Biochar: Beyond the Basics

Biochar: Beyond the Basics

Is biochar a fertilizer? The answer, oddly, is no. by David Yarrow

In soil, biochar hardly breaks down and doesn’t weather or chemically react. Up to 50 percent of biomass carbon can be captured as char. In five years, char loses only 3 to 5 percent more mass, mostly tar and resin residues consumed by bacteria. Beyond five years, the line is nearly flat. So, nothing eats it.

Yet, documented science shows charred biomass – properly made and put to rest in soil – delivers several immediate benefits including: increased porosity, lower density and better tilth; improved soil fertility; higher CEC and AEC; greater nutrient density; faster nutrient cycling; better fertilizer efficiency; enlivened soil biology and diversity; sequesterization of carbon. Substantial data confirms char in soil also improves water filtration, reduces leaching and curbs volatile out-gases.

Here are some unusual, useful services biochar provides to soil: Carbon insulates electric charges in cells and soil. In basic electricity, I learned carbon is a resistor. Carbon’s four half-empty orbitals absorb electrons to retard their movement – a semi-conductor that impedes electric current flow in a circuit. In soil, carbon’s first service is to surround ions to isolate their strong electric charge. Tiny char fragments arrange themselves in clusters around ions – much like chelation or chlorophyll – 20 Carbons in 8 rings around a Magnesium ion to capture photons for photosynthesis.

On a larger scale, carbon surrounds soil particles to separate their strong electric charges. Char inserts itself between soil granules to insulate their electric attraction. This isolates electrons to curtail stickiness and counteract tight, dense structure. Soil can more easily open, soften, with looser tilth, texture, air flow, water penetration.


Char is a sponge to soak up water. Plants are composed of at least 75 percent water, and plant structures are mostly plumbing to move water around. In a microscope, plants look like bundles of pipes and tubes. Properly charred, these microscopic pores are preserved. Char is very light because it’s mostly empty; it’s full of holes, and increases hollow spaces in soil. Thus, char weighs 1/6 as much as the same volume of sand.

Char’s micropores are sponges to soak up water, holding up to six times its weight out of soil circulation. This captive capillary water is slowly released back to soil. Biochar boosts soil capacity to absorb and digest water and resist drying out.


Char attracts ions out of solution. This slight electric attraction is known as adsorption. Atoms don’t bond, but their electric polarity attracts them into intimate relations. Char, like humus, has broken rings and embedded mineral ions that are areas of electric charge. Char’s charges attract ions with opposite charge, pulls them out of solution, loosely held by char.

Char’s vast inner micropores have hundreds of times more adsorption area than particles like clay. Char added in modest amounts – 5 to 10 percent – sharply boosts CEC. Char’s huge ion adsorption capacity is ideal to filter water. In soils, ions aren’t pollutants, but nutrients – cations, anions, and larger biological molecules, such as starch and protein. Like a magnet, char attracts ions out of the soil solution to retain them securely within its micropores. Char becomes a storehouse of loosely-held nutrient ions, ready to exchange with microbe or root.

But char also has positive charge sites. Char not only collects cations, but also attracts anions – nitrogen and phosphorus. This gives char an Anion Exchange Capacity (AEC) to complement and augment its large CEC. Thus, char stores ions of both polarities. Terra preta’s unusual AEC capacity caught the attention of soil scientists. AEC assures high fertilizer efficiency, reduced nitrate leaching, curbs nitrous out-gas, makes phosphorus available at any soil pH.

Substrate: Habitat for Biology

Char is now ready for a final preparation: inoculation. Char fresh from a burner is bone dry and sterile, having been heated to 500+ degrees C. Fresh char is truly an inert ingredient. But char is empty spaces, filled with water and ions. Microbes quickly move into the vacancies to become residents. We don’t eat our houses; microbes don’t consume char. They live in empty tubes and cell holes, sharing water, storing food.

Char isn’t annual fertilizer, but non-consumable infrastructure. Char’s empty space is condominium housing for soil biology. Micropores are a residential refuge from predators roaming soil.So, in soil, char comes to life.

Some microbes – notably fungi – send threads through soil to search for nutrients. In untilled soil, these networks extend hundreds of feet moving nutrients around. Char gathers scavenged nutrients from these networks; the microbial equivalent to supermarkets and shopping malls. Proper preparation of char for soil includes intelligent inoculation with a full spectrum diversity of microbes. This can be simple as blending with compost, or elaborate methods to take full advantage of char’s properties and microbial diversity. Char can also be sprayed with compost tea, biodynamic preparation or EM culture. Depending on particle size, microbes can colonize char in days, or a week.

