You've probably heard of compost – that thick chocolate-colored stuff that's an organic gardener's best friend and supplies plants with all kinds of succulent nutrients.
But what about biochar? It's another ancient farming material made from slow-burned wood (also known as charcoal) that holds nutrients and water into soil without them draining away. And lately it has enjoyed a certain revival because it can also pull and store the carbon in greenhouse gases from the air. Everyone from California grape growers to home gardeners on YouTube is trying it out.
Biogeochemist Caroline Masiello is pretty excited about biochar, especially what it means in an era when water resources are swiftly diminishing. "There's no question that there's been an explosion of interest in this," says Masiello. "And it's becoming clear that the water cycle benefits of biochar are at least as important, especially in drought-prone regions."
But there's a catch. As Masiello and her colleagues at Rice University write in the latest issue of the Journal of Biomass and Bioenergy, the temperature that you bake the wood chips or other plant matter that go into the biochar matters. They found that biochar baked at above 842 degrees Fahreheit (450 degrees Celsius) helped soil hold water and carbon better than biochar cooked at lower temperatures.
Scientists believe that biochar was first used by farmers in the Amazon Basin of South America to make terra preta, or fertile black earth, some 2,500 years ago. They buried wood in closed pits and let it smolder there for days. Nowadays, a lot of industrial-scale biochar is made in kilns.
Like compost, biochar takes some time and energy to make, but it can be a DIY project at home. The trick is to make sure no oxygen gets in while the wood is baking. Many people, like this small farmer John Rogers, build small kilns out of steel drums they seal off.
But Masiello says if you want to be sure get it right, you'll definitely need a thermometer.
In her humid hometown of Houston, she's tried to make biochar with magnolia leaves, which are abundant. But she recommends against it.
"A lot of plants around here have a waxy cuticle designed to repel water that will persist and create problems in the soil," she says. "If you're using wood, you're less likely to have problems. And the biochar will really improve the soil's capacity to hold water."
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