Some of the most famous fertile farm lands in the world are the black soils of the Canadian prairies, Terra Preta in South America and the farm land of Ukraine. The carbon in the soil is what makes it black.
Carbon is a basic building block of all life on earth.
How Does Carbon Get In The Soil?
The first way carbon gets in the soil is by plants putting it there. Plants make sugar through photosynthesis. This sugar contains carbon. The plants exude some of the sugar through their roots to feed microbes in the soil around the roots. Carbon is a product of the microbial food chain – when the microbes poop or die and are digested by other microbes, that carbon is left in the soil.
The second way carbon gets in the soil is when plant matter such as leaves and stems are worked into the soil. Dead plants mixed into the soil are eaten by microbes. Again, the carbon is now in the microbes and is released into the soil as explained. Plant matter left on the surface of the soil releases its nutrients into the air because the microbes cannot get at it. This is why proper tillage matters (***link to “NOT TILL”***) – it gets the plant matter to the microbes where they can eat it.
What Does Carbon Do In The Soil?
– Water Retention: Carbon holds up to 4 times its weight in water. Soil high in carbon can hold a lot of water.
– Microbe Shelter: Chunks of carbon, such as biochar, provide surfaces and nooks for microbes to live in. They love it!
– Nutrient Retention: Since Carbon loves to hold on to stuff, nutrients and elements cling to it. Microbes can use these nutrients to eat or pass on to plants.
– Fungal Growth: High-Carbon soil is necessary for soil fungi to thrive. Fungi are very beneficial to plants and general soil health. An ideal ratio of carbon to nitrogen is 30:1.
How Can I Add More Carbon to the Soil?
To build up the carbon in your soil, add carbon-rich material. Soil microbes will consume those materials and release the carbon into the soil. Some carbon rich sources include:
| Carbon Source | Garden Rate | Field Rate (per acre) |
| Sugar | ½ cup per gallon of water, watered in | 1 lb. – any time you pass over the field |
| Molasses | ½ cup per gallon of water, watered in | 1 quart – any time you pass over the field |
| Plant Residue | Your dead garden plants | Your field residue |
| Old Hay/Straw | 1-2 inch layer | Your field residue |
| Leaves | 1-2 inch layer | n/a |
| Sawdust | ½ – 1 inch layer | n/a |
| Woodchips | 1-2 inch layer | n/a |
| Newspaper | 2-8 sheets thick | n/a |
| Bio-Char / Activated Charcoal | A Heavy Dusting (wear a mask), or in furrow at time of seeding | |
| Humic Acid | 1 Tbsp per gallon of water | 2 gallons |
| Humates | 1-2 lb / 100 sq. ft. | 100 lbs. |
| Compost | ½ inch | n/a |
Make sure your carbon sources are worked into the soil with gentle tillage that goes slightly deeper than the aerobic zone. If the carbon sources are left on the surface, the soil microbes will not have access to them and the nutrients will oxidize into the air. If you do not want to roto-till, just use a garden hoe to gently work the carbon material in. Some carbon sources are watered into the soil instead of tilled (sugar, molasses, other liquids, etc.)
When To Apply
Sugar and molasses can be applied at any time soil temperatures are above freezing. And apply often – every week to 10 days is fine.
While the above carbon sources can be applied throughout the year, most of them are best applied in the Fall after harvest and lightly worked into the soil. This gives time for things to break down and have soil nutrients ready for spring planting.
If you have poor soil be sure to add compost or compost tea with your carbon source so that the carbon material will be broken down.
Sources:
Teaming With Microbes – Lowenfels & Lewis
Boots in the Dirt – request from Back To Your Roots Soil Solutions
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