In the virtual gardening circle, the terms “biochar” and terra preta have been receiving a great deal of attention. It all started when archeologists in the Amazon discovered the remains of indigenous settlements whose agricultural lands where distinctly different from the surrounding soils. They called this rich, black fertile soil terra preta, which is very high in carbon and humus, a complex soil community of bacteria, fungi, protozoa and earthworms.
Researchers believe these early agriculturalists cleared their lands by making charcoal from dense tropical biomass and then returning it to the soil to improve their crops. Making charcoal is an ancient practice for cooking, heating and agriculture. Wood and other organic wastes can be burned in a kiln without the presence of oxygen, which turns the material into charcoal – unlike ashes, which are produced by a regular fire.
You can view many tutorials online about building your own kiln for home use. The Ministry of Agriculture here in Costa Rica has also promoted a home garden system using bio-char made from rice hulls, which is used to grow plants in a nutrient-rich water solution. Red clay soils can also be improved by adding biochar to garden beds along with minerals and compost. As the red clay soil becomes fertile, it changes to a rich, dark brown color as the humus is restored. That brown to black color of soil indicates how much carbon (black) is present in the humus. Charcoal can act as a matrix building material for humus.
Many advocates have promoted biochar as a “carbon negative” initiative, stating that it could save the climate and boost food production by storing carbon in the soil, but many of their calculations have now been challenged. Addressing climate change turns out to be not just about curbing the rise of CO2, but also about keeping oxygen levels up, which only green plants and phytoplankton can do. Making biochar also has its downside: it gives off lots of heat and dense smoke, which is high in creosote. Pulverized charcoal can also pose a health risk for your lungs, so be sure to use a protective mask when handling charcoal. And remember that biochar alone is not a magic bullet for soil fertility. Biochar needs compost and biological cultures of soil organisms to activate the production of humus.