How to Add Carbon To Soil

So, of course, most gardeners would kill to have carbon rich soil! And that’s good, because you are going to need a lot of dead bodies in your soil if you want it to stay carbon rich long-term.

Humus

I know that sounds creepy. But the fact is, high quality soil happens when you have high humic content, also referred to as humus-rich soils. Humus is frequently described as decayed plant matter. However, that’s not exactly accurate.

Humus is made from the accumulated corpses of countless microscopic life forms that were involved in decaying that carbon-rich plant matter. In other words, enormous quantities of dead microorganisms are what sequester carbon and promote a healthy garden environment.

Let’s look at how that works so you can increase the carbon-rich carcass count in your soil.

Carbon Fixation

Plants take in airborne carbon that’s trapped between two molecules of oxygen (CO₂). Then, they separate that carbon from those oxygen molecules using energy from the sun.

To prevent those carbon molecules from reconnecting with oxygen and floating away, plants suspend carbon in a kind of syrup. Then, they use that syrup, plus nutrients and water from the soil, to make plant parts including roots, leaves, and stems.

That process of converting airborne carbon into plant useable carbon is called carbon fixation. It’s the first phase of the carbon cycle. Next, many of those carbon-rich plant leaves, stems, and roots that act as a temporary carbon sink must die.

Carbon Capture

When those plants or their parts die (annual leaf drop), if they stay in or on the soil, life forms like insects and bacteria will eat that plant matter. They’ll excrete some of it. But some will remain in their bodies and cells until they die.

Then, their bodies and waste products will be eaten and excreted by something else. Eventually, through countless iterations of living things eating once living things or their waste, all that carbon becomes more stably stored in microscopic carcass parts that don’t decompose so readily.

The plant carbon, turned into microorganism stored carbon, remains an active food source for soil life until it eventually has been used so many times that it transforms into humus. Then, it becomes the stable infrastructure that supports the well-being of all life the soil, including plants.

Both that still edible carbon and the humic content can soak up and retain moisture and nutrients that filter through the soil. They act as a root cellar/on-demand well pump for plants roots and soil life.

Composting

Composting is an alternative method of capture carbon for gardens. Instead of leaving plant material, or the waste of things that ate plant manure (like cows), on top of soil you collect it. Then you mix it with other organic materials to make a compost pile.

If you get close to the right amount of carbon (30 parts), nitrogen (1 part), air (40%), and water (60%) in a large enough pile, colossal numbers of thermophilic (heat-loving) microscopic organisms arrive. They quickly decompose those plant matter residues. Then, that can be reapplied to soil.

During composting, some carbon is lost to oxidization and methanization. However, a large part of it remains behind in the incredible quantities of short-lived thermophilic bacteria that died in that pile. As such, it’s a great way to heavy load your soil with microscopic carbon rich carcasses.

Carbon Exudates

Additionally, living plants send a regular supply of that carbon syrup into the soil via roots. That candy-like carbon concoction, called carbon exudates, gets picked up by microlife. While picking up, those microlife also drop off plant-usable nutrients, often as waste or by dropping dead in transit, near the root zone.

In many legumes and a few non-legumes, bacteria even deposit excessively large amounts of nitrogen directly into plant roots via nodules while picking up carbon exudates. Those exudates then get cycled through the soil life system like the way dead plant matter does. Plus, when the plant roots die, the nitrogen in the nodules gets released into the soil for other nearby plants to use.

Maximize Carbon Sequestration

With this background in mind, I am sure you can guess how to increase carbon in your garden. Yep, you got it! Grow tons of plants. Leave plant matter and roots in and on the soil. And add compost.

That’s the basic gist of how to get massive numbers of carbon-rich carcasses into soil. However, there are a few specific steps that can speed up that carbon-rich catacomb creation to support overall garden health. Here they are!

Step 1: Break Up Compaction

No till or no dig gardening is common in organic gardening. However, carbon-rich organic matter only gets sequestered in soil if it can move underground via roots, insects, and water.

If your soil is too compacted for roots to drill deep with relative ease, you’ll need to aerate, lightly till, or double dig to break up compaction before getting started.

Step 2: Populate

You need to ramp up the amount of living and dead microlife in your soil. The fastest way to do that is to add finished compost.

Compost that is over a year old is best. That way the heat-loving bacteria will have died out and the cool, soil beneficial bacteria will have started processing those hot-heads into humus.  A three- to four-inch-thick layer applied on top of your uncompacted bed is ideal.

Alternatively, you can use compost-in-place methods, followed by planting a cover crop like wheat or tillage radish to speed things up. Here are some options to consider.

• Cover soil with 6-8 inches of livestock bedding, cut grass, or other organic materials. When the materials have mostly decomposed, sow cover crop seeds into the area.
• Dig a trench and fill it with kitchen waste and cover with 4-6 inches of soil to keep digging pests out. Immediately sow cover crops on either side of the trench.
• Build a lasagna bed with layers of weeds, kitchen vegetable waste, livestock bedding, and other organic matter topped with 4 inches of garden soil mix. Immediately start a cover crop.
• Garden in straw bales to start and then leave the decomposed straw residues on the soil. Grow a cover crop in the spent composted bales.
• Build a compost pile on your new garden bed. When it’s mostly decayed, rake it out to 4-6 inches deep and grow a cover crop.

