In 2024 Leon County, Florida, became the first government entity in the nation to turn yard waste into a carbon-negative material called biochar. Carbon negative means the product or process offsets more carbon, through carbon capture, sequestration, or avoidance, than it contributes to the environment.
Biochar is essentially charcoal, created from wood or other materials which have been roasted at very high temperatures in anaerobic conditions to create a stable product which will sequester carbon for many years while rendering benefits to soil and plants. At the Leon County facility the gases produced in the process are captured and recirculated to provide fuel for the process, so very few emissions escape into the atmosphere.
Benefits of Biochar Additions in Soil
o Enhanced nutrient retention: Biochar has a porous structure that acts like a sponge, absorbing and holding onto essential nutrients like nitrogen, phosphorus, and potassium. This helps prevent nutrient leaching, ensuring that plants have a steady supply of nutrients for their growth and
o Improved water holding capacity: The porous nature of biochar also aids in retaining moisture in the soil. It can absorb water and release it slowly, reducing water runoff and evaporation. This means that plants have better access to water during dry periods, reducing the need for frequent irrigation.
Magnified Biochar produced
from wood
o Increased microbial activity: Biochar provides a habitat for beneficial soil microorganisms such as bacteria and fungi. These microorganisms play a crucial role in breaking down organic matter and converting nutrients into forms that plants can readily absorb. By promoting microbial activity, biochar supports a healthy soil ecosystem and enhances overall soil fertility.
o Carbon sequestration: Biochar is a form of stable carbon that remains in the soil for an extended period, sometimes even for hundreds or thousands of years. By adding biochar to the soil, carbon from plant material is effectively locked away, reducing the release of greenhouse gases into the atmosphere, and contributing to climate change mitigation efforts.
o Reduced soil erosion: The incorporation of biochar into soil can enhance its structural stability, preventing erosion caused by wind and water. It helps bind soil particles together, reducing the risk of soil loss during heavy rainfall or strong winds.
o Balancing pH and soil remediation: Biochar has the ability to adjust soil pH levels by acting as a buffer. It can help neutralize acidic or alkaline soils, creating a more suitable environment for plant growth. Additionally, biochar can aid in the remediation of contaminated soils by adsorbing pollutants and preventing them from entering the food chain.
o Long-lasting soil amendment: Biochar has a slow decomposition rate, allowing its benefits to persist in the soil over an extended period. This reduces the need for frequent reapplication and provides a long-term solution for soil improvement.
Biochar in Sandy vs. Clay Soils
o When biochar is added to clay, it makes the soil less dense, and increases hydraulic conductivity (the higher the hydraulic conductivity, the more easily water can move through soil).
o Adding biochar to sand also makes it less dense, but researchers have found that biochar-amended sand holds water longer.
o Sandy soils are generally more responsive to biochar than clay due to several key factors related to soil structure, water retention, and nutrient dynamics:
1. Low Organic Matter and Poor Nutrient Retention – Sandy soils have large particles with minimal surface area, leading to low organic matter content and poor nutrient retention. Biochar, with its porous structure and high surface area, helps retain nutrients and reduce leaching, making the soil more fertile.
2. Improved Water-Holding Capacity – Sandy soils drain quickly and struggle to retain moisture. Biochar acts like a sponge, enhancing water retention and providing a more stable moisture supply for plant roots.
3. Enhanced Microbial Activity – Sandy soils typically support less microbial life due to low organic content. Biochar provides a habitat for beneficial microbes, promoting soil biological health and nutrient cycling.
4. Cation Exchange Capacity (CEC) Boost – Sandy soils have a naturally low CEC, meaning they don’t hold onto essential nutrients like potassium, calcium, and magnesium well. Biochar increases CEC, improving the soil’s ability to retain and exchange nutrients for plant uptake.
