The rediscovery and ongoing experimentation with biochar, a charcoal specifically produced for use as a soil amendment and soilless mix component, is one of the most fascinating dimensions of contemporary horticulture.
Fueled in part by research on terra preta in the Amazon Basin, or ancient soils cultivated by pre-Columbian aboriginal populations, interest in this unique material is expanding exponentially worldwide. Academia, industry and individuals alike are investigating various methods of producing, processing and incorporating biochar into various soil ecologies and growing methods.
Properly manufactured biochar has a set of distinct and unusual properties believed to have a profound effect on soil dynamics, with benefits including increased nutrient availability and uptake; reduced watering and nutrient requirements; suppression of soil-borne disease; stimulation of systemic plant disease response; increased overall plant growth, vitality, health and yields; improved aeration, drainage, and porosity; improved soil flocculation and structure; prevention of nutrient leaching; stabilization of pH; and increased cation exchange and water-holding capacity.
Biochar can also decontaminate soil and is being studied and used for remediation efforts. Biochar will isolate and detoxify soil contaminants by absorbing heavy metals, allelopathic (plant-produced) and pathogenic (bacterial/fungal-produced) toxins, and chemicals such as pesticides and herbicides. This list is far from complete, and many of the more ardent and vocal advocates hail biochar as the ultimate horticultural, energy and climate panacea. Overall, biochar is best understood as a catalyst that continues over time to modulate and improve soil dynamics.
Biochar is being studied for its potential ability to sequester large amounts of CO2 gas, mitigating the effects of global warming by acting as an effective and long-term carbon sink. The scientific research is hardly 10 years old, and although there remains an adequate number of credible, peer-reviewed studies to validate the claims, some of the published literature shows conflicting results.
This ability to boost agricultural productivity without additional petrochemicals while ameliorating climate change has led to an increasingly impassioned debate. The pyrolitic process used to produce biochar can also be harnessed to provide cleaner energy, but aside from whether biochar might be the Achilles’ heel of global warming, it is scientifically verifiable and practically demonstrable that it does benefit overall plant health.
This multi-faceted material deserves increased focus and study. Without doubt, what is known about biochar pales significantly in comparison to what is not known; the science at this point has identified causative effects, but not necessarily the underlying modalities and mechanisms responsible.
This scientific deficit opens the door for opportunity and experimentation by the individual grower. Contemporary attempts to recreate terra preta soil, or engineer new versions, is known as terra preta nova. Use of biochar in hydroponic systems is a fledgling practice, virtually untouched by academia, but it is safe to assume many of the same results can be attained by using it in the grow substrate, mixing biochar powder in the reservoir, or using it as a foliar feed.
Pyrogenic, carbon-based agriculture is not a recent development—it has a long and storied history as a method used by indigenous cultures in many parts of the world including Africa, Australia, Europe and, most notably, by the pre-Columbian peoples in the Amazon Basin.
For decades, scientists and archeologists remained perplexed as to how these cultures could sustain such massive populations given the rapidly declining soil conditions immediately following removal of the native rainforests for swidden agriculture.
Scientists found patches of soil ranging from one to several hundred hectares in size and 7- to 10-ft. deep that exhibited exceptionally high fertility—some of the most fertile soil ever discovered on the planet. These areas stood in stark contrast to the nearly infertile reddish or yellowish clays left following removal of the rainforest.
After years of intensive research and analysis, scientists determined that these soils were intentionally cultivated by these pre-Columbian populations using waste agricultural biomass, fish and animal bones, pottery sherds, excrement and charcoal.
It was also determined that these peoples manufactured biochar specifically for amending soil, and did not simply include remnants from fire pits. They were practicing what we now know as organic, ecological or biological agriculture, far in advance of what their European counterparts were capable of at the time.
The dark earth of the Amazon exhibits near-mystical properties, including elevated levels of nutrients, minerals, active bacterial and fungal biomass, soil humus, humic and fulvic acids and elevated paramagnetic and cation-exchange capacity.
The terra preta also demonstrated the ability to regenerate and grow after mining and removal, and even the ability to support years of continuous crop cultivation without additional fertilizer inputs.
That the exceptionally high fertility remained despite 2,000 years of latent inactivity and exposure to unrelenting, torrential rains confounded scientists at the time. Ultimately, investigations into terra preta led to the identification of biochar as the principal change agent, although the research and debate continues.
Contemporary research has identified the properties that most likely make biochar so beneficial to soil ecology, though much remains to be learned. It is a light but solid material made up of 90% carbon, produced through a specific process known as pyrolysis, or the chemical decomposition of organic matter in an oxygen-deprived environment.
