Gardening With Biochar

Michael Urban


The implementation of Biochar (or horticultural charcoal) as a soil amendment has stirred significant  interest in the scientific community and the news has spread to the home gardener.  But what’s all the fuss? You may be asking:  what is Biochar, where did it come from, what benefits does it provide and a host of other questions. 

This article will attempt to shed some light on these and other questions about Biochar.

Professional landscapers and the Garden Centers are a bit behind the curve on this but that is no reason there should be a limit the spread of knowledge about Biochar for home use. 

Biochar is an organic soil amendment – essentially, Biochar is a fancy term for charcoal destined for the soil and not the grill.  Mostly, it is produced for researchers – more recently for broader markets, which include gardens of homeowners as well as those of commercial interests such as municipalities, owners of commercial buildings and golf courses, because of its sustainable characteristics and soil amending properties.

Historical background

Sometime before the late fifteenth century, Native South Americans learned that the use of charcoal would improve the notoriously weak soils in the Amazon River valley. Although rainforest and jungle soils are commonly thought to be fertile, these soils, in fact, are naturally very weak and unable to sustain crops for more than a single harvest because the thin amount of nutrients vital for agriculture.  A local farmer finds that soil nutrients are quickly consumed by the plants and he is forced to move on to support his family or tribe.  It is from this practice that the term “Slash and Burn” comes.

Later, when archeologists observed that when ancient locals infused charcoal into the soils to improve it and this practice allowed large populations to remain on the same land to farm – over generations, a whole new term emerged.  We now call the soil created with charcoal terra preta (Portuguese for “dark earth”)   These Terra Preta soils of the Amazon River and its tributaries, have dramatically improved fertility.   Terra Preta or the “dark earths” are believed to be responsible for mythical city of El Dorado.  In the BBC film The Secret of El Dorado (it can be viewed on, a modern archaeologist tells the compelling tale of how terra preta enhanced the earth and enabled the creation of a large city dependant on agriculture to sustain it.



Professor Johannes Lehmann of Cornell University’s Department of Crop and Soil Science is leading the way educationally; having conducted extensive research on terra preta and its effects in soils. He is, perhaps, the leading expert on the effects of Biochar as a soil amendment and a means for carbon sequestration and as a potential means to reduce greenhouse gas emissions.  The term Biochar is a modern term scientists use to define charcoal used in agriculture.

Dr. Lehmann has created a video called “The Promise of Biochar”, which explains that while making charcoal is perhaps the oldest manufacturing process known to man, it has only recently been rediscovered as a means to sequester carbon and to dramatically amend poor soils. In the video Dr. Lehmann shares the research of one of his students, Christoph Steiner and the very dramatic results of his field trials in the Amazon. 

Dr. Lehmann has authored a textbook:   Biochar for Environmental Management: Science and Technology.  He and others in the field are using it to teach a generation of soil scientists.   Today, all over the world many other individuals and agricultural institutions are conducting field trials, research and studies on the efficacy of Biochar as a soil amendment.


What we’ve learned is that charcoal has two significant properties:  permanence and porosity.  It’s permanence means it does not break down or degrade over time.  It doesn’t change.  Soil amended with Biochar stays amended.  No other organic soil amendment has this property.  And these small, permanent particles of Carbon wedge their way into tight clay and create airways and pathways for root development, worms and other organisms that otherwise would be absent.


When we say it is porous:  it means that under a microscope, charcoal is filled with microscopic pores.  These pores enable the charcoal to absorb not only moisture, but equally significantly – nutrients; especially NPK (nitrogen, phosphorous and potassium) vital for soil fertility. 


The benefit of porosity shows because in normal fertilizer or compost/peat/manure applications, many of the available nutrients quickly leach out or wash away, producing run-off and ground water pollution, and the need for repeated applications of fertilizer.  With Biochar present, however, the nutrients are retained – thus reducing run-off and the need for repeated fertilization.


The pores retain moisture as well.  In arid areas of the country, porosity means that moisture will be retained and will be available to nourish plants long after adjacent, untreated soils have dried out.


The microscopic pores in the carbon additionally provide a haven for microbial activity – thus enhancing the fertility of the soil.  Given the relative permanence of the carbon – its ability to remain in the soil over time (case in point: the pre-Columbian terra preta soils are still present in Amazonia) – this change in the fertility increases over time with subsequent applications and permanently enhances it.

While our understanding of Biochar is still evolving, one thing for certain:  because the carbon of Biochar is permanent when put in the soil, the soil becomes a permanent Carbon repository – reducing the potential of greenhouse gases.  Thus, while even in limited home or commercial use, Biochar in a small but real way provides all the benefits described above and justifiably earns the distinction as the most effective of all organic soil amendments.

Basic Q&A for the Gardener

“As a gardener, how do I put Biochar to work?”

·       If you live in an arid area, applying Biochar will help treated soils by retaining moisture due to its porous nature.

·       If you live in an area where soils are tight clay, then Biochar will lighten the soil, improving tithe - making soils workable with a trowel that normally in summer have the quality of cracked concrete.

·       The weaker the soils, the more dramatic the results – relative to enhancing soil fertility.

·       If you are interested in making a difference with a sustainable gardening practice that will demonstrate your good earth stewardship – gardening with Biochar will set you apart.  


