Using Biochar in the Garden

Biochar is a great amendment for any garden soil. I wrote about the wonder product a month ago (May 9, 2012, “What is Biochar?“), and have incorporated it in a few of my garden beds for testing. Biochar research is in its infancy so information on how to use it in gardens is still being developed by researchers and gardeners like me. While I encourage you to add biochar to your garden soil based on present data, be aware that my methods are one option developed only by me; there are other options and other undiscovered processes for using it.

100 percent biochar

Biochar acts as a repository and delivery system of beneficial soil bacteria. These microorganisms are an important component in soil that make nutrients available to plants by converting elements to the ionic form that roots absorb. Without an encouraging environment, bacteria and other microorganisms exist in soil but not at a level that can support sustained plant growth.

Compost is another repository and delivery system of beneficial soil bacteria and is the most common soil amendment used by home gardeners. Both biochar and compost enrich soil and provide similar structural, textural, and fertility improvement, but compost needs to be added regularly due to its steady decomposition while biochar remains relatively intact for decades.

To begin, biochar must be acquired and that is currently a major limiting factor. There aren’t many biochar manufacturers or distributors. I bought my biochar from Soil Reef Biochar through their website. Until wholesale manufacturing and processing becomes competitive you can expect the cost for biochar to remain relatively expensive, but you need to consider the long-term benefits. One application of biochar can save you the cost of multiple applications of compost over many years.

That being said, I’m pursuing a joint approach to soil improvement. My working hypothesis is that if compost is good for soil and if biochar is good for soil, then compost and biochar together should be really good for soil. My garden test beds are amended with both.

To be most effective, biochar should be prepared before adding it to your soil. Biochar provides a positive environment for beneficial bacteria but if just mixed with soil its effectiveness is minimized. It’s a “build it and they will come” approach that can be compared to building free housing in Europe for customers in the Americas; there is a big ocean that needs to be crossed before it can be occupied. Bacteria have difficulty transiting wide spaces.

Inoculating biochar overcomes this obstacle. Inoculation incorporates the bacteria directly into the biochar before you amend your soil. One method to inoculate it, and the one recommended by Soil Reef Biochar, is to mix biochar with an equal amount of compost and let them meld for two weeks. The bacteria in the moist compost gradually multiply and migrate to their new home.

I developed a speedier, and in my opinion more effective, method for inoculating biochar. Using a method analogous to the way we inoculate against disease, I essentially inject raw biochar with bacteria-rich fluid.

To begin I take equal quantities of compost tea and worm tea. Compost tea is made by soaking or steeping compost in water for two or three days; the resulting liquid is swimming with the bacteria that populated the compost. Worm tea is made in a similar process but using worm castings (manure) instead of compost; it has similar multitudes of beneficial bacteria.

About one gallon of compost tea and one gallon of worm tea are mixed and combined with one cup of dark molasses in a four-gallon bucket. This sugary concoction becomes an incubator for bacteria growth as the tiny creatures feed and multiply.

Oxygen is a critical component for the growth of beneficial bacteria; the “bad” anaerobic bacteria don’t need oxygen. To ensure adequate oxygen in the developing inoculant you can insert a fish aquarium pump with plastic tubing into the liquid to provide air bubbles. As an alternative I covered the bucket and physically twisted, bounced, and shook it multiple times during the day.

After three days the inoculant is a little foamy on top and ready to use. If left alone for a longer period the sugar in the molasses will eventually be totally consumed by the bacteria and the liquid will ferment, turning into alcohol. The lack of food and the presence of alcohol can kill the bacteria, which defeats the whole purpose of creating a bacterial incubator. Use the inoculant before it goes bad.

Biochar and inoculant

Two gallons of this concoction is enough for five gallons of biochar. Starting with small batches, I poured the liquid inoculant into the dry biochar using a rough one to two ratio.