Organic matter such as dead plant refuse, weeds, leaves, manure, bones and wood chips provide the biomass from which the charcoal is made. In a biochar reactor or kiln, the organic matter is heated to anywhere between 450and 900°F, and due to the lack of adequate oxygen, the organic matter does not fully combust.
During the thermal process, numerous physical and chemical changes take place. The cellulose of the plant material becomes fossilized, capturing the carbon that would otherwise decompose and enter the atmosphere as carbon dioxide.
The process is not as complicated as it may sound—biochar can be made in the backyard, though it can be dangerous due to extreme heat and open flame. Plans for backyard or small-farm biochar reactors can be found online and are typically constructed from 50-gal. steel drums.
Research suggests that the feedstock, firing duration and temperature play a significant role in determining the resulting physical-chemical structure and composition. After firing, what was organic matter becomes a pitch-black, crystalline, lightweight and somewhat brittle material.
Under the electron microscope, charcoal reveals a complex lattice structure with extensive nano, meso and macroporous tunneling throughout, from which the numerous desirable benefits originate. It is this subsequent porosity and the accompanying surface area, which can range from 4,000 to 12,000 sq. ft. per 0.04 oz. of char, from which the sorptive capacities are derived.
This powerful action allows biochar to absorb and store copious amounts of nutrients, minerals, humic substances, plant root exudates, oxygen, water and other essential elements for microbial and plant nutrition. The carbon structure of the char is highly resistant to decomposition, providing favorable habitat that is readily colonialized by beneficial bacteria and mycorrhizae.
It is believed that biochar can last thousands of years in the soil with minimal erosion, decomposition or lability. Many of the profound benefits resulting from the inclusion of biochar in soil blends are directly related to the powerful, dynamic relationship it has with soil microbes.
Following pyrolysis, the char is cooled with water but remains sterile. Whether you have made your own, or purchased raw char, several steps are necessary to accentuate the many properties that make it an exceptional catalyst for the rhizosphere-plant continuum, or the microscopic area at which biology, soil particles and roots interact.
It is not advisable to use raw char without first inoculating it with beneficial biology (mycorrhizae and bacteria). Methods of inoculation include blending biochar with compost and letting it sit for three or four weeks, soaking it in properly brewed compost tea or mixing in commercially available inoculant powders, the latter of which is my preferred approach.
This is a critical step, known as loading or charging, as the biology acts as a buffer to the extreme sorptive capacity of the char. Beneficial bacteria and mycorrhizae will immediately colonize the porous carbon structure and begin digesting residual tars and resins leftover from pyrolysis. Use a carbohydrate such as black strap molasses and rock powders to supply ample food for the microflora. Minerals in the rock powders will be broken down from the oxides into a form available to plants.
Other beneficial materials that can be integrated into the char are limited only by imagination. The resulting designer biochar is a highly advanced material containing virtually every element both plants and microbes need. Think of it as a technologically advanced compost, or biological growing on steroids.
Biochar can be intimately tailored to address any specific growing need, or to create a complete, slow-release plant and microbe fertilization system packaged in an ideal delivery module. Growing with char, you’ll notice plant root tips also penetrate the carbon, likely gorging on the cache of nutrients, air and water while forming symbiotic relationships with their microbiological accomplices.
The most important aspect is verifying quality of material. As biochar is often made from refuse, it is essential the biochar you select is free from contaminants. Biochar is easily crushed or broken into various sizes, from 1-in. pieces down to powders, and all sizes are useful for indoor growing.
I favor the 3-mm size down to the powder simply for ease of use. Biochar can also be added to existing plants by grinding it into a powder and mixing it with water, and there is some anecdotal evidence that it is useful as a foliar in this form. It is better to add char conservatively, as even a small amount of powder can have a significant effect.
There are few standard recommendations for quantity when blending soilless media, as the bulk of the research is centered on outdoor agricultural environments. In reviewing the available literature, biochar may constitute up to 20% of the total growing media, although in my personal experience, the benefits can be realized at far lower rates, even at 1%.
In addition, I have experimented with adding 5 to 10% topsoil or clay to indoor grow medias, thereby more accurately replicating what would be found in natural environments. The pH of biochar varies depending on feedstock, but typically has a pH of between 8 and 9, and has a stabilizing effect on soil pH.
Growers using conventional nutrients should use caution when fertilizing—biochar will absorb and store these salts. I recommend conventional growers cut back fertilizer use by 50 to 80% if using biochar in the soil media, which can save lots of money in the long run.
Biochar is a fascinating development in horticulture, with enough evidence to prove results, but also with limitless possibilities left to the individual indoor grower to experiment and develop. I recommend all growers try tailoring their own terra preta nova or designer biochar.