Terra Preta-Technical considerations

[Shultonus]

Hi All, I stumbled on this forum while doing some internet research. I thought I would give a brief synopsis of what I have synthesized so far. Some of these issues may have been already addressed, or probably by people more expert than myself. But none the less when people are confused by terms like CEC I feel the need to explain, I have a knack for those things.

 

Plant nutrients in soil is a tricky business. For starters I'll list some of the common concerns. Water, Oxygen (for the roots), Carbon dioxide (for photosynthesis), Nitrogen (either NO3- or NH4+ ions, Potassium (K+ ion), Phosphorus (PxOx- ions and complexes), Calcium ions (+2), Iron (Fe+2 or +3), and a handful of others.

 

Nitrogen can be lost by leaching and denitrification of NO3-, Ammonia is slowly convered to NO3- by bacteria in the soil. Fungi dominated soils have a lower conversion rate, and may actually store significatn amounts of it. Ammonia also can be volatilized by direct gasification to the atmosphere, this is especially critical during initial plowing of many forest soils or if ammonia fertilizer is not incorporated into the soil.

 

Phosphorus is very sensitive to changes in pH. Basically, phosophorus is only available between pH 6.5-8.0. Beyond this limited pH range mycrorizal species become infinitely more important in phosphorus aquisition. Iron is not available in high pH conditions or becomes leached over time in wet situations.

 

Now on the CEC and its relationship to pH.

CEC is cation exchange capacity. Clay, Organic Matter, and Charcaol all have CEC. Organic Matter also has a small but important AEC (anion exchange capacity). Agricultural charcoal has higher CEC and AEC then either of the other two options.

In clays and organic matter CEC is created by negative charge on small particles based on chemical structure and composition. Charcaol has CEC and AEC for entirely different reasons. The porous structure or the charcoal holds onto particles very well. The get trapped there with micropockets of water. This porous structure provides excellent homes for healthy soil microbes.

Each time it rains, nutrients can be lost in two different ways. The first is leaching. Leaching is simply water diluting the nutrients and carrying them off. This only happens to available and water soluble portions. The second way is erosion. Erosion carries away alot of phosphorus and organic matter.

 

CEC binds nutrients to the soil so they are less likely to leach. CEC is a measure of how much "space" the soil has to lock away the nutrients to prevent leaching. Base saturation is simply the percentage of the "space" filled. In the tropics, much of the soil is low in CEC (because of the type of clays--- good for pottery generally means bad for soil). THe parts that do have moderate CEC are still in bad shape because once it is lost, it is gone for good. In the northern latitudes we have had recent glaciation to provide a fresh supply of nutrients. The Terra Preta in the Amazon have HUGE CECs because of the charcoal in them.

 

On to part two of the discussion of the pH discussion.

Every chemical ion has an "acid rank". Sulfate is a strong acid (sulfiric acid), Potassium is a strong base (potassium and sodium hydroxides). Ammonia is a weak base. And so on and so forth. Nearly all positive ions are basic (either strong or weak). For plant nutrients that means we have Potassium(strong), Calcium(strong), Magnesium(strong), and Ammonia (weak) on the basic side. On the acidic side we have organic acids (weak), carbonic acid (weak) and phosphoric acid (very very weak). Most anions are acidic.

Therefore, the more CEC you have the more basic ions you have (meaning higher pH), because CEC specifically holds onto these basic ions (that also happen to be plant nutrients). Soil can have a high pH for other reasons as well. Left over limestone from glacial periods will have high pH. Ground water seeps and irrigations with groundwater can over time create high pH. Ancient inland seas can also be a source of high pH.

 

And finally,

As I discussed earlier, it is the porousity of the charcoal that makes it a suitable soil amendment. When creating the charcoal both TIME and TEMPERATURE are important factors, most of the literature I"ve seen thus far is concentrating on TEMPERATURE. TEsting will need to be completed for results.

