Benefits of Biochar in the Garden

Summer is here at last and I have charcoal on my brain.  Not for summertime grilling, though… for the plants! In the quest to encourage balance in my two gardens, I’ve become enamored with a new-to-me addition: biochar.

As home gardeners learn more, always seeking better ways to boost production and increase their enjoyment of the garden, a key idea will repeat itself: balance.  Biochar is gaining notoriety as a simple and effective way to achieve soil health and get more bounty from the ground.

What is biochar? The first definition I found made me laugh a little: “a granular substance produced by pyrolysis of organic matter.”  What a funny way to say “crushed charcoal bits soaked in fish guts.”  At least, that’s what I’ve got on my homestead.  Your biochar might be different.

As I explained it to my kids, who are always eager to help me with any task that involves lighting stuff on fire and the possibility of marshmallows, the basic idea is to take lightly crushed charcoal, then “charge” it with nutrients and work it into your soil.

The porous structure of biochar attracts moisture and nutrients and provides long-term housing for microbes and fungi.  Even if applied only once, the effects will last for decades!  Improved soil tilth, water retention, and higher crop yields can result.

The concept of burning organic matter to create Super-Soil may sound modern to us, but it’s actually quite ancient.  Experts have been studying the 2,500-year-old Terra Preta method in the Amazon Basin and come to a startling conclusion:  the native people had no marshmallows.  Also, they liked to slowly burn massive areas to create very deep, very fertile soil that is unmatched for growing crops.

So, can you just torch your compost pile and add it to the garden?  No.  The worms would protest.  You also won’t see great results by purchasing a bag of standard charcoal briquettes and soaking them in Miracle Grow.

Fortunately, making your own biochar is easy and cheap (or free).  The main idea is to burn your biomass slowly, with a minimum of oxygen.  This will create charcoal, a porous and lightweight product instead of a pile of ash.  (See this article to learn how to make your own charcoal.)

make your own biochar

Here is how to create your own biochar at home:

Buy or create an area that will enclose your fire and allow you to control the airflow; a metal barrel, a trench or cone dug in the ground, or a commercially-purchased “charcoal retort”.

I went with the trench method, for one reason: I have teenage boys to do all my digging.  I instructed them to dig the trench three feet deep and three feet wide, decreasing to become narrow at the bottom.  (If you are unsure how to start a fire and ensure a safe burning environment, ask a local Boy Scout. Keep a hose handy).

Fill it with wood and light it on fire.  Let it burn hot.  Then, smother the fire with dirt and continue to monitor the coals to prevent them from turning to ash.  Extinguish with water if necessary.  I had to douse my fire at least ten times!

When oxygen is introduced, the coals will likely reignite.  Continue to smother them, add water, and monitor until you have a nice trench/cone/barrel full of black, crispy matter.

Crush the larger bits with a shovel—aim for particles smaller than what you’d buy in a bag, but not powdered.  Allow these to completely cool and dry over a few days.  Now “charge” your pile with nutrients.  I prefer diluted fish emulsion.  You can also use compost, manure, or any liquid organic fertilizer. (Hint: we all contain liquid nitrogen.  Little boys are often pleased to be able to contribute).

I sent my kids on a treasure hunt around the property, mainly to give me time to dispose of the remaining marshmallows.  They returned with deer droppings, a dead bird, a pile of mullein leaves, and some crispy bones.  I crushed the bones to the same consistency as the char.  Free phosphorous!

The rough ratio for “charging” is half char, half nutrients.  The longer you soak the char, the better.  An airtight container will provide an anaerobic environment, and leaving the lid off will provide an aerobic one.  Choose your own adventure.

You want to give it time for a microbiome to start flourishing—one or two weeks.  It’s best to do this far from your house, protected from curious animals unless you like cleaning up puddles of black, fishy dog vomit on your kitchen floor.

Will the charging sludge also attract flies?  Develop mold?  Make you question all of your life choices?  Yes.  Press on.

I gave my char two weeks to charge, then dumped the remaining liquid in my compost pile and dried the char.  I have a friend who creates such large amounts of char that the only way to crush it is to spread it under a tarp and drive over it with her truck.  I handed my children rocks and shovels and sat back to watch the fun.

But you employ whatever method you feel is best.

How much and how deep?  Spread the char over your soil and work it in at least 5 inches deep.  It is recommended that, for poor soils, a quarter pound of biochar per square foot of soil will boost regeneration.

I wondered, as you probably are, if there could be disadvantages to using biochar.  An internet search revealed the truth: maybe. Most of the potential negative impacts that have been discussed are for large-scale operations (concern for cutting too many trees to create char, for instance.)  Much of the research I found regarding the small-scale use of biochar was positive.

As with any input to your garden, you will want to have a basic working knowledge of what you’re starting with.  A good soil test will tell you a lot.

Depending on what you charge it with, Biochar will tend to raise the pH of your soil, so keep that in mind.

I decided to test small areas before adding the whole wheelbarrow full.  Lacking any real scientific method, my casual observation has been that the areas with the most biochar are growing like crazy!

If the idea of making your own biochar does not appeal to you, happy news: you can buy it.  This is also a good resource for those with small patio gardens or neighbors who frown upon balcony fires.

Can you just buy charcoal and charge it?  Yes, but you probably shouldn’t.  Not unless you can be absolutely sure that you are buying slow-burned chunks (there’s that fancy “pyrolysis” term again), not hard-pressed briquettes, and containing no binders or fillers.  In general, lighter fluid is a poor fertilizer.  Whatever you do, don’t just add plain, uncharged char to your garden—it will act like a magnet to nutrients and suck the life from your plants.

I hope you’ll give biochar a try in your home garden.  You can improve soil health, water retention, and vegetable production practically for free.

No marshmallows required.


  1. Excellent article in layman’s terms, especially the importance of properly charging your finished product ( I have found worm tea works great). You covered all the bases without over complicating it with sciency jargon. The only thing I would add is if you are using wood, use hard woods when possible and the slower and hotter you burn, the better. There is a big difference between biochar and charcoal, and pyrolysis is what separates the two. For actual pyrolysis, you need temperatures of at least 500°C in a strictly anoxic environment so the closer you can get to achieving this, the better your results. The usefulness of biochar isn’t limited to increased organic matter and labile nitrogen reserves but also vastly improves water holding capacity and the soils ability to act as a carbon sink, significantly reducing carbon and nitrogen loss to the environment so it can used as food for microorganisms who in turn convert nutrients in the soil organic matter into plant available form. It’s no wonder they didn’t need chemical fertilizer 2500 years ago!

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