Overview of Biochar and its use in Food Gardens

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 health.

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 from wood feedstock

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: The effect of biochar on soil acidity or alkalinity depends on the starting pH of the soil. In acidic soils, biochar usually increases pH, while in alkaline soils, it may have little effect or may raise pH unless modified.  Once incorporated into the soil and stabilized, biochar can act as a buffering agent, preventing drastic pH changes.  This buffering ability is particularly useful in preventing acidification from fertilizers and stabilizing pH-sensitive microbial environments.  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.  Following is some of the information we have compiled in researching its use.  

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.  However, biochar produced from woody materials at temperatures above 500°F tends to have a higher pH because of its ash content.  (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 apparently doesn’t matter, they all are reported to 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. Add biochar in any amount up to a 1:1 ratio to existing compost and allow 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 conventional 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 and process 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.

Reducing Biochar pH—Biochar typically exhibits an alkaline pH due to its ash content and production conditions. For soils with a pH over 7.5, or biochar with excessively high pH, lowering biochar pH before charging and application may be desirable.  To lower biochar pH, several strategies are suggested:

1. Acid Treatments: Applying acids, such as diluted sulfuric acid, to biochar can effectively reduce pH. This can also enhance biochar’s phosphorus fertilizer value. This is not as important in our area since Florida soils usually contain plenty of phosphorus.  A more organic approach is applying acetic acid (vinegar) or citric acid to the biochar and then rinsing with water until the desired pH level is achieved.  This should be done prior to applying other charging methods to avoid damaging microorganisms.  

2.  High sulfur amendments:  Elemental sulfur can be used as an acidifying agent.  When sulfur and biochar are incorporated into soil, soil bacteria (Thiobacillus spp.) oxidize it into sulfuric acid (H₂SO₄) over time.  The recommended application rate is 1% - 5% sulfur to biochar by weight, depending on the initial pH of the soil, the biochar pH, and the target pH.  Add to inoculated biochar when applying to soil or mixing with compost.  Start with the lower amount and if the target pH is not reached after several months, add more.  

3. Co-Composting: Integrating biochar into composting processes can naturally decrease its pH. The organic acids produced during composting interact with the biochar, leading to a more neutral pH suitable for soil applications. Sulfur or gypsum could also be added to lower pH while composting.  

4. Water Leaching: Soaking biochar in water, especially over extended periods or through exposure to rainfall, can leach out alkaline ash components, resulting in a reduced pH. 

5.  Peat Moss:  Most research and information about combining peat moss and biochar relate to container substrate mixes.  However, there is a possibility that mixing peat moss with biochar will help to reduce the pH, although it is likely this is a long term solution. More information is needed.

Methods for Charging/Inoculating Biochar

A mask should be worn when working with biochar, particularly when dry, as the dust can be harmful if inhaled or swallowed. It can also cause eye irritation.  If using chlorinated water in the charging mix, best practice is to let it sit for 24-48 hours to outgas chlorine. Setting the water in direct sunlight and stirring occasionally will speed the process,. Before you begin, calculate the volume of biochar you want to inoculate, depending on the area you want to cover.  Any amendments to the biochar should then be added at half the application rate normally recommended for that amount of area. Cumulative adding of biochar to soil should be closely monitored as too much biochar can be detrimental to soil health.  A maximum of 10% biochar in the top 6 inches of soil is recommended.  This is a one-time soil amendment and should not need to be replenished.   Try to apply charged biochar 2–4 weeks before planting to give nutrients and microbes time to integrate.
Method	Details	Notes
Compost	Add water to biochar until it leaks out Mix biochar and compost up to 1:1 ratio, but beginning with 10% biochar is recommended.   Let it sit for at least 14 days. Other sources suggest 4-8 weeks up to 3-4 months. Keep moist	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.
Compost (alternative method)	Fill container such as 5 gal. bucket halfway with 2 parts compost/1 part biochar (or desired ratio). Fill rest of bucket with water. Stir well every few days for a month Pour off liquid and apply to plant roots or foliage Use the wet compost/biochar mixture on garden soil to top dress plants or work loosely  into top layer of soil	Once water is poured off, use biochar/compost at once.
Fertilizer	Mix liquid or dry fertilizer with biochar and add water to form a thick slurry.  Soak the biochar slurry for 24–72 hours. Add compost tea, molasses water, or microbial inoculants for better biochar activation. For better results, soak 1–4 weeks.	Microbial proliferation will depend on fertilizer and optional additions used.  Synthetic fertilizer can be used, but microbial population may be considerably less.
Aerated Compost Tea	Fill barrel or large plastic bin with water. Add biochar Add desired nutrient and microbe boosters  Insert aquarium aeration pump.  Weigh down hose. Aerate 12 to 24 hours Drain off liquid and apply to plant roots or foliage Use inoculated biochar on the soil or combine with compost first	Use at once  Quickest method for increasing microorganism population.
Worm Castings	Combine worm castings and biochar 1:1 Add 5% flour or molasses to promote fungi and microbe increase Add water to create a thick slurry Activation period 2-4 weeks; or up to 8 weeks	Seven times richer in major plant nutrients than compost. Excellent source of microbes Optimum air temp 70°F Keep out of direct sunlight
Co-compost	Add biochar to compost pile over time along with carbon and nitrogen feed stocks Total biochar should be less than 50% of pile It is charged when compost is ready	Use as needed
Optional nutrient and microbe boosting additions	Finished compost for microbes and nutrients Compost tea for microbes and nutrients Greensand for potassium Seaweed extract or kelp meal for trace elements Molasses to feed microbes Worm castings for microbes and nutrients Flour to feed fungi Healthy soil from target area for native microbes Azomite for trace minerals Mychorrizal fungi Bokashi bran  Any combination of these	Add when assembling charging mixture
Other possible charging materials	Urine Livestock manure or bedding Grass clippings

To calculate the amount of biochar needed for a 1-inch layer in a 4x4 bed, use the formula: (Length x Width x Depth).  For instance to calculate the amount for a 1-inch layer added to a 4 x 4 raised bed, the formula 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

Research and recommendations are just theory and we have no practical experience with biochar.  Our goal at the VegHeadz garden is to try at least some of these options to compare in trying to determine what works best with our soil and our unique north Florida growing conditions.  We intend to do a 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. The soil will be tested prior to application of biochar 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, although it is not clear how much copper actually accumulates in plant tissues. Copper is included in the feed for most farm animals for 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 which could 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 incorporated, it can’t be removed and most of 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. 

https://www.mdpi.com/2073-445X/12/9/1717#:~:text=The%20combined%20application%20of%20desulfurization,the%20application%20of%20biochar%20alone.  Application of gypsum and biochar to lower pH.  

https://www.sciencedirect.com/science/article/abs/pii/S0960852424004103. Application of elemental sulfur and vermicompost to lower biochar pH.  

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: