Charcoal in Soil

Terra Preta

• Scientists 60 years ago found fertile black soil deep in the Amazon jungle and called it terra preta, which means black earth. Tropical rain forests have poor soils because the high temperatures and heat flush nutrients from the soil and break down organic matter. The areas of good soil were man-made: Pre-Columbian Indians filled holes with charcoal, pottery and organic matter. Over time these materials accumulated nutrients and made sustainable farming possible in the rain forest.

Black Gold Agriculture

• A five-year study at Delaware State University, which wrapped up in 2008, found that adding charcoal to the soil can increase its fertility for a long time. The researchers heated different sources of organic matter such as manure and wood to make a biochar material. When added to the soil, this material traps nutrients and makes it possible for farmers to use fewer chemical fertilizers. This study was based on the terra preta found in the Amazon jungle.

Amending Your Soil

• You can buy charcoal from the store, grind it up and add it to your soil. Mix the ground charcoal particles into the top 1 foot of soil, where most of your plant's roots will be. Add about 5 percent of charcoal by weight to the soil. If your soil has an average density of 1.82 tons per cubic yard, you could add 0.091 tons or 182 lbs. of charcoal.

Drawbacks of Using Charcoal

• A 10-year study, ending in 2008, by Swedish scientists David Wardle, Marie-Charlotte Nilsson and Olle Zackrisson found that adding charcoal to the soil increases certain microorganisms that feed on organic matter. The microbes released the digested material as carbon dioxide gas. This finding contradicts the claim that burying charcoal is a good way to rid the atmosphere of excess carbon and enrich the soil.

  This study was done in the boreal forests of Sweden. The findings are thus not conclusive for all soils. Adding charcoal and ashes to the soil in states such as Colorado, Nevada and Utah is not good because these soil are high in dissolved salts and calcium. Ash and charcoal increase the alkalinity of the soil, which raises the pH. It is hard for plants to extract the necessary amount of Iron nutrients from soils with high pH.