Scott Burrell is the horticulture specialist in the horticulture program at Reynolds Community College in Goochland. He is a frequent contributor to this magazine and a well-known speaker on horticultural topics. His wife Beth is also a contributor and landscape designer. They maintain 1½ acre in gardens at their home in Mechanicsville.

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Biochar to the Rescue!
by Scott Burrell       #Environment   #Fertilizing   #Sustainability and Self-Sufficiency

Watching and waiting. Once the “jet engine” noise of inner barrel low oxygen charring and volatile gas consumption begins, the smoke becomes minimal. If enough oxygen were available one would see complete combustion with only ashes produced. Former Vice President and 2007 Nobel Prize recipient Al Gore noted, “One of the most exciting new strategies for restoring carbon to depleted soils and sequestering significant amounts of CO2 for 1,000 years and more, is the use of biochar.”

Biochar – you may have never heard of it, but in many research circles, and in a few select backyard lots, biochar is the stuff dreams are made of, particularly given our need for better soils, better air, better plants, and better climates. Biochar is a type of charcoal very unlike the grill’s charcoal briquettes, which are a mixture of powdered devolatilized coal, a small portion of raw or carbonized sawdust, and intentional ash additives. Biochar is the result of heating biomass under the exclusion of air – a process known as pyrolysis. Renewable lignin-based resources from nut shells to manures to wood, switchgrass, wheat straw, corn shucks and other green materials, can be the fuel used to create a very stable, very porous carbon rich product that can last hundreds of years. Biochar’s primary use is for soil enrichment, but it can do much more than that.

In the 1950s, Dutch soil scientist Wim Sombroek discovered dark, carbon-rich soils in the Amazon basin that supported productive farms in areas – many of them typically unproductive jungle cutovers – that previously had poor, even toxic soils. These dark soils known as terra preta, or black earth, had been “cultured” sometimes over hundreds, sometimes even thousands of years by the addition of biochar, by accident (wildfire mainly), or by intention, allowing the soil to retain vital organic matter, plant nutrients and moisture, essential for good plant growth. Unlike other raw materials we’re more familiar with, such as fertilizers, additives, composts or manures, biochar is not assimilated, transformed, or broken down, but is thought to remain unaltered in the soil through successive generations of biomass. Still enriched hundreds of years after they were amended, these dark soils have generated great interest in using biochar not only to improve soils but to sequester carbon by tying up the carbon in solid stable form from otherwise unused biomass as well as increasing plant growth, which consumes greater amounts of CO2 in the process then slower-growing plants. Thus, potentially, mankind could reduce – if done on a massive scale – the greenhouse effect of elevated carbon (CO2) levels currently in the atmosphere. And consider this, increased plant growth could also result in increased oxygen production at a time when oxygen levels in our atmosphere are falling. Wow, that’s big picture thinking for a gardener just wanting a better backyard!

A homemade biochar maker. Note the drilled air holes at the top and bottom of the 55-gallon metal barrel. Also note the removable metal seal rim attached to the removable smokestack/cover combo. The seal allows for lower oxygen ingress once the burning process is well along. Commercial production of biochar uses a number of alternative processes including kilns and gasifiers to achieve the same end product.

Biochar and compost mix three weeks after initial combustion. Some impoverished nations use urine to complete the preparation of biochar for use in the soil. Whether compost or urine, the aim is to stabilize it. This initial one time application will ensure the biochar does not compete for nutrients while setting up a good environment for the future microbe/nutrient/water/carbon matrix.

Just a wee bit more science. Why is it so stable? Mainly because the aromatic rings that make up the structurally altered carbon in biochar are so difficult to break. Voila, a long lasting, incredibly stable, soil amendment. If biochar is indeed the same product found in terra preta soils, which have also been discovered on other continents (for instance, Japan has a long history of using charcoal in soil), it is the realization of what gardeners and environmentalists dream of: It maintains balanced moisture levels during wide climate changes; it improves air permeability in otherwise dense clays soils; like humus and clay minerals it increases cation exchange capacity thus increasing soil fertility; it decreases leaching of essential nutrients making them available for microbial use while its pores provide a great habitat for microbes; and all the while it increases the buffering capacity and doesn’t itself deteriorate and have to be annually amended like fertilizers, which is all too good to be true. But, there is more: Like activated carbons, some biochars have activity levels high enough to act as detoxifiers of poisoned, sterile or dying soils. Imagine if we’d had biochar reclamation following the dustbowl years in the 1930s!

Let’s move beyond the science to the practical. How do we make it, how do we use it? Since we are talking about combustion, the material used needs to be reasonably dry (20-30 percent water weight or less). At Reynolds Community College in Goochland, Virginia, where I worked, one of my very capable volunteers, Bill Swanson, built a simple two-barrel “retort,” basically a stove of sorts. Ours was made of two metal drums dedicated to the pyrolysis, i.e., low oxygen burning, of dry wood and other dry biomass to make biochar. Ours used twigs and short pieces of excess untreated lumber up to 2 inches thick that we packed into the open end of a 30-gallon metal barrel. Wood chips or sawdust will not work well in this design as they pack too tightly and not enough heat is developed. There are other suitable designs such as kilns, pits, and gasifiers. Our 55-gallon open-end empty drum was inverted and placed over the open end of the 30-gallon wood -filled drum. The two barrels, now one unit, were turned over and the space between the two barrels was filled with dried wood. Air holes – cut or drilled – around the base as well as just below the upper rim of the 55-gallon barrel allowed for limited O2 access. Once the wood between the barrels was ignited and vigorously burning the outer barrel lid, complete with a stovepipe was seated to retain heat and make the process more efficient. The heat from this burning wood chars the wood in the inner oxygen-restricted barrel (if there was too much oxygen it would burn to ash), which gives off volatile gases while drawing in oxygen through the space where the barrels meet at the bottom. Once that charring process really gets started you’ll hear what sounds like a jet engine rushing sound of exiting hot gases and water vapor heading out the smoke stack with virtually no smoke. In the space of 90 minutes burn time you’ve created biochar! When the process is complete and cooled, remove the lid and dump out the inner barrel of biochar. If it’s been “cooked” right it will have a clinking sound, almost glassy when dropped or shaken.

Properly carbonized wood forms a rigid, easily crushed material lacking any pockets of undercarbonized material. The biochar will not feel greasy and the black dust will wash off one’s hands with just water – no soap necessary unless it was incomplete charring.

To prepare our biochar for garden use, we wrapped it in a tarp and ran over it repeatedly with the truck to crush it into 1⁄2-inch or so pieces. Fresh biochar needs to be further prepped by mixing it with compost (in some countries urine is used to stabilize the biochar) or a balanced fertilizer. We mixed it with compost and let it sit for three weeks. The biochar will attract and hold nutrients and microbes from the compost. This only needs to be done once to prevent competition with plants for nutrients.

The study and refinement of biochars for reintroduction of endangered or threatened coastal plant species to coastal barren habitat in Massachusetts is just one of the many ongoing biochar initiatives. The biochar we made at the college was introduced to the new educational vineyard recently established. Who knows, the results may well lead to stronger plants with less need for chemical controls. Who yet knows the full promise of biochar?



A version of this article appeared in a September 2015 print edition of State-by-State Gardening.
Photography courtesy of Scott Burrell.


Posted: 08/01/18   RSS | Print


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