Co-composting’s Carbon Multiplier Effect

When black carbon from biochar is added to compost, it greatly improves the soils ability to retain water, retaining 6 units of water for every unit of carbon added. Not only does the soil retain more water, but the biochar also greatly improves the soil systems’ ability to create and retain other nutrients. This enhanced environment supports the development of the plant’s root system and structures thereby increasing the plant’s exudates in the rhizosphere, adding as much as 10 units of green carbon for each unit of black carbon, which also fosters the microbiome in the soil and increases the growth and extent of fungal mycelium, thereby adding as much as another 10 units of white carbon.

These results can vary according to a number of factors such as the initial soil quality, with poorer soil seeing much greater benefit. They are also the subject of ongoing research and study, with some studies suggesting as much as a 40 fold increase in total carbon sequestration for each unit of biochar that is properly applied. However, given these variables, our current conservative claim is for a 10x multiplier effect.

Even this represents a massive amount of carbon capture triggered by the addition of the black carbon in biochar. So it is not just the carbon from the biochar that is sequestered, but all the carbon from these other biological processes that also gets removed from the atmosphere as a result of these multiplier effects from co-composting with biochar. 

Recommended practices for co-composting Biochar

1. Prepare compostable materials with a good ratio of carbonaceous to nitrogenous materials. This is commonly referred to as the ratio of “browns to greens”, though this is potentially misleading as certain “greens” are actually brown in colour. You can refer to this webpage for a thorough explanation and chart. Chop up any pieces which are excessively large so that the pieces can mix well with the rest of the compost.
2.  Portion out enough biochar to constitute 10% of the compostables mixture (by bulk volume)

3. Water the biochar until it is all thoroughly wetted.

4.  (Optional but highly recommended.) Mix a quart (100ml) of finished compost with the biochar to seed it with compost microbes

5.  Mix the wet biochar with the compostables material, and either load it into the composter or place it into the compost bin or pile. Compost piles can be built out of wire mesh or wooden pallets, or can be simply piles on the ground covered with tarps but the must have a minimum size requirement of 3ft per side (1 meter cube). Smaller piles will likely require an insulated tumbling composter.

6.  Aerate the compost twice per week for the first three weeks, and once per week afterwards.

The compost should take 8 weeks to finish, but it also benefits from an additional month of resting and ‘curing’ where the temperature gradually declines until it matches ambient temperature. The curing phase stabilizes the compost and favors fungal growth, which enables the compost to form stable aggregates when it is incorporated with soil.

Proper blend of compostable materials:

Items that tend to be rich in nitrogen include green leafy materials, grass clippings, and kitchen scraps. Items that tend to be dominantly carbon-rich and have very low nitrogen include wood shavings, bark, twigs, dead dried leaves, straw, and hay. The nitrogenous materials are often referred to as “greens”, while carbonaceous materials are referred to as “browns”, as a rule of thumb to help gardeners estimate, but the color rule may be misleading due to several prominent exceptions. Some brown-colored materials such as manure and coffee grounds are brown in color but are nitrogenous. The ratio can range from 1:1 down all the way to 5:1, and is quite forgiving.

Precision is not easy due to the high variability of how much nitrogen is found among various “greens” and how little is found in various “browns”. If the nitrogenous material is particularly concentrated in nitrogen, more carbonaceous material is needed to balance it. The higher the nitrogenous portion, the more aeration is needed. If your particular blend does not compost actively enough, more nitrogenous materials are needed; if it begins to stink and smells like ammonia, this indicates that there is too much nitrogenous material, and more carbonaceous material should be added to balance it.

To simplify, one bucket of kitchen scraps goes with:

  • 2–5 buckets of dry leaves
  • 2–3 buckets of wood chips (finer chips are better; extra time may be needed)
  • 3–5 buckets of wood shavings (usually used as animal bedding)

Be cautious with dried lawn cuttings, they should be considered as a “green” rather than as a “brown”

Items to exclude from compost

Avoid the following items, due to the risk of fouling the compost, producing pungent stench, and attracting pests:

  • Meat
  • Fish (a small quantity of fish bones and scales can be added if you add more carbonaceous material, and if you bury these deep in the middle of the pile)
  • Eggs (egg shells are fine, just crush them well)
  • Dairy products
  • Invasive weeds

Invasive weeds can be used if they are first burned, steamed, or boiled to ensure that they are totally dead. Otherwise, they can propagate using the compost.

Piles of tarp-overed compost

Expected results

Biochar amended compost tends to compost hot. Expect the compost to produce a significant amount of heat. The heat comes from aerobic decomposition and is a good thing. Hot compost kills weed seeds and pathogens. Core temperatures can reach as high as 68˚C / 155˚F.

Thermometer in compost pile

In the garden, the compost with biochar that has gone through a high-temperature decomposition process will be much more stable, and will last longer in the garden. Consider the following example from Gill Tract Community Farm.

Gill Tract Community Farm prepares raised beds with open sides, with the beds packed using compost mixed with a little bit of soil. In the following photo, a walkway separates two raised beds of comparable age. The one on the left was prepared using co-composted biochar. The one on the right, which is barely even recognizable as a raised bed, was prepared using conventional compost. Prior to adding biochar, the compost at Gill Tract Community Farm failed to heat up past 54˚C /130˚F; the resulting compost was not fully decayed, and continued to decompose after being applied to the soil. The co-composted biochar decomposed into a stable solid, and remained. One of the expected effects of composting with biochar is that more of your compost will remain, enabling you to gradually but steadily build up your soil organic carbon levels.

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