Saving The Planet While Saving The Farm, How soil carbonization could save the planet while it saves the family farm

Tom Miles

Saving The Planet While Saving The Farm
How soil carbonization could save the planet while it saves the family farm
Scott Bidstrup, Arenal de Tilaran, Guanacaste, Costa Rica, 2004

For a long time, soil geologists and archaeologists faced a mystery.

The mystery was a nagging one, but not an urgent problem that impelled soil scientists to travel to the Amazon jungles to solve. But it was an intriguing problem which was finally solved by an archaeological survey - a survey which involved soil geology.

For many years, it had been known that among the extremely weathered and infertile soils of the Amazon basin, some of the least-fertile soils on the planet, there are large, widespread patches of highly fertile soil. Soil that is not just fertile, but extremely fertile - so fertile and so valuable that for many years it has actually been mined and exported. What was the difference? The only visible difference was that the fertile soil is black, pitch-black, and grew just about anything with ease, and the infertile soils, just a few meters away, are a pale yellow color, and are so infertile that almost nothing except native weeds can grow in them.

Archaeologists had long noted that in the black soil, there were also abundant pottery fragments, apparently left behind by pre-columbian aboriginals. Not just a few, either, but in many places in the black soil, the pottery fragments were so numerous that they made up as much as ten percent of the soil volume - but the pottery fragments only occurred where the soil was black. The source of the pottery fragments was a considerable mystery to the archaeologists, as nothing was known about where they came from or how they came to be there in such abundance. So a few years ago, archaeologists undertook a survey of the black earth ("terra preta" as it is locally known) areas to determine just where the pottery came from and how many indigenous peoples must have been living in the area.

The results of the survey were startling indeed. The black earth areas, about twice the size of Great Britain, together had supported as many as three million people - more than had been believed to have ever inhabited the entire Western Hemisphere at any one time. They had realized that the black earth was fertile, but had never imagined that the Amazon basin could be so hugely productive.

The survey solved another mystery, too, this one a historical mystery. In the year 1541, the first European exploration of the Amazon river occurred when Francisco de Orellana, a Spanish conquistador, floated down the Amazon river from a tributary in the Peruvian Andes to the mouth in the Atlantic, a distance of more than five thousand miles. The privations of the journey, lack of food, prevalence of jungle diseases and the like took their toll, and the crew of Orellana's expedition died, almost to a man. By the time he returned to Spain to tell his tale, he alone remained. His epic history included stories of huge cities, with markets bulging with foods of every description, and of course, gold. Orellana's tales contributed greatly to the legend of El Dorado. But when the Spanish finally returned to the Amazon eighty years later, all they found was empty jungle, with a few scattered natives subsisting off of the rain forest. Oreallana's descriptions were dismissed as fantasy by generations of historians. Impossible, they said, couldn't have happened.

What we now know is that the Amazon did, indeed, support large communities of aboriginal peoples, which seem to have mysteriously vanished into the hunter-gatherer tribes seen by later European explorers. But we also know, from archaeological evidence, that they may (and probably did) survive into post-Columbian times, only to quickly disappear. And it was apparently the disease brought by Orellana and his men that caused the sudden decline, as European contact did in so many other places. When the cities were swept by disease, they were abandoned, and the jungle quickly reclaimed the sites, leaving nothing behind for explorers to find eighty years later. So, as we now know, the legend of the rich cities of the Amazon did have a basis in fact. And it was the terra preta, the black soil, which sustained that vast culture, that was the real gold of El Dorado.

All this got the curiosity of the archaeologists really going. What could possibly make the difference - why was the terra preta so fertile, when the soil around it was so sterile? They finally felt compelled to call in soil geologists to find out. And the discovery they made astounded them.

The soil scientists studied every possible aspect of the black soil - its minerology, its geological history, its chemistry and its physical structure, but what they discovered was truly amazing to them as well as to the archaeologists. The mineral content of the soil is identical to the sterile yellow clay - there was no geological or mineralogical difference. The only difference between the sterile yellow clay of the Amazon river basin and the incredibly rich and fertile terra preta of that region is the presence of finely divided charcoal powder in the terra preta.

Apparently, the indigenous farmers of the region had taken to carbonizing their farm waste, grinding the charcoal to a fine powder, and adding it to the soil. The richest soil samples, those with the greatest fertility, were between twenty and forty percent charcoal by volume, and the charcoal was powdered to a fine powder - a few hundred microns was the average particle size. There are few bits of charcoal any larger than a quarter of an inch in size. The charcoal was produced in a low-temperature process, not heating it too excessively. It contained within its molecular structure, plant resins that were heat stabilized.

Because nobody had ever bothered to investigate powdered charcoal's effects on soil fertility carefully, soil scientists had simply always assumed that charcoal when added to the soil, was inert and its effects primarily mechanical. Chemically, it is very stable at ambient temperature - even on geological time scales - and does not participate in chemical reactions, so it was simply assumed that any nutrients it trapped were simply unavailable to plants. Close investigation of the terra preta situation proved this to not be the case. Not at all.

What the soil scientists, working with microbiologists, discovered was that a community of bacteria exists in symbiosis with the root hairs of plants. The bacteria produce enzymes that release the mineral ions trapped by the heat stabilized plant resins in the charcoal and make it available to the root hairs of the plant as nutrients. In return, the plants secrete nourishment for the bacteria. Not only that, but the resins within the charcoal act like an ion exchange resin, adsorbing traces of mineral ions onto the charcoal particle surfaces from the rain water, and trapping it within the charcoal's molecular structure, where it can be held for centuries - until the soil bacteria associated with a root hair come along and secrete the enzymes necessary for it to be released once again. So the trace minerals always present in rainwater actually act as a fertilizer - providing the nutrients needed by the crops, year after year. The secret of the soil fertility of the terra preta was finally understood. And it was understood how the indigenous farmers were able to produce bumper crops year after year, decade after decade without a single application of chemical fertilizer and without wearing out the soil.

This was confirmed when the soil scientists grew some test plots. The results were seen recently on a Discovery Channel special about this Amazonian mystery. Viewers saw three plots - the first, the control plot, was natural Amazon yellow soil from which the native vegetation had been removed. The second was identical to the first, except that chemical fertilizer was added. And the third was a plot identical to the first, but to which charcoal was added along with a normal dose of chemical fertilizer. The results were dramatic. On the entire control plot, there grew only a single plant, pathetically stunted, which did not flower. On the fertilized plot, there was a small growth of stunted plants, few having produced seed heads - clearly what could only be described as a failed crop. The charcoal plot was dramatic - lush growth with an abundant crop of seed heads - a bumper crop indeed.

This discovery also solves a mystery that has puzzled farmers in tropical regions for years. It has long been known that growing sugar cane increases soil fertility. Over the years, soil in which sugar cane has been grown can become quite fertile - the opposite of what happens with nearly all other crops, which tend to exhaust soil. We now know the reason why - sugar cane fields are normally set alight before harvesting. The flames sweep through the field, burning off the thicket of leaves and leaving only the cane behind, making it much easier to harvest. What is left behind also includes a small amount of charcoal, which finds its way into the soil, gradually adding to its fertility, year after year. Where I live in Costa Rica, sugar cane, which is a low-value crop, is often grown simply to keep the farm alive and sustain the soil, while the farmer tries to find an alternative use for his land. It is a sad situation, but now there is an alternative. It is to make the land economically productive once again, by doing deliberately what the cultivation of sugar cane does accidentally.

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