SEVERAL WAYS TO USE BIOCHAR
By Charlie McIntosh and Josiah Hunt | June, 2022
Biochar has been found naturally occurring in soils around the world as a long-term stable component of soil organic matter. Using the most common testing method for measuring organic matter, loss on ignition, biochar is impossible to differentiate from other forms of organic matter. Furthermore, because biochar does not easily move through the soil profile, generally remaining where it is applied / incorporated, we have found it most useful to discuss biochar applications in terms of the percent organic matter (% OM) equivalent applied in the volume of treated soil. To simplify these calculations we developed a worksheet called the “Biochar Application Rate Calculator_% OM”. This tool allows users to calculate how much biochar is needed to achieve the desired increase in % OM for a specific project, including options for both broadcast and in-row applications.
Download the Worksheet here (xlsx): Biochar Application Rate Calculator_ % OM
The figure below shows an infographic displaying three different options for a 10 ton/acre biochar application;
- Broadcast application cultivated to 6” depth (left)
- Broadcast application cultivated to 12” depth (middle)
- In-row application cultivated to 24” depth x 24” wide x 8’ row spacing (right)
Note, dark shading represents 0.6% OM equivalent (left and right) with the lighter shading representing 0.3% OM equivalent (middle).
We encourage you to play with the Biochar Application Rate Calculator_% OM, adjusting the values in cells with an orange background to fit your specific project. There are sheets for broadcast and in-row applications that can be customized to fit your project as well as a matrix (displayed above) for use as a quick reference guide. Feedback is greatly appreciated so feel free to send us a message suggesting ways that we could improve this tool. We hope you find it useful!
The matrix included below shows the % OM achieved for various biochar application rates (tons per acre) and cultivation depths (inches) when biochar is broadcast applied (i.e. spread evenly over a given area).
(Assumptions: Biochar at 45% Moisture Content and 95% Organic Matter Content).
In-row applications can be useful to apply biochar only where it will be most effective for establishing a crop (i.e. in the planting row), reducing the total amount of biochar needed to achieve a given increase in % OM. Below, we have included a screenshot from the Biochar Application Rate Calculator_% OM showing a common example of an in-row application. In this example, the planting rows are spaced 8 feet apart and each row is cultivated 24 inches deep and 24 inches wide with an average of 800 plants per acre. The calculator shows that we need 4.33 tons of biochar / acre to achieve 0.25% OM equivalent and that a 10 ton / acre application rate will achieve 0.58% OM equivalent.
GENERAL APPLICATION RATE SUGGESTIONS:
- 0.5% to 1% OM equivalent is recommended for realizing maximum ecological benefit
- 0.25% to 0.5% OM equivalent is often more economically feasible in the near-term
- 2% OM equivalent is the suggested upper limit for initial applications
Orchards and vineyards (see Biochar Application Rate Calculator_% OM)
- If deep cultivation of the planting row will be done, incorporate biochar to depth before planting
- Incorporate biochar in aisles between planting rows for better cover crop growth
Broad acre, tilled (see Biochar Application Rate Calculator_% OM)
- 10 tons (wet weight) per acre tilled to 12” depth is about 0.3% OM equivalent
- If you’re tilling deep, till the biochar deep; If you’re tilling shallow, till the biochar shallow
To improve composting (% by volume)
- 5% to 10% biochar added at start of compost pile
- 10% to 20% when compost N is excessively high
Blended with compost (% by volume)
- 10% to 20% biochar blended with compost for regular maintenance
- 20% to 50% for major events (deep tilling, field prep, etc.)
- Allow the blend to cure for several weeks with adequate moisture before planting, if possible
Potting media, planting backfill, landscaping, or engineered soils (% by volume)
- 5% to 10% is common in potting media. If pH is balanced and nutrients are provided, biochar can be used as the primary potting media
- 10% to 20% of backfill soil is a blend of biochar, compost, and minerals mixed evenly with the removed soil to achieve desired rate of regrowth
Some words of wisdom
- Biochar matures over seasons in the soil or in just weeks in a compost environment. While maturing, organic acids, minerals, soil particles, and living organisms further complicate, or change the surface of biochar, aiding in its functionality and beneficial qualities.
- When combined in compost piles at an early stage, relatively small amounts of biochar can significantly reduce N-loss during composting and help to support thriving microbial communities.
- Adding fresh and raw biochar to soil can create a temporary nitrogen tie-up, however, this is markedly different from adding materials such as sawdust or wood chips. Because biochar is almost entirely non-reactive carbon, due to the reactions that occur during pyrolysis, the carbon is not easily accessible as a food source for microbes, minimizing issues with nitrogen tie-up.
- When biochar is made, there is always some amount of ash (minerals) present in the finished product. Although the majority of biochar is a pH neutral and non-reactive carbon, ash is alkaline. Ash can be a great source of many plant nutrients, except nitrogen, but in alkaline soils it must be used only sparingly. However, the pH of biochar won’t tell you much. “Liming capacity” or “calcium carbonate equivalent” is the most useful measure of a biochar material’s potential impact on soil pH, as it can account for both the pH and concentration of ash in a biochar material.
- Biochar does not travel through the soil profile easily. Where biochar is applied to the soil is more or less where it will stay for centuries unless disturbed by soil life or human activity.
- When storing biochar, keep it wet or keep it covered. Dry biochar is a nuisance to work with and can potentially become a fire hazard.