Tools and Techniques- Basic Biochar

**Fire is potentially dangerous to people and property. Ensure you take all proper precautions before doing your own experiments. This account is not meant to represent exhaustive advice in this regard**

Recently a big buzz has been made about the terra preta soils discovered in the Amazon. In a region which typically has thin, acidic and nutrient leached soils under the rainforest canopy particular patches of soil created by human activity were discovered that were deep, black and nutrient rich. The soil features large amounts of charcoal, along with pieces of broken pottery and bone and other components. Despite the many wonders of this discovery no one knows for certain how they were created and used since the Amazonian culture that produced them collapsed with little trace after epidemic diseases were introduced from the old world hundreds of years ago.

In response there has been a flurry of activity experimenting with producing and using charred plant residues. Plants are predominantly composed of cellulose, a carbohydrate made of carbon, hydrogen and oxygen. Heating this up causes release of unreactive water along with flammable hydrogen and carbon monoxide gases that can burn when mixed with oxygen in the air. If oxygen is limited then the remaining carbon can survive the fire, creating charcoal. Depending on the exact conditions the carbon can have negative carboxylate groups on its porous surface, allowing it to hang on to positively charged mineral ions that are often needed as plant nutrients. This allows terra preta soils to retain nutrients under high rainfall conditions much better than unimproved mineral based soils. Other forms of organic matter like compost and manure also have negative groups that help retain positive mineral ions, but under tropical conditions they tend to break down very rapidly. Charcoal on the other hand is stable potentially for thousands of years. When oxygen is not limited and the fire not extinguished the plant material will continue to burn until it is reduced to ash, the trace residue of the minerals that were present in the original plant material.

One way to control the charring conditions is to build complicated metal apparatuses to exclude oxygen and manage the temperature. I’m sure for people who love welding and engineering that sounds like fun, but I couldn’t imagine anything less pleasant. These machines with metal chambers need to be carefully packed with wood and unfortunately my available feed stock consists of twiggy branches that would take too much time to cut down and pack into a confined space. Instead I stack these branches in long rows, about 1 m wide and high, along my veggie garden beds next to the goat pen to limit the distance I need to move them once the goats have stripped them down. My most recent fire was delayed many months by the prolonged fire bans, so combined with the extra branches collected for my hungry goats to get them through the drought I ended up with a bigger pile than normal.

When weather conditions are suitable I light the row from one end and the fire gradually works its way along the pile. As the flames begin to die down in one section I use a watering can to douse the flames and prevent the charcoal from turning to ash. If the pile is too big the radiant heat can be quite intense so pay attention to this hazard. From my perspective if I douse the embers a bit late it just means a bit less charcoal and a bit more ash, so not a big deal since ash is also useful. Leaves and twigs turn to ash quickly, while large logs are often left only partially burnt, so I need to judge the timing of dousing to maximise charcoal production from the mostly intermediate sized sticks. Most crucial is to use a metal hoe or similar tool to stir the charcoal immediately after dousing to ensure the lower layers don’t continue to burn. I suspect a thin layer of damp ash ends up smothering the pores leading into the embers and prevents them reigniting. Large unburnt branches are kept aside for the next fire. Ensure the charcoal is completely cool before handling since even slightly warm charcoal can reignite when disturbed.

From a pile of thin branches about 2 m wide, 2 m high and 6 m long I used 80 L of water (carried in two 10 L watering cans from a nearby water barrel) to extinguish and recovered seven large 15 L buckets of charcoal (105 L from this burn). This was spread over the center of the rows of the vegetable garden under construction, a total space of about 130 square meters. I will probably burn about as much again soon, plus some fires that will be left to produce ash for the space. If I add a total of 210 L of charcoal to the just the beds covering about half the area and ignoring the paths, if it is incorporated to a depth of 30 cm into the soil it will represent about 19.5 cubic meters of soil (19 500 L), so the 210 L of charcoal represents only 1 % of the total soil volume. Terra preta soils contain up to 9 % charcoal, but I am not in a rush to push the soil to this level. Various growing trials with biochar have produced mixed results so far, especially with different species and on different parent soil types. It is definitely not a magic bullet to improve all soils forever. The roughly burnt and ash rich charcoal I produce is very alkaline, which in moderate doses is good for most crops on my fairly acidic soil, but I could easily add too much all at once.

Fresh charcoal can also absorb nutrients from the soil to fill its sponge like capacity, potentially depleting the soil in the short term. Some people “charge” their charcoal by soaking in various nutrient solutions but my approach is to simply apply it in combination with goat manure direct to the soil surface. I believe allowing some of the material to turn to mineral rich ash reduces the nutrient absorption problem. Some people also crush their charcoal to a fine powder and dig it in to achieve an instant result. I do neither. In my oldest garden that has had added rough uncrushed charcoal to the surface of the soil the material has disintegrated into small fragments in a few years and disappeared into the soil profile due to the constant churning of soil life. The original clay soil which tended to set solid during dry spells is now much more friable and easier to work over a wider range of weather conditions. The soil has changed from reddish brown to dark brownish-black (though I think just looking at colour of soil doesn’t tell you a lot).

In terms of crop response by best results have been with capsicums, and to a lesser degree their relatives of eggplants and tomatoes. These crops can tolerate fairly alkaline soil and can use the large amounts of soluble potassium to foster fruit growth. Previous problems with fruit fly in my capsicums simply went away when I applied an inch thick layer of fresh biochar around established plants, followed by a large surge in fruit production. Other crops seem a bit more indifferent to even modest doses of biochar in the short term, but perform better on soil with aged biochar that has gradually incorporated into the soil. That is one of the reasons I am only adding about 1% charcoal before each growing season, combined with long fallows between adding any more. I figure this way I can gradually approach the optimum level for my soil, and avoid pushing the pH to very high alkaline levels that would damage most crops.

Attentive readers will have noticed that real terra preta is more than just charcoal. I make use of any spare bones from animals on the farm, but usually put them through a fire as well in order to make coarse bone meal. This phosphorus rich ash is applied to heavy feeding crops like potatoes, but if left uncrushed will create fragments that act as a slow release form of this key nutrient for many years. I have had good results from putting unburnt bones under young fruit trees as well at the time of planting. The pottery fragments aspect of original terra preta is one I haven’t seen explored by anyone. The microbial communities living in the pores of charcoal in terra preta soils are also only partially explored and may be a key missing ingredient. At any rate the people who knew how to make those soils are long gone, and the technique may not be translatable to other soils, crops and climates. Just like the original Amazonian settlers we have to get out there and do our own experiments to develop techniques that work for us. Biochar might be a useful component of soil management systems but initial enthusiasm for lost ancient secrets is giving way to the hard slog of figuring it out for ourselves, one fire and one crop at a time.

Burn number one nearly complete. The smaller pile on the side didn’t ignite. The third on the left has been quenched, while the center still shows white ash indicating it is still actively burning. The radiant heat on this one was a bit too high to quench at the ideal time.

The first burn completed. This just needs spot quenching after half an hour of resting.

A end view of the second burn with just the final section awaiting quenching. Large branches have been pushed aside by the hoe to allow more thorough stirring of the smaller charcoal fragments.

Cooled charcoal applied to the top of next season’s beds. I am doing a mix of charcoal or goat manure first to see if it makes any difference.

Ready, set, go! A basic biochar burn underway with full watering cans and trusty hoe at the ready.