Peak Oil Soil

Terra Preta is an increasingly prominent term in the world of permaculture. Leanan at The Oil Drum posted a link to a good article from Discover magazine the other day introducing the suspected origins, composition, and benefits of Terra Preta soils. The article serves as a great first look at the complex nature of soil science.

Terra Preta (“black earth”) refers to certain dark soils found in South America dating from 500 to 2500 years ago. What is so exciting about these soils is their amazing (and persistent) fertility, which far surpasses any of the surrounding soils. They are characterized by high levels of carbon (150 gC/kg soil), high levels of phosphorus (200 – 400 mgP/kg soil), and soil depths of 1 - 2 meters. Researchers believe the prime contributor to Terra Preta soils is bio-char (charcoal) with important contributions from clay pottery shards and composted organic material.

The introduction of burned organic material is not a new concept in permaculture (or agriculture), but there is a marked difference between charcoal and ash. Charcoal can be characterized as incompletely burned wood whereas ash is fully burned and has a very low carbon content.

There are several ways to make charcoal, some consisting of complex furnaces or solar heaters and others as simple as dousing a wood fire once the logs are sufficiently charred. The usual “slash and burn” process for clearing land typically produces mostly ash. While the soil is temporarily enriched, the nutrients are not retained and wash away after just a few years. Charcoal, on the other hand, can trap nutrients in the soil for thousands of years.

These ideas can easily be incorporated into your own garden to producing long-lasting effects. The finer the charcoal grains, the better, so it’s worth your while to pound on the lumps of char for a while before adding it to your soil. Charcoal will wash away if you're not careful, so be sure to either till it into the soil or cover it with compost. And voila: you’ve added a bit of fertilizer that will still be readily available after Peak Oil.

One curious aspect of Terra Preta is that it appears to reproduce autonomously. There are some suggestions that this is due to high microbial activity, but there is little research to confirm or deny the effect.

From a sustainability standpoint, it seems that converting wood into charcoal rather than allowing it to decompose prevents more carbon from reaching the atmosphere. The natural decomposition process releases more carbon over a long period of time whereas converting to charcoal releases less carbon over a short period of time. I imagine this balance is different for different types of feedstock (e.g. pine vs. oak) and it’s only applicable if the wood is already dead or dying -- burning up perfectly good trees is rarely a sustainable practice.

There is some concern that Terra Preta is the subject of a great deal of hype. Since the main component of it is charcoal, it’s possible that some people may misunderstand the message of its carbon sequestration potential and use it as an excuse to deforest and burn.

There’s a lot of unknowns with regard to Terra Preta, but the preliminary indications are that we stand to benefit from studying this 2000-year old creation. Whether the results are as profound as a solution to carbon sequestration or as locally important as improved garden productivity, Terra Preta has some serious help to offer. (For a further source of many good links and discussion, try this thread at permaculture.org.au)