Secret to lowering acidity is the introduction of black carbon

Some years ago in early spring, I burned a fairly large pile of brush. Rain came before all the glowing embers in the resulting ash pile were consumed, and I was left with a pile of ash and a bucket or so of charcoal, or black carbon, that I could have easily used in my barbecue but instead spread and tilled into the soil where the burn was made.

The general run of the soil here is not naturally fertile and tends toward acidity. I was surprised later in the summer to see how lush the vegetation was on the spot where the burn had been, including a volunteer broccoli that looked better than anything in my garden.

I decided that the ash from the pile had lowered the acidity sufficiently so that the plants could get to some nutrients that had been otherwise locked up in the soil.

I’d forgotten about it until recently when I stumbled across what may be an explanation and a source of interest to any gardener.

Some few decades ago, anthropologists investigating the native cultures of tropical Brazil happened upon well-defined and isolated areas of soil that were black in contrast to the red and brown soils immediately surrounding them. Tropical soils tend to be poor in nutrients, and it was soon noticed that these isolates were considerably more productive than surrounding soils when seeded and cultivated. Crops such as papaya and mango grew at three times the rate seen in surrounding soils.

Today, these soils are known as “Terra Preta” or Amazonian “Dark Earths.” Beyond their black color, they are also high in organic matter. Further investigations eventually determined the blackness of the soil was due to an extremely high content of black carbon that long vanished cultures had initially introduced into the soil. This charcoal was produced much as I accidently did, by burning wood and other vegetation in a low temperature, oxygen-poor environment, something that anyone who routinely heats a home using a wood stove conveniently and naturally does each night for use to stoke up the stove with new fuel the next morning. In Brazil, the method was named “slash-and-char” to relate and distinguish it from the more widely known “slash-and-burn” practices that have quickly made a waste of dangerously large portions of the tropical rain forest and are affecting weather worldwide.

“Slash-and-char” has quite opposite and highly beneficial results.

It seems that the innumerable small bits of carbon in Terra Preta serve two functions, the first of which reminds me of the use of Haydite as a soil amendment, as discussed in an earlier column. Each bit of carbon — just like a charcoal briquet — is first of all a natural sink for moisture and for nutrients. The Amazonian Dark Earths indeed have a high phosphorous content and higher than normal cation exchange capacity, both of which fundamentally serve soil fertility.

But a second process is at work. Soil amended with black carbon is observed to increase the activity of arbuscular mycorrhizal fungi that first form in the charcoal and then spread into the soil, these fungi then fixing yet more molecular carbon into the soil by stabilizing the soil with glomalin, a glycoprotein that both stores a great deal of carbon in its own structure. It also permeates and binds organic matter to silt, sand and clay particles, forming soil aggregates that give structure to the soil and that further keep carbon from escaping the soil.

That is, the introduction of “slash-and-char” carbon into the soil begins a virtuous circle in which soil fungi in turn fix yet more carbon — taken from the atmosphere and decaying life forms, and now called “biochar” — into the soil. Some Terra Preta soils extend to a depth of 5 or 6 feet and their carbon content is seven to 10 times that of immediately surrounding soils. This biochar black carbon is highly stable in the soil, for reasons not yet fully understood, and will not return to the atmosphere as a component of the greenhouse gas CO2 for centuries; indeed there is a small group of enthusiasts who see Terra Preta as a solution to global warming, if it were widely practiced as a method of carbon sequestration as a by-product of both food and energy production.

Our concerns here are smaller and more immediate. If you want to experiment with your own biochar as a growing medium, you can begin with a bag of wood charcoal — this looks like and is charred wood, not charcoal briquets. Before adding it to your soil, crush it into smaller pieces (try using a hammer or wood post or rolling it around in some container that also contains a concrete block).

You might then try introducing your black carbon into any amount of good highly organic soil that you have prepared as usual, perhaps just enough for just one potted plant. Then observe the results in that plant, especially perhaps by the second year.

This spring, I will do so using a soil mixture of crushed woodstove charcoal in an amount of 10 percent by volume. You can find Web postings by Terra Preta enthusiasts that will give you much more information and the various things they have tried.

Dennis Sentilles, MU professor emeritus of mathematics, is a Missouri Master Gardener and a member of Katy Trail Slow Food International with a love for working outdoors and eating simply and well every day. He can be reached at sentillesd@missouri.edu.