Add Biochar to Your Garden

Next Thursday I’m speaking at a meeting of the La Grange Garden Club.

One of the members asked me to talk about biochar. If you’re unable to attend, here’s a preview.

Biochar is just charcoal the black carbon residue left after burning wood in a lowoxygen environment.

If you look at it under a microscope, you’ll discover it has an incredibly complex structure – porous with a huge surface area. This allows it to capture a lot of water and any nutrients absorbed therein - up to five times its own weight.

It also has the ability to bind positively-charged ions of ammonia and ammonium to its surface, making them available to plants and microorganisms.

The ancient inhabitants of South and Central America may not have understood the science I just briefly mentioned. But they did know that biochar was an incredibly powerful soil amendment. They used it to practically terra form a big portion of the continent.

Anthropologists have somewhat recently discovered that the rich black soil found in the Amazon Basin is not a natural feature. Rather, it was built up by the early inhabitants of the area. They mixed charcoal, crushed bones, broken pottery, and manure to the relatively poor clay soil of the Amazon. The result is an incredibly rich black soil known as “terra preta.”

Charcoal is key to it all. It helps keep nutrients present in the soil. This soil remains fertile thousands of years after the ancients created it.

A while back I watched a video from a speculative anthropologist who attempted to recreate terra preta. He basically dug a hole in some clay soil and threw in some chopped up wood and branches, which he set on fire. He let it burn for a while and then smothered it out, covering it with clay from the hole, broken pottery, crushed bones and manure. He then threw in some more wood, started a fire and repeated the process a few times until he filled up the hole with multiple layers of charcoal, broken pottery, clay, bones and manure.

I’ve never tried this, but it sounds like a really good way to start a new garden bed. I do often add some charcoal to my compost pile. Whenever I get close to emptying a bag, I crush it up as much as I can and add it to the pile. But I only use lump charcoal in my barbecue pit. I wouldn’t try this with charcoal briquettes since they contain petroleum products and binders (which is also why I don’t like cooking with them).