Question arise of compost versus char. Actually, the two processes complement each other. Compost cooks best with wet biomass, while char ‘cokes’ best with dry. Char needs compost to inoculate its empty micropores. Compost benefits from char’s super-stable matrix to house microbes, transfer cultures, and supply minerals and metabolites. When biomass decays, most carbon oxidises to return to air, coming full circle in the carbon cycle. In photosynthesis, solar energy caught by chlorophyll is harnessed to fix CO2 with water into sugar. This carbohydrate energy powers most of Earth biology.

Digestion releases this energy to microbes, which share it with soil biology and roots. Slowly, plant and animal residues revert to CO2, water and energy, feeding soil biology. In five years, even woody biomass is digested. Large logs left in living soil vanish in a decade or two.

Humus: Carbon Indigestion

But a small percent of biomass is indigestible. Depending on biomass, moisture, soil, temperature, microbial diversity, and more factors, 10 to 15 percent remains. Mostly, this residue is carbon. Lignins, fats, celluloses, waxes and chitins resist chemical and microbial breakdown, so a small fraction remains as dark, crumbly, soft, spongy humus.

Humus has  chemical and biological stability – amorphous dark matter left from microbial decay. Humus persists for decades, even a century, as a soil conditioner, nutrient reservoir and microbial habitat. Humus, like biochar, is huge, complex, mega-molecules – chains and rings – from 50 carbons, up to thousands. Like biochar, only 5 percent transforms lifeless rockdust into rich loam. It can both bind sand and granulate clay. Humic and fulvic acids are acid and alkali extracts of lighter, smaller, soluble fractions.

Humus science isn’t much older than biochar, but clearly the two types of recalcitrant carbon interact to build soil structure and supra-structure, supply microbial habitat and store food. While they share functions, they also specialize. Stable, healthy, functional soil benefits from a balance of both. […]

Community: Soil Food Webs

[…] Science calculates [char’s] soil life is 1,628 years. […] Char allows microbes to blossom into a full function soil food web. […] Soil microbes are among Earth’s most ancient organisms. They form complex communities with interactive, inter-dependent, functional cultures. A teaspoon of living soil has thousands of species – from simple bacteria, lacto-bacteria, up to fungi, protozoa, algae, nematodes, and more. Extreme diversity and broad specialty assure wide adaptation to changing soil and climate.Nitrogen Cycle Bacteria are a key concern to inoculate soil and char. […]

Nitrogen Cycle: Nitrate, Ammonia, Amino Acid

The nitrogen cycle begins by fixing N2 gas into NO3 nitrates. The entire N cycle is driven by bacteria. These least of all organisms regulate this key biological nutrient in soil.

We know Rhizobia, an N-fixing bacteria, each specialized in specific host, climate, enzyme, trace element, and ecology. Scientists believe terra preta has rich stores of N due to several strains of N-fixing bacteria living in char. Char not only adsorps N ions out of solution, its resident microbes sucks N2 out of air into solid substance. This opens additional enzyme pathways to fix more N, and to increase soil’s N-fixing potential. […]

Nitrogen forms oxides (NO, NO2) that out-gas from soil, and are 250 times more potent than CO2 as greenhouse gas. By similar adsorption and metabolism, char reduces nitrate leaching. […]

Symbiotic Synergy: Microbial Economy

[…] We now know 20 percent – a full fifth – of carbon fixed by plants leaks from roots into soil. The loss is no accident, but a deliberate deal for fungi, who accept solar sweetness in exchange for water, minerals and metabolites. Trade in a soil food web economy simplifies a plant’s search for soil’s solid substances. […] Microbes form networks to share energy, nutrients and information. Living soil is millions of individual organisms in intimate cooperative relationships for mutual benefit.

The whole is greater than the sum of its parts. Myriad cell metabolisms have functional unity that unleashes an astonishing synergy of symbiosis. When all elements are present in balanced ratios, and the soil community develops its full diversity and unity, then growth and health are optimized. Distress and disease vanish as plants reach full genetic potential. […]

Twenty-first century farmers are, first of all, soil stewards – caretakers of soil food web biology – this living tissue that is the skin of the land. Biological farmers bring true culture back to agriculture by learning to regenerate these intelligent communities to continue their billion year successful service in evolution: to create fertile soil and favorable atmosphere. It begins by restoring soil Carbon. […]

For over 30 years David Yarrow has taught and organized about building sustainable food systems in the northeast United States, how to create abundance and health from fully fertile soil, and how to use simple food and cooking to restore health. For more information visit and

For the full article, please see ACRES USA magazine, May 2011, Vol. 41, No.5. Reprinted with kind permission from ACRES magazine.