5. Structural Stability and Aeration – Unlike clay soils, which can become compacted and poorly aerated, sandy soils benefit from biochar’s ability to enhance aggregation and provide better root penetration without causing excessive compaction.
o Biochar application rates as low as 0.5% appeared to reduce saturated flow in sandy soils, but only >1% application rates appeared to increase saturated hydraulic conductivity in soils with high clay content.
o Application: To improve aeration in clayey soils, biochar can be tilled into the soil benefitting the entire root zone. For most other soils, including sandy soils, biochar can be top-dressed and blended into the upper layer of soil.
Biochar in the VegHeadz Garden.
In the VegHeadz Demonstration Vegetable Garden and Edible Food Forest at the Leon County UF/IFAS Extension, we have been provided with a big bag of biochar from the local facility, approximately one cubic yard, and we are learning how to use it in our garden. It is confusing since most gardeners are just becoming aware of this soil amendment and there is much conflicting and complicated information available from gardening sites, producers, and research reports. Research about its use in small gardens is still sparse, with little consensus, standards, or repeatability across the biochar spectrum.
Biochar Feed Stock and Processing— The best source material for application in vegetable gardens appears to be lignin-rich biomass from hardwood like oak and softwood such as pine. The best processing temperatures for creating biochar from these materials appears to be 400-700°F. (For reference: The biochar processed in Leon County is produced from untreated wood materials at temperatures between 500-600°F.)
Applying untreated Biochar to Garden Soils—Most sources agree that the addition of untreated biochar to the soil may temporarily immobilize nutrients, potentially leading to a short-term reduction in plant growth. Avoiding this result would require several months up to a year of fallow time after application of untreated biochar to allow the soil and its organisms to stabilize before planting.
Treating Biochar Prior to Application—The purpose of treating biochar, called activating, or inoculating, or charging, is to fill the nutrient storage capacity of the biochar and pre-load it with microorganisms so it will cause the least disruption in the growing medium, and will immediately begin doing its work in the soil and providing nutrients to plants.
There are a number of approaches for the home gardener to charge or activate biochar. Which you choose doesn’t really matter, they all work. You can even use a combination of methods. The important thing is exposing the biochar to nutrients and microorganisms for a sufficient period of time to become established.
Charging with Compost—This seems to be the easiest method if you have compost available, preferably some you made so you have control of the input. This method involves adding biochar in any amount up to a 1:1 ratio to existing compost and allowing it to “marinate” for a number of weeks. The same thing can be accomplished over a longer period by adding biochar to nitrogen and carbon materials when feeding your compost bin. The nutrients and microorganisms that are created in the composting process will inhabit the biochar at the same time.
Charging with liquid—This method produces quicker results and biochar that can be applied sooner. It follows a common pattern—add nutrients, add liquid, and let it sit. Some proponents of this method claim that aeration of the liquid mixture is essential for the flourishing of microorganisms in the biochar since the beneficial organisms we want need oxygen to proliferate. If aeration is not used, biochar can be activated by mixing in a source of nutrients and letting it sit before adding it to your soil. The liquid is usually water, and the source of nutrients can be organic or synthetic fertilizer, compost, manure, fish emulsion, worm castings, etc.
The llquid method will also store microorganisms to a greater or lesser degree, depending on the nutrient material used. Shorter soaking periods may not allow microorganisms to develop to the same extent as the compost method, and access to oxygen is necessary for aerobic microorganisms to proliferate. Also, using this method to activate biochar in any substantial volume would be cumbersome for a home gardener.