Too low of temperature or too short would result in incomplete charcoal creation. There would be less pores. Also the unburned portion would provide food for the microbes and would eventually be released back into the atmosphere rather than stay in your soil where it does the most good. Too high of a temperature or too long probably leaves too large of pores, and generally would lack the complex chemical structure that the microbes find most inviting. Wood and other materials do have nutrients in them (0.1-2% generally). When burned in a fire the ash residue has lost much of the nutrients. But remains a good source of potash and depending on plant species calcium and magnisum. Ash is highly erodable. Charcoal holds onto those nutrients much better. Other sources of nturients would include any compost or urine, or bat guano or a handful of other sources that would be available to tropical farmers.

 

I theorize that Terra Preta developed out of a couple of possibilites:

1) Burning of middens (trash heaps) to reduce rodent and pest infestations. Trash heaps generally burn poorly and can create signifcant amounts of charcoal with enough fuel. The density of the heap would naturall restrict oxygen to prevent complete burn. The trash is an extra source of nutrients.

2) Slow roasting of animals and tubers in a pit, this would also sustain the necessary time and heat for charcoal prevention.

3) Roasting of trees in several situations: in order to build boats (dugouts), or harvest grubs/termites from. Again these processes controll time and temperature farily evenly.

Once people got onto the idea, they would no doubt rapidly expand on it.

Its easy to imaging once a season having just one family add 100 sq ft. Over several centuries this would result in the huge fields we see today. Even more could be accomplished with a heirachy that developed. A group could be no doubt comminted to full time conversion into Terra preta.

[kmcolo]

This is a very interesting post. Thank you.

 

Do you know much about the impact that charcoal production has as a methane source to the atmosphere? I would think it considerable. I have heard though that terra preta soils produce less methane than do traditional agricultural soils. This of course mitigates the production problem but does not eliminate it.

[Shultonus]

Charcoal production doesn't usually produce methane. Instead it produces a wide variety of hydrocarbons, which can lead to smog and respitory problems. These products themselves are combustable. Any real solution to global warming via charcoal production would require capture and use of this fuel.

 

The two companies seeking to commercialize bio-char have created efficient and robust systems that provide their own fuel. Esentially this hydrocarbon gas is burned to produce the charcoal. ON the internet you can find information about this process under gasifer or wood-gas. There are even conversions to run machines on this fuel.

 

In the third world, where Terra Preta will have the most impact, I see a different sort of solution. Using improved cook stoves (instead of tradition or open pit methods), charcoal is created and the actual cooking is performed largely by the smoke (aka wood gas). Several non-profits advocate this new cooking technology. The Juntos stove is just one example.

Imroved cook stoves use a lot less fuel, and provide health benefits as most of the problem causing pollution is burned instead.

[kmcolo]

Hi Shultonus,

 

I'll have to locate the refs but, counter-intuitively, any process that burns biomass produces methane. Biomass burning is actually one of the major anthropogenic sources of methane to te atmosphere. My interest is in reference with the developing world where the technologies you mention are mostly impractical to implement.

 

Ken

[Shultonus]

I apoligize for my error. You are right. Biomass burning does indeed contribute large amounts of methane to the global carbon cycle. The research I found indicated that burning in Savanah areas (grass) was produced more methane than burning forests. And even more disenhartening is charcoal production produced even more per volume (3.5 times roughly) as burning other biomass.

 

However, that being said. You mentioned that these technologies won't work in the tropical (poor) areas. This is were I again disagree. There are a couple of non profits spreading this technology in the 3rd world (central America and Africa). Look up Aprovecho.org Many of these improved stoves reduce both non-methane and methane hydrocarbons when used compared to traditional methods. The stoves can be built using local materials for next to nothing. Most of the stoves are less than $2. The main advantage is of course reduced fuel requirements frees children and women for more production rather than looking for fuel.

 

This improved cookstove can and has been modified to produce 'clean' charcoal without the associated hydro-carbons. Interestingly enough, soils that have cio-char give off less methane (reportedly... although I doubt it significantly off-sets the normal charcoal creation process.

[kmcolo]

Shultonus,

 

No need to appologize for errors. If one does not make them then one is not trying.

 

And good news on the non-profits efforts. I like being wrong in favor of good news!

 

Cheers,

 

Ken