Adding inoculant

I stirred it with a garden trowel and let it soak for about five minutes. Then I mixed and stirred it again because some of the liquid drained to the bottom. After the second mixing and another five minutes, the biochar was evenly moist.

Stirring the mixture

Liquid inoculant achieves two important steps in biochar preparation.

First, it incorporates the bacteria directly into the microscopic crevices and holes that constitute biochar. While contact with moist compost can take two weeks for this crossover to be accomplished, inoculating with a liquid achieves the same results in minutes.

Second, it moistens the biochar so it can immediately provide the moist environment that bacteria need. Pure 100 percent biochar from the distributor is very dry with a large portion of it very powdery. It needs to be wet before adding to soil to be most effective, both to benefit bacteria and to keep the dry powder from blowing away.

The inoculated biochar can be added to soil immediately or covered and stored for later use. However, it should be used within a day or two of inoculating for best results because the bacteria will go dormant or die if the biochar dries out or if their food source is diminished.

There is no universal standard developed yet for biochar distribution in garden soil. The Soil Reef Biochar 5-gallon bucket says it will cover 36 square feet when distributed one-half inch thick. The Soil Reef Biochar website says the same bucket will cover 18 square feet at one-half inch thick. A biochar handbook developed by EcoTechnologies Group recommends a maximum of one pound per square foot; biochar is very lightweight so at this rate the five-gallon bucket would only cover about five or six square feet.

I spread my inoculated five-gallons of biochar over about 48 square feet. The addition of the liquid increased the mass slightly. I added the biochar to one half of two separate 32 square-feet beds and into one 16 square-feet bed. The spread depth of the biochar varied between one quarter and one half inch thick.

Spreading wet biochar

Using a garden spade I turned the biochar into the soil to incorporate it about five to six inches deep. Biochar works its magic with the bacteria that provide nutrients to roots. To be most successful it should be resting at root level. A garden tiller can also be effective in distributing it evenly in soil.

Turning it into the soil

An alternative to adding 100 percent inoculated biochar to soil as I did is to combine it with compost at the time of application. If inoculated as Soil Reef Biochar recommends, it is already mixed with an equal amount of compost. If inoculated as I recommend, it can be mixed with compost for easy soil amending.

The amount or organic material present in your garden soil will determine whether you need to add compost at the same time as biochar. I add compost at the beginning or end of the growing season based on the quantity available. One of the nice things about biochar is that once it’s amended to soil it never needs to be replenished.

Another option is to add biochar to your compost pile. Biochar can shorten the time it takes for compost to develop and is already incorporated in the mix when you amend your soil. You’ll get the benefits of biochar but it may be difficult to determine how much of the mass is biochar and how much is compost. For accurate distribution and for testing, I suggest adding biochar as a measured amount directly to the beds you want amended.

Once added and turned in or tilled, water the garden bed right away. You want the inoculated biochar to stay moist for best effectiveness. Moist soil also gives the bacteria a new potential environment. Think of the biochar as an urban center and the surrounding soil as the suburbs. Eventually the bacteria will inhabit all of it. Because biochar retains moisture better than basic soil, it will continue to propagate the beneficial bacteria even when the soil dries out between waterings.

Watering the bed

To date there is no definitive research that recommends the ideal amount of biochar for home gardens. One test by EcoTechnologies Group showed no difference between plants grown in a 10 percent biochar application rate and that of a 20 percent rate. My distribution is much less than that, but I still anticipate positive results.

Already my test bed is showing benefits of using biochar. Cucumber seeds on the biochar side germinated a full two days earlier than those on the unamended side. I’ll continue documenting its effectiveness and report on it in the future.

Consider biochar in your garden and let me know how you do it.


11 responses to this post.

  1. Scott, This may be a stupid question but would Lump Charcoal work? I know that briquettes are full of binders and all sorts of other nasty things but from reading BBQ sites Lump is supposed to be nothing but wood.


