Dr. Paul's Research Perspectives

Research in the Amazon region suggests that ancient farmers dramatically improved poor soils through the addition of charcoal-based carbon

By Paul Hepperly

April 20, 2005: The yellow and red soils of the Amazon Basin are infamous for their low fertility and poor ability to provide the vital nutrients needed for optimal plant growth and development. In addition, they are notoriously leaky, rapidly shedding applied nutrients from fertilizers and/or manures. If this wasn’t bad enough, when they become acid these yellow and red soils make high amounts of aluminum and manganese available to plants, thus causing acute plant toxicities resulting in poor crop production and crop failures.

Despite these seemingly insurmountable deficiencies, pre-Columbian indigenous farmers in the Central Amazonian plains converted these infertile and incipiently toxic soils into fertile black terrain covering an area the size of modern-day France. Today, these relict black soils have high levels of organic matter and A horizons as deep as one to two meters—in contrast to surrounding yellow and red soils, with A horizons of just 10 to 20 centimeters. Amerindian black soils have important implications for agriculture’s attempt to feed exploding world populations and for mitigating the rise in atmospheric carbon dioxide levels.

Despite having been abandoned hundreds of years ago, these soils have retained their fertility and crop productivity. And this productivity holds up under renewed cultivation. In fact, Petersen and his collaborators (2001) have shown that the productivity of some of these black soils in the West Amazon region has been sustained over 40 years of continuous cultivation even without fertilization. This ability to retain plant nutrients and release them slowly has researchers from the US, Germany, Japan, Brazil and elsewhere scurrying to unearth the mechanism behind this black soil conversion. Their results clearly show that carbon is the key.

Testing of these tropical black soils suggests that their high carbon content comes from charcoal. To test this theory, researchers artificially amended yellow and red soils with charcoal and found that plant productivity and nutrition were drastically improved. Lehman and co-workers (2003) showed that cowpea shoot and root growth and development were doubled in a yellow soil to which 20 percent charcoal had been added. In addition, phosphorus, potassium, calcium, zinc, and copper were doubled and toxic manganese levels halved.

These results fly in the face of prevailing agri-chemical theories which hold that simple nutrient salts alone are the key to plant production and nutrition. Evidently, the carbon content of these soils plays an important role in their fertility. Practitioners of organic agriculture have long argued that soil organic matter and carbon content is critical to plant health. These findings point once again to the wisdom of nature and biology over industrial chemistry.

We look forward to seeing advances in natural technologies, such as composting, that will support real, long-term improvements in soil and plant productivity.

References

Lehmann, J., J Pereira da Silva, C. Steiner, T. Nehls, W. Zech and B. Glaser. 2003. "Nutrient availability and leaching in an archaelogical Anthrosol and Ferralsol of the Central Amazon basin: fertilizer, manure and charcoal amendments." Plant and Soil 249: 343-357.

Petersen, J. B., E. Neves, and M. Heckenberger. 2001. "Gift from the past: Terra Preta and prehistoric Ameriindian occupation in Amazonia." In Unknown Amazonia, C. McEwan, ed., pp. 86-105. London: British Museum.