. Whether they're bugs, bacteria, or fungi, your soil needs them all. They help turn the rough carbon in your garden into cakelike soil by producing microaggregates (tiny, crumbly nuggets) that stabilize and store carbon. "Soil aggregates are clumps of material that are called stable because they can't decompose much further," explains Magdoff. The stability comes from the action of microorganisms, which produce a sticky gum that glues the soil particles together. The soil is a sink, or storage place, for this kind of carbon.
Grow lots of stuff. To store the maximum amount of carbon, always keep the surface of the soil covered with growing things: trees, turf, vegetables, flowers, and cover crops. The more different kinds, the better. "Grow plants that have deep, extensive root systems and produce coarse, woody roots, because shallow, fine roots decompose too easily," says Naomi Pena, a policy analyst at the Pew Center on Global Climate Change. She also advises planting trees, since they accumulate carbon in amounts two to four times higher than grass.
Sink Locally, Act Globally
If improving your soil wasn't a good enough reason to increase the carbon in it, then how about playing a role, albeit a small one, in combatting global warming? Soils have the ability to store huge amounts of carbon. Indeed, there's more carbon in soil than in all plants, all animals, and the atmosphere. If we don't sequester carbon, too much of it can be pulled out of the soil and into the air, where it becomes a liability instead of an asset.
As Pena notes, "soils contain a large amount of organic matter, and depending on environmental conditions and human activities, soil organic matter may be converted to carbon dioxide or methane, powerful greenhouse gases that contribute to climate change." Yet another reason to grow a garden.
Don't let the term carbon cycle bring back bad memories of high-school science classes; there's no quiz. It's simply a term for the merry-go-round of changes that this element undergoes in the atmosphere, in plants, and in the soil.
1. In the air, carbon is a gas. Carbon dioxide (CO2) gets a bad rap as a greenhouse gas, but it's a normal product of respiration from animals, including humans. We inhale oxygen and exhale carbon dioxide. Plants do the reverse as they photosynthesize: They take in carbon dioxide and release oxygen.
2. Without carbon dioxide, plants not only can't photosynthesize; they can't even stand up. The process of turning sunlight into food occurs within a plant's leaves, where carbon dioxide and water change into oxygen, glucose, and fructose. These form chains of starch and cellulose. Cellulose is a vital component of cell walls, so plants don't stay upright without it.
3. When plants die, the flora and fauna of the soil (such as bacteria, fungi, and larger scavengers like millipedes) break down the carbon in the dead plants, using some of it for food. During this process, the decomposing plants release carbon dioxide and nutrients, which growing plants take up. Essentially, the dead parts of your garden feed the living parts.
4. As gases naturally diffuse in and out of the soil, it gently "breathes," releasing air with a high percentage of carbon dioxide and taking in air with a higher concentration of oxygen. Plants above the soil reuse the carbon dioxide, grow, and bear fruit. In this way, the carbon cycle is completed.
Carbon. An abundant nonmetallic element (atomic number 6) that is present in many inorganic compounds and in all organic ones.
Carbon cycle. The combined processes, including photosynthesis, decomposition, and respiration, by which carbon atoms move in a cycle involving the atmosphere, oceans, and living organisms.
Carbon sequestering. The processes by which carbon dioxide is removed from the atmosphere and, through photosynthesis, turned into plant material, then stored in the soil as organic matter.
Greenhouse gas. A gas, such as carbon dioxide (CO2), that traps heat in the atmosphere by absorbing radiation.
Methane. CH4, a greenhouse gas that is produced through anaerobic decomposition of waste, animal digestion, and industrial processes.
Organic matter. The remains, residues, or waste products of any living organism.
Photosynthesis. The manufacture of sugar by plants through the action of sunlight.