Biochar is made when biomass is burned in the absence of oxygen, a process called pyrolysis. The substance was first discovered in the Amazon, and it is believed that agricultural groups there used it to fertilize the rainforest's nutrient-poor soil, somewhere between 2,500 and 6,000 years ago. Biochar is a great fertilizer, because it contains high levels of nutrients vital for plant growth, like nitrogen, phosphate, and calcium. It is also highly porous, which helps soil retain water, and provides a nice environment for various microbes that are beneficial for plant growth. And it locks carbon dioxide away, possibly for thousands of years. Any biomass can yield biochar, including wood, as well as agricultural waste like hulls and stems. The pyrolysis process also yields hydrocarbons, which can be made into fuel.