JARS v63n4 - Soil Carbonization and Its Implications

Soil Carbonization and Its Implications
Scott Bidstrup
Tobosí, Costa Rica
Reprinted, with modification, from the Nov/Dec 2008 Bulletin of the BC Council of Garden Clubs. Copyright 2009 by Scott Bidstrup. Used by permission.

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 that was finally solved by an archaeological survey - a survey that 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 as potting soil for gardeners. 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 colour, and are so infertile that almost nothing except native weeds can grow in them.

Archaeologists entering the Amazon in the 1870s noted patches of dense forest surrounded by savanna scrublands and noted that where the forest patches grew, the soil was black and rich, and there were also abundant pottery fragments, apparently left behind by pre-Columbian aboriginals. But outside the forest patches, the earth was the yellow clay oxisol that is dominant in the Amazon basin. They discovered that there were not just a few such places either, but rather there were many such places. In the black soil, the pottery fragments were often 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 do indios ," "black earth of the Indians," or simply " terra preta ," in Portuguese) 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, possibly as large as France 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.

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 oxisol clay in the surrounding areas - 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. Terra preta soils are of pre-Columbian nature and were created by humans between 450 BC and AD 950 (http://en.wikipedia. org/wiki/Terra_preta). The richest soil samples, those with the greatest fertility, were between nine and 40% charcoal by volume, and the charcoal was powdered to a fine powder - a few hundred microns was the average particle size. There were few bits of charcoal larger than 8 mm (1/4 inch in size). The charcoal was produced in a low-temperature process, i.e., was not heating too excessively, and contained within its molecular structure plant resins that had been heat stabilized by the pyrolization process.

Because nobody until recently 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.

What the soil scientists, working with microbiologists, discovered was that a community of organisms exists in symbiosis with the root hairs of plants in terra preta soils. Bacteria and fungi (mycoorganisms) live and die within the porous media, thus increasing its carbon content. Until recently there was no scientific evidence for a particular micro-organism to be responsible for the formation of terra preta , but a significant production of biological black carbon is produced by the fungus Aspergillus niger (Glaser et al. 2001), especially under moist tropical conditions (http://en.wikipedia.org/wiki/Terra_preta#cite_ref-glaser07_23-0). In addition, Ponge et al. (2006) show that the peregrine earthworm Pontoscolex corethrurus , widespread in all Amazonia and notably in clearings after burning processes thanks to its high tolerance of a low content of organic matter in the soil, ingests pieces of charcoal and mixes them in a finely ground form with the mineral soil. The authors, who experimentally verified this process, point at this as an essential element in the generation of terra preta soils, associated with agronomic knowledge involving layering the charcoal in thin regular layers favourable to its burying by P. corethrurus . The microorganism community then produces enzymes that release the mineral ions trapped by the heat stabilized plant resins in the charcoal, making it available to the root hairs of the plant as nutrients. In return, the plants secrete nourishment for the micro-organisms.

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 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 yellow clay 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.

Carbonization of farm waste has traditionally not been practiced, because it has traditionally been slow, inefficient, messy, labour intensive and produces a product for which, until now, there has never been enough use to justify all the trouble - and it has always been assumed to have limited value as a soil amendment. It has always been easier to just plough farm waste under, and allow it to rot. This is fine, but it produces only modest improvements in soil fertility, and they are largely temporary.

Implications Today
If practiced on a truly large scale, the carbonization of soil in the tropics and subtropics can help reduce and possibly reverse the problem of global warming. To understand how this is possible, one needs to understand the biospheric carbon cycle. The carbon cycle begins with atmospheric carbon dioxide, the greenhouse gas we all know and are growing to hate. It is a natural part of the atmosphere and prior to the wide-scale use of fossil fuel in industry, the concentration of carbon dioxide in the atmosphere was roughly 260 parts per million by volume. Digging up carbon-based fuels and burning them, adds to atmospheric carbon dioxide, since carbon dioxide and water vapour are the primary combustion products of burning fossil fuels. A century and a half of burning (oxidizing) fossil fuels has raised the concentration of carbon dioxide in our atmosphere to date to roughly 360 parts per million. We are burning fossil fuels at such a rate this value is increasing at roughly 10 parts per million per year. This does not sound like a lot, but carbon dioxide is extremely efficient at trapping heat in the atmosphere, and that is why its influence is all out of proportion to its concentration, and why increasing its concentration even a little bit matters a whole lot.

Plants breathe in carbon dioxide and through photosynthesis, combine it with water and turn it into the carbohydrates that make up the plant -the sugars, cellulose, lignin and other materials. When the plant dies, this carbon, which was sequestered (locked up) temporarily in the plant's tissue, is oxidized by decay organisms and is released back into the atmosphere in the form of carbon dioxide. The cycle is complete - the carbon dioxide becomes available for another plant to absorb. The theory of planting trees to slow global warming is that trees sequester this carbon in their plant tissues - wood - for centuries. But of course, they eventually will all die and rot away, and so the carbon dioxide they are supposed to sequester will eventually be released back into the atmosphere. Other schemes have been suggested for sequestering carbon dioxide - usually by liquifying and dumping it on the ocean floor (with unknown and unpredictable consequences for ocean ecology) or pumping it into abandoned natural-gas wells (again with unknown and unpredictable ecological consequences).

