Why are there pottery shards in terra preta?

When I did my video on my attempt to recreate terra preta, many people wondered what role pottery shards played in the original mix.

Here’s my video on our attempt to make terra preta:

“Why add broken pottery?” I was asked.

Perhaps the pottery shards were incidental, and not part of a plan to improve soil. Heck, maybe terra preta itself was a complete accident of primitive waste dumps.

Or maybe not. We don’t know anything for sure about how terra preta was created. We just know that it stays rich and fertile centuries and that it contains bones, pottery shards and charcoal.

I added all three to my mix. Without an understanding of “why,” sometimes you just have to “do” and see what happens.

This morning I received an email from Jordan that appears to fill in one piece of the puzzle: the ubiquitous pottery shards found in terra preta soils. They do have a use!

The Effect of Pottery Shards in Terra Preta

Jordan writes:

I’ve been making biochar for a while now, and studying terra preta profusely. Pottery shards are a key aspect often overlooked, but they are hard to come by in abundance.

He attached this video, demonstrating how the use of pottery shards effects terra preta:

Jordan continues:

We’ve been digging a retention pond to stock fish, and I now have plenty of soil. I’ve made pottery clay a few times now. I roll it into long thin strips and put it inside my bio char retort, and usually by the time I have char, the clay has been fired, and being rolled long and thin it’s cracked and shattered down to a good soil aggregate size, ready to use.

Sounds like a simple method that anyone could do at home. Someone in the comments of my video on making terra preta noted that the plants growing on top of a pile of old bricks did much better than those growing without bricks. Can you imagine? The first thing most of us would do in our gardens is to take out the bricks!

In the video Jordan sent, a group of people make clay patties for their soil with low-tech methods, probably not dissimilar from how the Amazonians created their pottery.

Making Pottery for Terra Preta

At the beginning of the video, the wicking of water up a clay brick is shown, leading to the germination of a seed which then proceeds to grow healthily despite its very tiny amount of soil.

From there, it progresses into the making of low-fired clay patties to use in terra preta soils.

The Process

First, two women take dry clay and smash it down into powder, removing stones as they go:

Then water is added to the dry clay mix and kneaded by hand.

After the clay is at the right consistency, small handfuls of it are pressed into clay patties and set aside to dry.

After drying, these patties are placed into a pyre of dry wood, which is then lit.

When the fire burns out, the resulting fired clay patties are removed to be used as a soil amendment in creating terra preta.

At the end of the video, the wicking power of a stack of fired clay patties is shown:

So why are there pottery shards in terra preta? The most obvious answer is for their wicking effect. The shards almost act as sub-soil irrigation, moving water through the root zone of plants and making it easier to access.

I have a bed out in the garden that needs some work to bear better crops. The garlic I planted in it last fall did poorly, so I’m going to re-do the bed with lots of amendments to improve it for this coming fall. It’s obvious now that I need more clay. I’ll get the kids making patties, plus I’ve got some bisque-fired pottery shards from a local pottery studio I can add. Heck, maybe I’ll even throw a few bricks in it. There are old bricks scattered all over the yard.

Stay tuned. We’ve got experimenting to do.