Methods for Charging/Inoculating Biochar
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Method |
Details |
Notes |
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Compost |
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Use as needed, applying as you would normally apply compost. Good for microbe proliferation Turn the pile each week. Keep out of direct sunlight. Optimum air temp 70 degrees. |
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Compost (alternative method) |
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Once water is poured off, use biochar/compost at once. |
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Fertilizer |
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Microbial proliferation will depend on fertilizer and optional additions used. Synthetic fertilizer can be used, but microbial population may be considerably less. |
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Aerated Compost Tea |
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Use at once Quickest method for increasing microorganism population. |
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Worm Castings |
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Seven times richer in major plant nutrients than compost. Excellent source of microbes Optimum air temp 70°F Keep out of direct sunlight |
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Co-compost |
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Use as needed |
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Optional nutrient and microbe boosting additions |
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Add when assembling charging mixture |
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Other possible charging materials |
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To calculate the amount of biochar needed for a 1-inch layer in a 4x4 bed, use the formula: (Length x Width x Depth); in this case, it would be (4ft x 4ft x .0833ft) = 1.33 cubic feet of biochar. (1 inch is .0833 of a foot)
There are .43 cubic feet in a dry gallon. Therefore it would take 3.1 gallons to add 1 inch to a 4 x 4 bed. (1.33 / .43).
There are 128 ounces in a gallon. 3.1 x 128 = 397 oz. x 10% = 39.68 oz. or 1.25 qts. biochar in 3 gallons of compost or 10.4% biochar. Double this to add 2 inches of compost and biochar combined to a 4 x 4 bed, or to add 1 inch to a 4 x 8 bed. .
Biochar for Raised Bed Vegetable Gardens
The rate of biochar application in vegetable gardens can vary depending on the type of soil, organic matter content of the soil, and the plants being grown. A general rule of thumb is to apply 5 to 10% biochar by volume to the soil. For example, a 4 ft. by 4 ft. by 6 in. raised bed holds about 8 cubic feet of soil. Therefore, you could add .4 to .8 cubic feet or about 1 to 2 gallons of biochar per 4 x 4 bed. Dividing the total into several applications would facilitate integrating soil nutrients and microorganisms with the biochar.
Before applying biochar, thoroughly mixing it with compost helps protect it from erosion by water or wind. Adding mulch secures the mix to the soil and helps prevent exposing initial microbes to the sun.
Biochar Trial in the VegHeadz Garden
Our goal at the VegHeadz garden is to run our own trial by planting some of the same plants in soil that does not include biochar, in soil amended with biochar charged with compost, and in soil amended with biochar liquid charged with organic fertilizer. Any differences in pests and diseases, and plant health and growth, will be noted. Several gardeners have also taken small amounts home to try in their own gardens.
We have several concerns or questions.
According to some sources, biochar tends to increase soil pH, and our soil test results reflect we are already operating at the high end of the desirable pH range. Other sources indicate that while biochar can indeed raise pH if it is extremely low, such as 5.0 or below, if the pH is close to neutral or alkaline it does not further raise the pH. This would tend to support the claim by some sources that biochar regulates pH. So will be tested prior to application to provide a benchmark with which to compare biochar amended soils. This is highly recommended when adding amendments to your soil..
Second, in general, biochar seems to be more beneficial for sandy or poor, nutrient-depleted or acidic soils than healthy ones. Our soils are already healthy, and not acidic, so our main purpose in using biochar would be to utilize its water and nutrient retention and microbe boosting capabilities, and its ability to sequester carbon and some soil-borne pathogens over long periods of time.
A third consideration is whether biochar can help remediate the copper concentration in our garden soil. Biochar is reported to be effective in sequestering copper contamination in soil. Our recent soil tests indicated that our soil contains more copper than recommended. It has been determined that it is probably from the use of animal manures in the garden mixes that we have applied, which is further concentrated by composting plant material from our garden that has been grown in the higher copper soil. Copper is included in the feed for most farm animals because of its antimicrobial qualities as well as for nutrition. We believe the buildup of this element in our soil may have affected seed germination and plant growth to some extent and will get worse if copper levels continue to rise.
Finally, it is important to proceed carefully when adding any amendment to your soil, and particularly biochar because once it’s there, it cannot be removed and it will not degrade naturally in our lifetime.
