Learning how to make biochar is a no-brainer if you’re trying to improve your soil without spending any money.

We became big fans of biochar years ago when we went to this workshop. Then we hosted a workshop at our farm to learn how to make biochar vessels out of 44-gallon drums.

That technique is useful but complicated enough to put us off making it ourselves. It seems like a lot of effort to produce a small amount of biochar.

Discovering the bathtub method was a game changer.

A simple method for making biochar

We were lucky enough to be invited to a small, privately run workshop that introduced us to a much simpler system for making biochar in a bathtub.

It was hosted for us by Grow Great Fruit members Win Westerhoff (below left) and Clare Claydon, and the workshop was presented by biochar enthusiast Jim Sansom (below right).

Jim started by talking us through the main steps of the method.

Jim Sansom explaining the bathtub method
Jim Sansom explaining the bathtub method

A simplified Kontiki Cone Kiln

The bathtub method is based on the ‘Kontiki Cone Kiln’ but is a much simplified version. The Kontiki Cone Kiln is a large (about 1.6 m across) kiln for making biochar.

One of the challenges with this method is that there are no hard and fast rules about when to stop the burn. “Judge when to stop” was Jim’s advice, “when a stick has lost its identity as a stick”.

Another thing to know before you start is don’t use a cast iron bathtub, or it may crack when you light a fire in it. And obviously, don’t use a modern plastic or fibreglass bath either. You need to find an old steel bath at your local tip shop.

Biochar making method

At the workshop, we went through the actual process of making biochar under the expert tutelage of Jim, Clare, and Win. It was surprisingly fast and easy.

Here’s how it works:

Step 1

Fill the bottom of the bathtub with light, dry fuel.

Starting the fire in the bottom of the bath with light, dry material (and a bit of metho if needed!)
Starting the fire in the bottom of the bath with light, dry material (and a bit of metho if needed!)

Step 2

Light it on fire. (Pro tip: use a bit of metho if needed). Aim to get a good strong fire going along the whole length of the bathtub.

Step 3

Once it’s burning, pile up with bigger stuff, but make sure it’s burning the whole length underneath. (Pro tip: this is much easier on a still day than a windy one.)

Step 4

Stack the bath until it’s 3/4 full. If the fire is burning well, it shouldn’t be too smoky. It’s fine to use bigger wood in the bathtub as long as it’s all roughly the same size so it burns at about the same rate.

Hugh keeping the fire going evenly along the whole bathtub with a rake
Hugh keeping the fire going evenly along the whole bathtub with a rake

Step 5

Use a metal rake to pack the wood down hard. This also keeps the fire burning evenly and everything burning at the same rate.

The gases will be pushed out of the wood and are then burned in the bathtub. This prevents oxygen from getting down into the fire. If too much oxygen gets into the fire the wood will just burn away to ash.

New research has shown that if the biochar is made at a lower temperature you end up with biochar that has a higher level of organic matter.

Hosing the bath to put the fire out when you judge it's finished
Hosing the bath to put the fire out when you judge it’s finished

Step 6

When you judge that most of the material is blackened to char, quench the whole thing with a hose.

Hugh and Clare admiring the finished product
Hugh and Clare admiring the finished product

It’s completely fine to have sand, dirt, clay, or mud on the wood and in the cooking process. In fact, this may be beneficial by creating a more highly mineralised final product.

Using your biochar

How do you apply the finished product? We’ve covered this before in other blogs, but here are the methods that Clare and Win use:

  • dig a trench, tip in a barrow load of biochar, add compost and a scattering of blood and bone;
  • broadacre – spread with a fertiliser spreader and then go over it with harrows.

The point is to find some way to incorporate biochar into your soil where it will work its magic. And as we’ve covered in this blog, it’s crucial to “charge” your biochar first to make it really effective.

Of course, it’s not magic, it’s science!

Biochar has remarkable effects on soil by providing both a stable form of soil carbon and an extensive habitat for soil microbes.

And the best bit? If you make it yourself, it’s completely free!

Related Articles

Wannabe an organic farmer?

Wannabe an organic farmer?

Do you have a secret dream to be an organic farmer? We asked people about their dreams and barriers – with some surprising results!

read more

Get our FREE ebook – 10 Key Steps to Growing Great Fruit

This useful ebook will give you answers to all the topics you need to know, from pests to pruning, and it’s completely free!

You'll soon be enjoying abundant harvests.

When you download the ebook, you'll also get our free Weekly Fruit Tips newsletter to help you stay on track with the little jobs that keep your trees healthy and fruitful.

Just hit "Get my ebook!" to download your free copy.

You have Successfully Subscribed!