Building the soil and improving it with Biochar is a sustainable undertaking and a long-term investment – compared with the short term beauty – the instant gratification - resulting from the addition of fertilizers and even compost alone that have temporary benefits.


Coastal soils are thin and have poor fertility because of their sandy texture, low pH, and low organic carbon content.  Biochar or horticultural charcoal can permanently reverse those characteristics as proven by Dr. Jeffrey Novak (USDA).  His success improving these soils with the use of Biochar is published.


The State of Florida is initiating an effort to encourage its residents help reduce chemical run-off, reduce the use of fertilizers and pesticides, reduce ground water exploitation.  Biochar can be part of a well-considered campaign by homeowners with the help of landscapers to support the Florida State government’s Florida Friendly initiative.


Learn about permaculture and the role Biochar can contribute to the creation of a permaculture experience.




How is Biochar made?

Pyrolysis (see web definition) is thermal reduction of bio mass in the absence of oxygen.
Bio mass can be a variety of plant materials including tree waste, corn stover, rice hulls, the shells of a variety of nuts.  It can also be produced from the manures of cows, chickens etc.  Ideally the pyrolysis process should occur between 450 and 600 degrees Celsius, because it is in that temperature range that the optimal soil amending characteristics are achieved.   Once the organic material has been reduced to char, but before it turns to ash, the process is stopped and Biochar results.




One ancient method utilized a wood pile covered with moist earth with a vent on top and a heat source on the bottom





This is the TLUD or top load updraft technique which can be replicated by the home gardener at a low cost




Modern production equipment such as these portable systems that enable medium to large scale commercial production.


Ideal production methods include the capture and recycling of all energy and emissions and preventing any greenhouse gases to escape.  Some have manipulated feedstocks, time and temperature, to produce "designer” or “boutique” chars with differing Ph levels and nutrient absorption characteristics that meet the needs of particular soil types.


More can be found on the production of Biochar by looking up Josiah Hunt (of University of Hawaii)’s blog online.



The cost of producing and shipping Biochar is high – and boutique Biochar’s cost is even higher, making the adoption of Biochar in the home garden somewhat challenging; and yet, the magic of it —the history of terra preta, its sustainability, and its soil amending properties – once understood can inspire trials.  Once the material has been adopted, even on a limited scale, and results observed, expanded use is very likely. 


In the short-term, is an early adopter, you will be demonstrating a unique insight into sustainable gardening by using Biochar and, potentially, to educate your friends and neighbors who might be stuck in a rut that requires continuous fertilizing while polluting ground water and adding greenhouse gases to the atmosphere. 

Many people feel a responsibility towards the earth, and the use of Biochar allows that desire to be fulfilled. As a long-term investment, once you get started, you may be able to persuade other homeowners as an eager audience to also foster the stewardship of a greener, healthier planet.

Because so few people know about Biochar, the opportunity from homeowners’ perspective to make a difference, is relatively limitless; thus, those who use Biochar are “early innovators of sustainable agriculture” at a local and very personal level.


Other Benefits:

Resistance to Plant pathogens, molds and mildew has been shown in roses, tomatoes and peppers. The mechanisms for these broad benefits are currently in great debate by soil scientists. What are not debatable are the fabulous results in temperate and tropical soils which is fueling this debate.
Biochar has also found great utility as a bulking agent in active compost processing. The absorption characteristics produce compost that retains much of the nitrogen that is normally lost to the air as the microbes do their composting work.
 Economical benefits over time

Biochar’s permanence enables one to apply the material over time. For example: on a lawn that has been aerated or thatched, or had a small amount of Biochar applied under sod could have it applied in subsequent applications over time to spread the cost.  The initial investment will not be wasted or lost because the material is essentially permanent.

Application Rates

Much of the research done today is aimed at determining best application rates for what plants in what soils and in what climates. Here are some tips in Biochar application:

  • As little as 5-10% Biochar in the native soils in blended in combination with other organic materials such as manure mixed into the prospective root zone of the plant.   In a planting mix, incorporate 15 – 25% Biochar with peat, compost, humus, etc.
  • Biochar works best if it is pre-treated and not put raw in proximity to a seed or a seedling.  Mix it with a store-bought or homemade planting or potting mix and allow the blend to ‘sit’ together for a time, and give it a rinse to wash out some of the ash and mobile matter.  This will enable the charcoal to blend and absorb nutrients and moisture before exposure to the plants.
  • More Biochar can be added to your soil over time.  If transplanting, work the Biochar blend into the prospective root zone of the plant – no need to cover the entire bed initially.
  • Top dressing beds will require time to enable the charcoal to work its way down to the root zone.  Results will be better if the Biochar can be worked into the root zone.
  • After thatching or lawn aeration, charcoal alone or in combination with fertilizer can be applied with a spreader.  It can also be applied under sod.
  • Keep records.  Do your own experiments.  The Biochar community will be grateful for results and reports on trials.

The opportunity for the gardener then, clearly, is to educate him or herself; to experiment; to enjoy the results knowing that feeding the soil with Biochar will improve the fertility of the soil, the health of plants and of the earth.    

Special thanks to Mr. Hugh McLaughlin and Mr. Erich Knight for their assistance in developing this article.  Consider stopping at for additional information on Biochar.

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