    • Max, it’s not a stupid question. Biochar is basically specialized charcoal used for the explicit purpose of amending soil. I would not recommend plain, lump charcoal in the garden as compared to commercial biochar, and never use briquettes. First, lump charcoal is too big; you’d have to grind the charcoal or pulverize it to get the small pieces you need to be an effective soil amendment. Second, the process of making commercial charcoal may not remove all of the caustic gases and chemicals in wood; the process for making commercial biochar burns the wood in a process that eliminates most of the negative components in basic charcoal. Third, the charcoal may still have ash as part of the processing and wood ash can raise soil pH; biochar is ash free. All that being said, if you washed, drained, and crushed basic wood charcoal, you could use it in the garden and achieve the same effects as retail biochar. Ideally you want the pieces of biochar/charcoal to be the size of a pea or smaller. At current costs, charcoal would be a cheaper alternative if you’re willing to take the time to prepare it. Just be sure it is completely free of any chemical additives.


  2. […] written about biochar before and about the beginning of this experiment (see my article “Using Biochar in the Garden“, June 4, […]


  3. Posted by Terry Black on April 28, 2013 at 8:44 pm

    I do believe lump charcoal is the same as biochar, but for the “green” manufacturing of it. It would take a little extra labor to cleanse and make it finer, but probably worth it.


    • Terry, lump charcoal is similar to biochar, but there is a difference in how they’re made. The pyrolysis processing of biochar tends have higher temperatures and results in a product that eliminates most of the volatile chemical compounds still present in lump charcoal. That extra processing is what makes biochar more expensive and a better idea for garden soil.


      • Posted by Chris Haynes on May 28, 2014 at 8:09 pm

        Also, lump charcoal is highly compressed, while biochar is very light and exceptionally porous. Much of its utility comes from its vast network of pores.

      • Thanks, Chris. That’s an excellent point. The pores and the haven they offer to soil bacteria and microorganisms are important reasons why biochar is so efficient.

  4. Posted by Roselle on August 16, 2014 at 3:34 am

    Hi there, thanks for your post.

    I noticed that when I googled Inoculated biochar (even the ones that are inoculated with compost tea) the end product looks gray not black. Which makes me think that this gray look is what indicates inoculation. How come yours still looks black? Or is it maybe that it needs to dry out first?


    • Roselle, it could be the moisture content. I took pictures of my biochar while it was still very wet. Most commercial biochar products are delivered dry. As you’ve probably noticed, when something gets wet it deepens in color. When I first unpack the biochar it is very dry and does have a gray tint, but definitely turns black when wet.


  5. Posted by Christopher on August 22, 2014 at 3:39 pm

    I’ve been doing pretty much the same thing as you describe here, only I use the commercially sold EM inoculant and ferment them anaerobically in doubled-up garbage bags to make 50/50 bokashi/biochar- Works great! and yes seedlings germinate way faster with the right bacteria.
    As for the concentration to use in your garden soil- keep in mind that the Amazonian “terra preta” from which the whole biochar idea has come from, was made- possibly unintentionally- by people digging pits in the jungle to toss their waste- they dumped years and years worth of home-fire ashes, charcoal, excrement, animal parts, kitchen and garden scraps, green waste, broken clay pots, etc- in random layers, forming essentially a concentrated nucleus of microbial life in the forest, Over hundreds of years it expanded, as fungi colonized the pits and grew their mycelia (which next to biochar is also a highly stable form of carbon) outwards into the surrounding soil. The people probably liked adding the left over fire coals because dumping it on top of all that rotting stuff they threw in there helped cut down the smell and keep the flies away.
    Any way that is just to say that if you only have a small supply, maybe it would be more effective when using the inoculated biochar in the garden, to bury a shovel-full here and there next to or under a plant- especially the perennial types- instead of spreading it out in a thin layer or evenly tilling it into a whole bed. That is what I’m experimenting with anyway


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