Nikolaus Foidl reports of waste wood burns

Nikolaus Foidl has given us an excellent report on the experience gained attempting to exploit the products of an open burn of waste wood. It also brings home my ongoing disquiet surrounding the drawing of conclusions from this and many other similar tests. In this case particularly, a huge amount of ash was produced that was not fully incorporated into the surrounding soils. This made the soil initially very rich in soluble salts which had to be leached away before any benefits could emerge. The soil is actually ‘burned’ by an overload of nutrients.

I suspect that this is a problem with traditional slash and burn protocols also, however it may be obviated.

When we set out to produce a uniform end product of either charcoal or bio char or Terra Preta, it is necessary to manage the variables of temperature, airflow and end product production.

This can be done in an industrial kiln to great satisfaction. Tight packed wood with restricted air flow also seems to work okay. The earthen kiln that I have proposed for corn culture fits in between in terms of its ability to manage the process.

First and most important, the air flow must pass through an earthen wall several inches in thickness in order to reach the hot zone. This strictly limits the amount of oxygen and its velocity.

Second, the combustion is primarily fed by the heat generated from light gases such as methane which ignites first closest to unburnt material producing the most heat directly were it is needed, continuing the reduction process. Heavier unburnt volatiles enter the chimney were they may or may not be consumed if there is any remaining oxygen. These hot gases are then forced back into the stack at the top of the chimney traveling into the corn and back through the soil cap. Two things happen. A lot of the produced heat is absorbed by the unburnt corn stover preparing it for combustion. The gases then enter the soil giving up much of the unburnt volatiles including most pyrolysis fluids. They are also well distributed in the process and depending on the thickness of the soil cap, most are captured.

Once the fuel is totally processed, the capping soil is mixed with the reduced bio char and ash to properly distribute the combustion products throughout the soil. This virgin terra preta soil blend can then be taken in baskets or shovel loads to produce seed hills. Biological agents will quickly destroy any complex organics not already reduced by the heat leaving a carbon enriched soil that can hold nutrients for years as demonstrated in the Amazon. Rather importantly, they must also succeed in quickly reducing the high acid content of the pyrolysis fluid. That will need to be studied in field tests.

What I find particularly beguiling about this earthen kiln protocol is that it allows for quite a bit of variation in the air flow through changing the thickness of the earthen shell itself. This allows for a maximization of output over time. More importantly, this method is completely within the skill set and capital resources of every subsistence farmer in the world. He and his family merely need to be shown once.

From: Nikolaus Foidl <>

Date: Wed, 09 Jan 2008 16:28:52 -0400

To: ""<>

Conversation: Charcoal in soil

Subject: Charcoal in soil

Dear All!

Looking on the trials done so far with Charcoal in soil and terra preta, the most common plant used was corn so far. I do trials with charcoal since one year and I have as well soils at hand where huge amounts of forests after clearing where piled up in long rows and burned down, leaving behind ashes, charcoal and torrefied wood and all the condensates from the burning.( as well a good amount of soil burned together with the wood because the soil was on the roots and part of the logs and branches where covered by soil when they pushed the chained down trees to a row with caterpillars.

In the first 2 years only certain grasses ( brachiaria) would grow on those stripes. After some 3 years the planted corn and soy and sunflower show pronounced growth in the beginning but after about 60 to 70 days all plants in the field reach the same height and have the same state of development.

Looking at the harvest data there is no significant difference between charcoal and non charcoal in fertile well fertilized land not suffering drought. If there is drought during the development of the plants then the charcoal plot is more sensible and shows earlier drought damage in the plants.

If you make a mass balance over the amount of forest cut down and dragged with a caterpillar from a stripe of 50 meter each side to a small long heap of about 15meter width then you accumulate some 5 times the volume of the intact forest in the stripe of 15 meter or you concentrate the amount of 5 ha forest in one ha area and burn it . If we suppose a average dry mass yield of total biomass per ha (including roots) of some 200 to 250 tons this would be some 1000 to 1250 tons of dry biomass burning in this ha.

From sampling I can estimate that there are some 150 to 180 tons of partially or fully charred material per ha in the burning zone. So this leaves us with a huge amount of ashes in the same area. As most of the material are trees with an average diameter of 15 to 20 cm ( some are more then 60 cm, but most are smaller brush like trees) we have a good amount of barks with quite a high ash content. Wood without bark is in the range of 0.3 to 0.8 % ash and barks are in average around 7 to 8 % ash, some more.

Do you have any idea what this naturally reduces to in terms of elemental charcoal? Otherwise the amounts appear excellent and suggest that modern land clearing could be judiciously used to sequester a lot of carbon.

We urgently need to make mineral mass balances about the ashes and we need to know as well in which chemical form those ashes are in the soil and to what chemical form they convert. From the first look it seems to me that potassium and calcium and then magnesium and phosphor would be the mayor constituents.( someone has figured out the plant availability of those ashes?)

Now imagine that the indios additional used these burn and char areas as waste disposal and most of there waste where ashes from cooking fire and rests from there meals like fish heads and spines or bones or non edible parts of the animals beefing there diet ( as well needed a mass balance over at least a period of several tens of years to get a grip on quantities and content of minerals) then you easy can imagine that the terra preta sites are an enormous accumulation of minerals in different chemical forms. The adding of biologic material enhances whatever biology is working there and for sure will enhance growth of whatever plant you grow there.

We all believe that this is likely, but I also think that the land needed by a family was at least four or five acres. That means a pretty broad distribution of human and fish waste into the field. Has anyone mapped distribution over several acres to find out if it was consistent. If I were personally handling high grade wastes in such a setting, I would focus on the household garden to get the biggest bang for my effort. Of course a communal village could well have shifted this every several years.

Now the charcoal does not play an important active role in the beginning but degradation over the centuries transforms the charcoal into more stable chemics like humic acid and fulvic acid etc. which have high interchange capacity and high chelating capacity.

Is this a derivative of pure carbon or remnant organics? Does terra preta show such an acid profile? If not why not?

Maize reacts very strongly to high amounts of potassium ( the mayor ingredient of ashes)as well does soy and sunflower. Brachiaria as well is addict to high potassium. Other grasses do have problems with high potassium and do not grow in the first years in those burned areas.( dont think that this is a coincidence)

Conclusion is that we may be get distracted by fast visible effects on corn and other potassium and only relate those effects with charcoal but not with ashes and other micro minerals accumulated in waste disposal sites.

I believe in several enhancing effects of charcoal like vigor enhancing from the liquids produced during charring but I think there is very low direct short time effect from charcoal itself on growth of plants ( first 10 to 50 years). There is without doubt a indirect sink and source effect by its capacity to adsorb micro and macro nutrients.

Ancient terra preta soils are been continuously cropped over decades without significant fertilization. This implies that the carbon essentially fixed nutrients in the growing zone. Otherwise fertility would have collapsed.

Best regards Nikolaus

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