Charcoal consists almost entirely of carbon, making up between about 70 and 98% of it by weight. Charcoal is chemically stable - it does not react readily with either water or atmospheric oxygen at ordinary temperatures, so once it is well mixed into the soil, it will remain there for geological time scales. Indeed, scientists routinely find charcoal from forest fires in sediments laid down during the age of the dinosaurs and before. So once added to soil, carbon in the form of charcoal will remain there until it is physically separated out or oxidized in the interior of the earth. Charcoal is also easy to make - simply heat up any solid biological product to a temperature of about 243° C (470° F), and volatile organic compounds come off as gases, leaving behind charcoal. The carbon atoms simply reattach themselves to each other, and what we have left is elemental carbon - in the form of charcoal. If farmers knew what it could do for them, they would have a major financial incentive to create charcoal from their farm wastes and add it to their soil, sequestering it not for decades or even centuries, but effectively permanently. How could a bunch of today’s farmers, carbonizing their farm waste and ploughing the charcoal under, possibly have this much of an effect? The answer is in numbers. Millions of farmers, all doing this together, could have an enormous impact, bringing atmospheric carbon dioxide potentially right back to its pre-industrial levels.

As it turns out, we need not rely just on tropical farmers, who stand to benefit the most from this soil carbonization. Recent work by Cornell University scientists has shown that agriculture in temperate and even subarctic regions can also benefit from soil carbonization. In the temperate agricultural zones, soil carbonization can greatly assist in improving soil texture as well as considerably improving a soil's fertilizer absorption and retention - making application of fertilizers far less frequently necessary. Since this is one of the temperate-zone farmer's largest costs, it can be of great benefit to a temperate-zone farmer seeking to improve his profitability. Since temperate-zone farmers can benefit too, if even a third of the world's farmers were to carbonize their soils, the problem of anthropogenic global warming would be greatly reduced. Finally, a few people are beginning to recognize this in the academic community, and new proposals are coming out to include soil carbonization as a scheme to be subsidized under the next version of the Kyoto Protocol.

As an addendum, it has recently been discovered that permanent soil fertility enhancement by using charcoal amendments alone in temperate-zone soils is achieved only through good soil management. The dramatic and permanent improvements seen in terra preta is probably achieved only by colonization of the soil by highly specialized micro-organisms, and this may take decades or centuries to achieve in temperate zone areas. But even in the absence of this terra preta effect, the immediate improvement in the ability of the soil to absorb and retain fertilizers is more than worth the trouble of adding the char. And someday, after decades of good soil management, the temperate-zone farmer may be able to wean himself off of fertilizer altogether as his soil becomes true terra preta .

Glaser, B. L. Haumaier. G. Guggenberger, and W. Zech. 2001. The 'Terra Preta' phenomenon: a model for sustainable agriculture in the humid tropics. Naturwissenschaften 88: 37–41
Ponge, J.F., S. Topoliantz, S. Ballof, J-P. Rossi, P. Lavelle, J-M. Betsch and P. Gaucher 2006. Ingestion of charcoal by the Amazonian earthworm Pontoscolex corethrurus : a potential for tropical soil fertility. Soil Biology & Biochem . 38: 2008–2009.

Editor’s Observations
The obscure discovery made by archaeologists and soil geologists working in the Amazon basin a few years ago is being claimed by some to have potentially huge repercussions - that it might save millions of farmers in the tropics and subtropics from lives of poverty and destitution, and that learning from this discovery by aboriginals many centuries ago could help the world today from some of the follies of modern man. According to a research team under the direction of Christoph Steiner of the Bayreuth University, average poor tropic soils are easily enrichable to terra preta nova by the addition of crumbled charcoal and condensed smoke. Efforts to recreate these soils are being undertaken by companies such as Biochar Energy Corporation, Eprida and Best Energies. Research efforts are underway at Cornell University, the University of Georgia, Universität Bayreuth, University of Florida, Iowa State University and Geoecology Energy Organisation. Biochar, or agrichar (new terms being used for this soil carbon), is the main (and likely key) ingredient in the formation of terra preta . One focus of these researchers is the prospect that if biochar becomes widely used for soil improvement, it will involve globally significant amounts of carbon sequestration, remediating global warming.
Looking at this topic on the internet, I find that there is some debate on this, not that biochar won’t help plant growth, but that the process of creating the biochar itself may have some negative environmental effects. Regardless, its an interesting idea that is worthy of further exploration and evaluation.
So what has this to do with rhododendrons? Well, it sounds like an interesting idea for some of us to try with our temperate rhododendrons grown in soil, as it might minimize or reduce the need for chemical fertilizers while enhancing growth. For those of us that grow vireyas in a soil-less medium (peat, coir, perlite, and bark mixtures), we could perhaps note that orchid growers have for many years often added charcoal to their culture media, or even used it exclusively, as with vandas in hanging baskets. Adding ground carbon may thus also improve the growth of vireyas in soil-less media too, and I at least for one am planning to try it. Stay tuned!

Scott Bidstrup