Terra Preta

Adding pure organica to your garden.

Terra Preta, or "Dark Earth," refers to an end process of soil enrichment that is centuries old and linked to Amazonian agricultural practices. Commercially available char called "Bio-char" can be purchased, but why pay for something you can create yourself?

I have been burning garden refuse for several years, and just this year began adding it in large amounts to my garden beds after watching a documentary on the Amazon. The documentary focused on how vast numbers of ancient peoples were able to support themselves in a region with notoriously poor soil quality.

One answer was the discovery of areas of blackened earth surrounding cities and individual living areas. These pits contained the charred remains of food items, pottery, wood, animal bones and feces, and household refuse. In some regions, particularly along high mountain tops within the scope of ancient volcanic eruptions, Terra Preta can be found occurring naturally.

In doing research, I found reports of increased fertility in gardens utilizing biochar ranging anywhere from 350%-880%. This is quite a claim, but even if the numbers are much lower, investing in this process seems more than reasonable. And so far I have seen dramatic results!

The trick to using char is in the manufacturing process. I burn a lot of agricultural refuse in my compost piles. But you want to avoid adding too much wood ash. Rather, you want to create charcoal, which requires smothering the fire before it can use up all the wood fuel.

Burning high fiber grass clippings is OK, but won't add any charcoal to your compost. There is the option of buying untreated charcoal and crushing it, but that's so much more expensive, especially on a large scale, and ooooh way too much work.

So I have been practicing and have come up with a fairly simple solution. First, you want to burn wood products. If you can get hold of bamboo or other types of canes, you are in magic land! I grow sunchokes that make a tall stalk that is ideal for this purpose. The fuel is utilized quickly in the fire but it does leave a huge amount of charcoal behind.

Otherwise use small sticks, pine cones, corncobs, anything that has a woody content that will take a little longer to burn. In the piles I am working on in prep for Spring I am using woody shrub trimmings on one pile and old corn cobs on the other. As you can see in the image of the charcoal in my hand, this wooden branch has been successfully charred to the point that it crumbles easily, but does not turn to ash or powder.

When the refuse is dry enough to burn, lay a single layer of cardboard on top of the pile. This will keep the fire hot underneath while limiting oxygen, which is a great way to get your charcoal. To ignite this pending massive bonfire, add a little dry grass or leaves in between the wood to get the fire going.

I usually start at one end of the pile, and as the fire burns it's way toward the other end, I follow behind with a hose to dowse the embers. Remember, you don't want to burn it all the way down. Drenching the fire after the major flames have subsided is important. If you used horse manure, you will also notice that the manure balls that were exposed to the fire are now little charcoal briquettes. Perfect!

So now you have your burned piles of manure. Now what? Initially there are some issues that you will have to address. Charcoal can be added to the beds at very high levels, as much as 75%-80%. However, at first the charcoal will be water repellent, and may actually deprive the soil of nitrogen.

In order for the charcoal to be of use, it must be mixed into the bed and populated with nutrients and microorganisms. This may take as long as 6 months to a year in drier climates. To assist the charcoal in it's process of soil supplementation, you can apply fish emulsion to the beds which will solve both problems.

Charcoal is porous, and will retain nutrients for centuries. This is how it benefits your plants, by retaining nutrients in the soil that would otherwise be lost to leaching or degradation. So while you may have to give your charred bed a kick in the pants in the beginning, you will only have to do this one time to receive continuous benefit.

Also, Terra Preta actually replenishes itself by about 1cm per year if properly managed. What garden soil do you know that can do that?

IMPORTANT!  PLEASE be sure to have your cell phone, a hose turned on and at the ready to put out any fires that try to escape from the pile, and DO NOT burn in areas or at times where this activity is restricted!