Some References:
USDA site: https://www.climatehubs.usda.gov/hubs/northwest/topic/biochar, lots of info and a wider view of potential uses and benefits of biochar
https://edis.ifas.ufl.edu/publication/SS585, UF/IFAS introduction to biochar and its uses in agriculture
https://www.rhs.org.uk/soil-composts-mulches/biochar. Biochar overview and use in gardens from the Royal Horticultural Society of Great Britain (RHS).
https://climate.mit.edu/explainers/biochar. A good explanation of how biochar works to sequester carbon dioxide over centuries.
https://www.southlandorganics.com/pages/faqs/the-power-of-biochar-how-long-does-it-take-to-charge-in-compost#:~:text=Optimal%20Charging%20Period%3A%20For%20optimal,of%20nutrients%20and%20beneficial%20microbes. Details about compost charging of biochar.
https://www.geoengineeringmonitor.org/growing-concerns-about-biochar-safety-and-carbon-markets#:~:text=A%20review%20of%20259%20studies,water%20availability%20and%20soil%20erosion. A more negative or cautionary view of biochar.
https://www.permaculture.co.uk/articles/how-to-charge-biochar/#:~:text=The%20simplest%20and%20most%20efficient,by%20maturing%20in%20your%20compost., A permaculture view of biochar charging and application.
https://www.lancasterfarming.com/farming-news/organic/biochar-helps-vegetable-grower-improve-soil-vitality/article_38026246-f44b-11ef-89e8-d7b6f3247fb0.html. How vegetable growers in the Lancaster, Pennsylvania, area used biochar with their compost and in their soil.
https://pmc.ncbi.nlm.nih.gov/articles/PMC10821463/ A more complete analysis by the National Inst. of Health
https://ohioline.osu.edu/factsheet/anr-36. Role of soil bacteria and aerobic vs. anaerobic bacteria.
https://www.mdpi.com/2071-1050/11/22/6215?utm_source=chatgpt.com. The effect on plants of copper toxicity in soil.
https://pubmed.ncbi.nlm.nih.gov/16566157/. The effect on plants of copper toxicity in soil.
https://www.sciencedirect.com/science/article/abs/pii/S004896971732942X The ability of certain biochar to sequester copper contamination in soil.
http://nmsp.cals.cornell.edu/publications/factsheets/factsheet113.pdf. Fact sheet about copper use in agriculture and cattle operations.
https://drive.google.com/file/d/17hb5We4T_93UmyU-JB05gHi8G6ORQucS/view?usp=drivesdk Minnesota and Iowa Vegetable Garden Trial 2013-14. Biochar was not inoculated and results inconclusive.
https://drive.google.com/file/d/1QUOkSiA42KuRyMiVZhgnZ8TZytt3rxMz/view?usp=drivesdk. Some information and links about how biochar adsorbs pollutants.
Several sources with information about liquid activation of biochar.
https://bottom-up-biochar.com/7-ways-to-charge-biochar/
https://biochar6.com/activating-biochar/
https://soilfixer.co.uk/Biochar-activation-inoculation
https://chatgpt.com/share/679fa1cf-8680-8011-b9a9-05f913898ff4. Charging Biochar with fertilizer
Biochar Educational Materials
These organizations have ongoing research into biochar and/or good summaries/bibliographies of some of the more popular research that has already been conducted. The first two are the trade associations for the biochar industry.
- International Biochar Initiative
- U.S. Biochar Initiative
- Yao Lab at Yale University School of the Environment
- Bioeconomy Institute at Iowa State University
PUBLIC/END USER ARTICLES
- Biochar: A home gardener’s primer, Washington State University Extension
- Biochar: An emerging soil amendment, Michigan State University Extension
- Guide to Making and Using Biochar, University of Arizona Extension
- How to Use Biochar, Wakefield
In addition to the above, the International Biochar Initiative undertook a fairly significant campaign in 2018 focused on research and public education as part of their Biochar For Sustainable Soils (B4SS) Campaign. There are some good resources that run the spectrum from full scientific articles to cartoons on their campaign site.
Visit the VegHeadz Garden web page:
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