Step 3: Stabilize

After breaking up compaction and heavy loading with microscopic carcasses, the goal going forward is to cycle in new carbon at a stable pace. Here’s how to do that.

• Grow as many plants as your soil will sustain, for as much of the year as possible, to keep up the constant flow of carbon exudates.
• Leave plant parts on the soil to decompose or compost them, age compost for 1 year, and then apply. (Buy aged compost in the interim to use until yours is ready.)
• If you harvest plant materials from your garden (e.g. as vegetables or cut flowers), replace what you remove with light layers of green manure (grass clippings, cover crops), compost, or other organic mulches (leaf mold, straw).

Also note, whenever rapid carbon loss happens, chemical reactions in the soil will automatically awaken dormant weed seeds. If you don’t want weeds to come help you garden, avoid creating carbon loss through the most common routes: oxidization and methanization.

Oxidation

Oxidation happens when the carbon in soil is exposed to large quantities of air. Digging, tilling, and ripping out roots cause it.

To avoid it, remove weeds when they’re small and have shallow roots. Harvest crops carefully to keep soil in place, even when removing root vegetables. Repeat mow mature plants to the crown to kill them rather than ripping them out. Or cover them with cardboard to rob them of access to light.

Also, when you cut off the supply of carbon exudates by leaving soil unplanted, soil life will consume carbon already in the soil. As they use up soil carbon stores, air moves into the emptied space. That can oxidize the soil from within even when you don’t till or rip up roots.

If you happen to need a break from active gardening, fill the beds with cover crops or low-maintenance short-lived perennials to maintain that critical flow of carbon exudates. Or, expect weeds to arrive and do this work for you.

Methanization

Methanization happens when carbon rich soil becomes anaerobic or combusts.

Anaerobic Activity

If you apply lots of carbon rich material right before a soaking rain, the material becomes soggy. That keeps air from reaching the soil. Then, air breathing bacteria go dormant.

Anaerobic bacteria wake up. They turn carbon into methane gas that evaporates. Additionally, as the soil dries, the air spaces left behind after the carbon was mechanized may lead to further oxidization.

Combustion

Soil-based combustion happens when you apply carbon-rich materials like mulch or compost, and then get an influx of nitrogen that triggers hot composting. The nitrogen often comes from fertilizer, a nitrogen rich rain, or a lightning strike. It can also be released by carbon loving fungi.

If combustion occurs, it raises soil temperatures and cooks plants. It robs soil of water through rapid evaporation and evapotranspiration. Plus, it releases carbon into the atmosphere as methane, triggering weeds.

To avoid anaerobic activity and combustion issues in soil, apply light layers of carbon rich materials in cool, dry weather. Move mulches out of the way before fertilizing. If soil starts to heat up or become boggy, rake out those carbon rich materials. Then apply in lighter layers going forward.

Step 4: Enjoy the benefits!

Once you’ve successfully deployed those first three steps, over time your soil will sequester lots of stable carbon. Your humic content (and the rate at which it is produced) will increase. That will lead to compounding benefits. For example, soils with high humic content develop significant beneficial mycorrhizal colonies.

Mycorrhizal Colonies

These special fungi play a role in providing nutrients and water to plants on demand. Plus, they may control the soil-based signaling system that warns plants of impending pest or pathogen invasions.

Even when plants don’t form specific relationships with mycorrhiza, those little fungi keep the soil cooler during hot spells reducing plant stress. Plus, they retain stores of water near plant roots to help during droughts.

The Nitrogen Connection

Carbon rich soils with high humic content and extensive mycorrhizal colonies also retain more plant-usable nitrogen. Nitrogen is abundant in nature. It enters the soil via these routes.

• Mammals and bird drop it as manure.
• Insects drop it as frass.
• Rain drops pick up nitrogen as they fall into soil.
• Airborne nitrogen in the soil is converted into plant useable nitrogen by lightning strikes.
• Bacteria process airborne nitrogen into plant-ready formats.
• Nitrogen contained in microorganism bodies is freed up on their deaths.
• Nitrogen in dead plant matter gets recycled for use by living plants.

Unfortunately, these sources or nitrogen are water soluble. That makes them prone to washing away or evaporating before plants get a chance to use them. However, carbon and mycorrhiza acts like sponges to keep the nitrogen in the soil.

That means gardens with carbon-rich soil won’t need much (if any) fertilizer. Since fast-acting fertilizer applications almost always trigger some dormant weed seeds to germinate, not using it can cut costs and save time spent weeding!

Good for Gardeners

When your soil is carbon, humus, and mycorrhiza rich, plants are better able to care for themselves without constant gardener interventions. That means you can relax in your beautiful garden rather than trouble shoot plant problems and pull weeds. And, getting carbon out of the air and deep into our soil is one big way gardeners can help fight climate change doing something we love.

Tasha Greer is an Epicurean homesteader and writer living in Surry County, North Carolina. She’s the author of Grow Your Own Spices and Weed-Free Gardening. You can find her at Simplestead.com. 

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