Thousands of years ago, the Indians of the Amazon River Basin improved their unproductive tropical soils by mixing in charcoal made from smoldering wood. Ancient terra preta — or “black soil” in Portuguese — remains fertile today, rich in nutrients and organic matter.
Could biochar, which is similar to charcoal, provide a fertility boost for crops grown in Minnesota soils? That question prompted recent AURI-sponsored research by the University of Minnesota.
As Minnesota’s biofuel industry expands, there will be a growing supply of biomass ashes, including biochar, a fine-grained charcoal high in carbon and potassium, says AURI coproducts scientist Al Doering. “Utilizing these coproducts is critical to the future success, profitability and sustainability of the biofuel industry.”
In 2008, soil scientist Jeffrey Vetsch of the U of M Southern Research and Outreach Center, tested three types of biofuel ashes as corn fertilizer in replicated trials at Waseca. The ash sources included combusted turkey manure, combusted distillers solubles, and biochar from gasified wood and corn cobs.
Both combustion ashes proved to be good sources of plant-available phosphorus and potassium and produced corn yields comparable to commercial fertilizer, Vetsch found. However, “no measurable agronomic benefit to the char ash was found in this study.”
There could be several reasons for that, Vetsch says. Biochar resists decomposition by soil microbes, so the nutrients in the ash are not readily available to plants. On Minnesota’s highly-productive clay loam and glacial till soils, where organic matter levels typically reach 3, 4, 5, even 6 percent, biochar “may actually immobilize essential nutrients, especially nitrogen and phosphorus,” Vetsch says.
By contrast, biochar benefits “have been found on highly-weathered, low-organic-matter soils,” he says, where biochar may raise fertility, boost long-term organic matter content and prevent nutrient leaching.
Biochar might have agronomic benefits on certain sandy soils in Minnesota. But Vetsch says it is probably best suited to severely depleted, acidic soils with little humus, “which we don’t have in Minnesota. The closest place in the U.S. with those soils is the mid South.”
Carbon ‘sink’ potential
Biochar’s greater potential in Minnesota is for storing carbon in the soil, Vetsch says.
In fact, biochar is attracting interest these days as a tool to combat climate change, Doering says. “It’s one of the best ways to capture and sequester carbon.” Through plant photosynthesis and pyrolysis, carbon dioxide — a potent greenhouse gas — could be removed from the atmosphere and stored in the ground as solid carbon. And because biochar doesn’t easily decompose, carbon could be safely locked up for decades, he adds.
Injecting biochar into the soil could make biofuel production carbon neutral or even carbon negative, Doering says. But that depends on policies or economic incentives, such as carbon credits, that “would put a value on carbon sequestration,” he says. “If there is ever a value put on carbon sequestration, this use would be a big one.”