A black cloud of biochar applied to a sunny field looks as menacing as it is mysterious to soil scientists.
The soil amendment gets a lot of credit: improved water-holding capacity, nutrient boosts and even cleaner air.
But, researchers are still figuring out the secret of the fickle technique, which works differently in every field.
“We can run a demo and see differences, but a neighbor runs it and sees the opposite,” said Kurt Spokas, a USDA soil scientist with the Agricultural Research Service (ARS) Soil and Water Management Research Unit in St. Paul, Minn.
“We need to understand the mechanisms behind it.”
Chars in soil have influenced farming for thousands of years, from the Amazon River watershed to Great Plains prairie fires.
A new push to understand them started about 2007.
Biochar comes from a variety of products exposed to heat in a process called pyrolysis.
“We’ve looked primarily at ag waste — from veggie production, nut production with almond and walnut shell waste products, manure, hardwood pellets,” Spokas said.
The biomass is heated with limited oxygen, producing the char and trapping air-polluting gases. This relates to the most-recent area of biochar research — it can be carbon negative.
Spokas explained if the waste matter wasn’t used for char, it would degrade and convert back to carbon dioxide in the atmosphere.
“Processing through pyrolysis shortcuts that cycle,” he said. The carbon-rich biochar that results holds more of that element in soils.
ARS research also has found lab-tested, soil-emission levels of carbon dioxide and nitrous oxide greenhouse gases were suppressed when biochar was present at high levels (20, 40 or 60 percent). The soils had lower microbial production of carbon dioxide.
“Across various soils, it ranges from no impact all the way to 90 percent suppression of N2O (nitrous oxide) gas production,” Spokas said.
“The real mystery is figuring out how that’s working so we can further optimize and make it a more predictable process.”
David Laird, Iowa State University professor of agronomy, said fertility changes with biochar are equally unpredictable.
“Biochar is good for solving specific problems that may exist in specific soils,” Laird said.
“If it’s sandy with low-nutrient and water-holding capacity, biochar can be used to improve ability to hold water and nutrients. It helps increase soil pH, increase retention of phosphorous and other nutrients.”
Continuous corn fields that usually see a 20-bushel drop the second year also get a response from biochar, he said. Trials show yield increases of six to eight bushels.
Laird said he thinks the biochar absorbs toxicity from the previous year’s decomposing corn crop.
But, on productive soils, biochar doesn’t provide a boost.
“Here in Iowa we have some of the best soils in world,” Laird said. “It’s hard to improve on a good thing. We don’t see a lot of response on really good soils.”
Spokas said a quarter of all studies have shown a negative effect on yield. But, that hasn’t stopped ag businesses trying to capitalize on it.
“Right now on a typical Iowa farm, it’s just not cost effective,” Laird said. “It can be $500 to $1,000 a ton — way too high for typical farm application.”
Farms would have to see multiple benefits for applications to make economic sense, so researchers continue to work to understand it.
Spokas said the future of biochar lies in its water-holding capacity, and Laird said scientists are interested in using chars to capture nutrient runoff from animal feedlots or tile lines, then applying it to fields.
“There’s a lot of high-value char applications going on,” he said.