How Biochar Works in Soil
Thus, char made from plant biomass is hollow inside. Charcoal is lightweight because it’s empty. It floats because air is trapped in those tiny tubes and pipes. Those bundles of emptiness are like a sponge, and give char its huge capacity to soak up water and other substances - even ions and electrons. And even as char itself is empty, open and porous inside, it encourages soil to become lighter, looser, more open to air and water.
A bit of char has limited external surface area, but this is tiny compared to char’s vastly greater inner surfaces. The walls of all the inner chambers are many times greater than outer surfaces. The added internal capacity of char is estimated between a few thousand, to maybe a million, times more than external surface.
The combination of micropore sponge with soil microbiology in active symbiosis with roots enhances drought tolerance of plants, from annual crops to trees. In 2011, the US Forest Service released preliminary results of adding char to soils after forest fires in the West. Clearly, soils with char stay wetter, so soil biology, including shrubs and trees, regenerate faster.
Why Carbon is Black
Carbon’s valence electrons make four covalent bonds in tetrahedral symmetry, Nature’s simplest 3-dimension geometry. This 4-arm arrangement attracts and holds many kinds of energy and chemistry, from photons of light, to neutrons in nuclear reaction. Carbon’s perfect symmetry captures photons, atoms, ions, and electrons - even microwaves. Thus, Carbon is black, the color of complete absorption and zero radiation.
“Adsorption” is a technical term for how this slight electric attraction between atoms and molecules causes char to soak up and hold ions. Char’s “absorption” of water operates largely by capillary action from differential pressures. But “adsorption” occurs due to electric charges that cause atoms and molecules to be attracted, form clusters and align in groups.
Charcoal has abundant charged sites that give it tremendous capacity to pull molecules out of solution, and hold onto atoms on—and even in—the char. Charcoal’s very high adsorption potential is what makes it an ideal media for water filtration. In water purification, ions in solution are considered “pollutants.” But in soil, the important ions are “nutrients.” Biochar is a sponge that also soaks up and holds ions of elements and biomolecules.
Adsorped charges can also stack up in thin layers, and form rudimentary thin films—a special structural strategy of cells and microbes. Thin films allow nutrients and electrons to move around a cell in remarkable ways that are orderly and efficient.
Like most soil particles, char has negative charge sites that attract and adsorp positive ions—the cations. Adding char to almost any soil will boost Cation Exchange Capacity (CEC). Since metal cations are soil’s primary electron donors and charge sources, CEC is a valuable numerical measure of soil’s potential energy to support and sustain growth. The higher the CEC, the faster and better plants are likely to grow. Typically, biochar added to clay or sandy soils sees CEC rise by ten, twenty or more points.
After char was understood as a useful soil additive, scientists began a global search to re-examine soils. Ecologists discovered ancient prairie soils had significant fractions of charred Carbon—something no one had studied before. Significant charred Carbon is created by slow-moving, low temperature prairie fires, especially fires with green, moist biomass. Centuries of fires accumulated charred Carbon in soils, and improved those soils fertility.
July 2012, American Chemical Society’s Environmental Science and Technology journal published a paper by six scientists’ on the Midwest’s highly productive, grassland soils (Mollisols). These rich, dark soils were found to contain char from pre-settlement fires (40—50% organic C). Structurally, these Mollisols compare to Amazonian Terra Preta. Such soils are much more abundant than previously thought. Char residues are highly stable, abundant, fertility-enhancing forms of Soil Organic Matter. Further, if just 40% of SOC is char (low estimate for Mollisols), essentially the entire soil CEC is attributed to the char.
What was good for Amazon rainforest soils found a natural use in North American prairies.
But char isn’t just Carbon. Held in the Carbons are minerals. If you burn charcoal, the final product is whitish ash—mostly metal oxide minerals. In char, these mineral ions are held in the biocarbon matrix, and create embedded sites in the char with electric charge. Char also can have a few Oxygen & Nitrogen atoms that create other electric charges in the biocarbon matrix.
The embedded electric charges attract atoms, ions and molecules of opposite polarity. This electric charge isn’t strong enough to form bonds where atoms exchange or share electrons, but this charge is able to draw and hold atoms close to the char surfaces. This loose association is based on a subtle electro-static attraction, and is like a "hydrogen bond" in water. Nature makes many uses of these subtle electric fields and inter-atom attractions.
Biochar has an added adsorption capacity, far beyond other soil particles like sand and clay. Like any particle, a bit of biochar has a fixed, measurable external surface area to attract and adsorp ions. But biochar’s limited external surface is augmented by complex internal surfaces of its micropores. Those hollow, inner chambers provide a far greater ion adsorption capacity than almost all other natural materials. Calculations vary, as do biomass characteristics, but biochar easily has a few thousand to over a million times more internal than external surfaces.
Char draws ions to itself, separates them from solution, and holds them tightly at char surfaces by weak electric attraction, not fixed atom-to-atom bond. This loose association is easy to affect or alter, so adsorped ions can become mobile, or jump around, such as from char to root, or microbial membrane. This easy access ion exchange makes adsorped nutrients bioavailable.
Fertilizer Efficiency & Water Quality
However, biochar has a further feature to dramatically boost its ion adsorption properties. Amazon studies of Terra Preta reveal that char also has positive charges embedded in the biocarbon matrix. This positive polarity attracts negative ions and molecules: the anions. These atoms are electron acceptors, and are charge carriers in cells to move energy around and deliver electric power to metabolic reaction sites. So, unlike most soil particles, char has Anion Exchange Capacity (AEC). The two most critical soil anions are Nitrogen and Phosphorus—N & P of NPK fertilizers.
Anion adsorption is a potent tool in soil fertility. By gathering and holding anions out of the soil solution, biochar immediately curbs leaching and loss of these nutrients. Instead of washing out with rain or irrigation, Nitrogen, Phosphorus, and other anions are held on and in the bits of char. On the supply side, this holds critical nutrients in the root zone, and delivers more fertilizer to the plants, sharply increasing overall useful fertilizer efficiency.
Research in Japan, Australia, America, and Germany has documented that biochar added to soils curtails outgassing of greenhouse gases by 37 to 90 percent. Biochar seems to alter microbial activity in soil, reducing soil respiration (CO2) and conversion of Nitrogen fertilizer to nitrous oxide (NOx) that otherwise goes into the air. Nitrous oxides have a greenhouse gas effect over 300 times that of CO2. This is partly biochar’s adsorption power and capacity, but also microbial activity to manage the Nitrogen Cycle and keep these volatile molecules in soil. Methane emissions are also reduced.
The ability of biochar to curtail leaching, outgassing and pollution, and improve fertilizer effectiveness occurs at two levels. First is the physical chemistry of adsorption—char’s immense capacity to capture and hold nutrient ions. The second level is biological, involving soil microbes that process nutrients into biomolecules and protoplasm.
Multiple research studies in Japan, Australia, US, and other countries found that biochar added to soil cuts nitrate migration out of the root zone into groundwater by 50 to 80 percent. Since nitrate and phosphate from farm fertilizers is a primary source of non-point water pollution, this facility of biochar has great significance for watershed management. Biochar’s adsorption power is also useful in stormwater purification and management.
Federal-funded research at University of Vermont is testing biochar use as a phosphorus trap on dairy farms. Phosphorus pollution is a recurring problem fouling water in the Great Lakes. Protecting Lake Champlain, the sixth Great Lake, from phosphorus pollution from farmland runoff is an environmental priority for two states and two nations. Early efforts at Shelburne Farms near Burlington VT documented over 50 percent reduction in phosphorus leaving the farm to enter the lake.