The past decade has seen growing enthusiasm for an agricultural practice that involves burying charcoal—at first glance a seemingly odd thing to do. But rather than everyday barbecue briquets, this method employs a very special fine-grained charcoal, produced at very high temperatures under low-oxygen conditions. Proponents have designated it “biochar.”
The International Biochar Initiative, the industry’s highest-profile trade group, describes the product'seffects this way: “This 2,000 year-old practice converts agricultural waste into a soil enhancer that can hold carbon, boost food security and discourage deforestation. The process creates a fine-grained, highly porous charcoal that helps soils retain nutrients and water.” Furthermore, IBI notes, “The carbon in biochar resists degradation and can hold carbon in soils for hundreds to thousands of years,” curbing greenhouse-gas accumulation in the atmosphere.
Is it possible that high-tech charcoal could resolve what are arguably humanity’s two biggest challenges, by helping produce enough food while protecting the Earth from being grilled by global warming?
My Land Institute colleague Tim Crews, whose ecology research focuses on soil nutrient cycles, says biochar has two primary agricultural benefits: improving soil “tilth” (its physical condition, which affects plant growth) and increasing the soil’s capacity to retain nutrients and make them available to plant roots. “Biochar is not,” he stresses, “a significant source of nutrients itself.” If nitrogen, or phosphorus, or other essential elements are deficient in a soil, incorporating bichar into the soil won’t add enough of those nutrients to make a difference. But for certain types of soils that don’t hold onto nutrients or water very well, biochar can help.
Addition of biochar improves soil tilth and nutrient-holding capacity, according to Crews, only when it's in the technique’s “original context.” The practice was first used millennia ago to improve heavily weathered tropical soils in the Amazon basin. “There,” says Crews, “years and years of slash-and-burn cropping cycles, with long periods for regrowth of natural vegetation between episodes of burning and tillage, added charcoal that persisted in the soil for a very long time. Not just any old kind of charcoal-making will result in such durability, so it must have been done with some insight; the burning must have been done with a smoldering fire in an oxygen-deprived situation, which is required to make ‘good’ biochar.” And it was done again and again and again. Over the centuries, that charred plant material was incorporated into the soil and has remained there.
In those very old, weathered, acidic, iron and aluminum-rich soils with often low organic matter—known as Ultisols and Oxisols —the addition of biochar brought significantly better crop growth, because the soils were better able to retain essential nutrients until the crops needed them. Those soil types are found throughout the tropics, but in the United States they are common only in parts of the Southeast. Most soils in temperate latitudes were rejuvenated in relatively recent geological history by glaciers “rototilling” the Earth. These soils retain nutrients very well without any amendments like biochar.
Crews has seen small, neighborhood-scale operations in the U.S. that can produce high-quality biochar, but they tend to be “elaborate and expensive.” And maybe unnecessary. “If you’re cropping on soil types other than Ultisols or Oxisols and you manage your organic matter (residues, manure, compost, etc.) well,” he advises, “you don’t need biochar. It won’t do anything for your fertility.”
Another situation in which biochar could provide benefits is on very sandy soils in arid climates, because of its ability to improve such soils’ water-holding capacity, reducing drought stress on plants. But Crews warns that there’s a hitch: “Making biochar requires large quantities of bulk plant material, and the biochar factory needs to be close to the source of that material; therefore, the product would have to be manufactured in or around highly productive lands or on vast areas of unproductive lands, and then be hauled long distances to the, arid environments where it’s to be applied.”
That raises a key concern about large-scale biochar use in any environment, not just the desert Southwest: where will all that plant matter for making the biochar come from? Proposals to rely only on biomass left over after harvest are unrealistic. Not only is the available quantity of crop residue per acre too small; to haul residues off of cropland to a biochar plant would be to further rob the soil of organic matter, while paying a price in energy and other resources as well. Instead, the feedstocks for any large-scale biochar operation would have to come from the same source as those for industrial biofuels: large stands of natural vegetation or human-managed plantations. Indeed, biochar to be made most efficiently on a large scale, biochar would be mostly a byproduct of biofuel production.
The quantities that would be required per acre of cropland are staggering. Some controlled experiments have found that achieving a detectable improvement in crop yield required applying 8 to 20 tons of biochar per acre. Typically, according to Crews, adding 20 tons to the top 8 inches of soil would result in a soil that is close to 3% biochar by volume. But some of the same studies that found yield increases with such heavy application found that after a few years, soil carbon was no higher in biochar-treated plots than in control plots.
Meanwhile back at the factory, depending on the method used, production of 20 tons of biochar reportedly means burning between 50 and 200 tons of plant material. Research conducted in the Amazon concluded that treating one acre of land with biochar would require clearcutting, charring, and hauling all the biomass from two acres of secondary forest, which then would take many years to regrow.
Writing for the German development organization Miserior and citing biochar researchers’ own data, Almuth Ernsting gives an example of what such requirements would mean if biochar use were expanded to a scale that would not only improve soils but also make a significant dent in atmospheric greenhouse gases:
… an article by leading biochar advocates published in Nature Communications claimed that “sustainable biochar” could offset 12% of annual greenhouse gas emissions, which would require large-scale extraction and charring of residues (including 90% of pig and poultry manure and 25% of cattle manure worldwide), as well as the conversion of 556 million hectares of grasslands and so-called ‘abandoned croplands’ to produce crops and trees for biochar production … 556 million hectares would be more than twenty times the area of land used to produce biofuels at present …
Most likely the negative ecological and social impacts would also be twenty times as great as the already serious damage caused by today’s biofuel production. Furthermore, notes Crews, the fuel burned to haul such huge quantities of bulky biomass from plantations to biochar factories, to process the biomass, and then load up the still-bulky biochar and haul it to distant crop fields would put a big dent in the advertised climate benefits. And even the remaining benefits would not be fully realized until the harvested biomass plantations had regrown to their former size, pulling their share of carbon dioxide out of the atmosphere—a process of many years if trees were used. And, as we’ve seen, the length of time that the buried carbon stays sequestered in the soil and out of the atmosphere is far less predictable than advocates claim.
The Amazonian peoples of 2000 years ago improved the properties of their soils with charcoal because they not only had the specific type of soil that could benefit, but they also had vast stretches of available land, biomass, and most importantly, time. As Crews says, it was important that “they didn’t push the process too hard.” Today, human civilization has far less land and biomass per person and far less time to wait. Temporary industrial fixes, from synthetic fertilizers to weedkillers to biochar, can help alleviate specific local problems for a time, but they can’t solve agriculture's root problem.