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  • From: jas39a@earthlink.net
  • To: nafex@lists.ibiblio.org
  • Subject: Re: [NAFEX] Charcoal in Soil (and Terra Preta)
  • Date: Thu, 31 Jul 2008 13:26:51 -0400 (GMT-04:00)

( One member sent me a private e-mail, asking me for more information.  I thought I'd pass it along to the whole list, in case anyone is interested....EHE)

Charcoal is valuable as a soil amendment in two ways.  First, it has a very high specific surface (surface area per unit volume), and a lot of charge sites (dissociated molecules with a net negative charge), so charcoal increases the cation exchange capacity of the soil (in other words, it increases a soil's ability to hold, in a manner that is available to plants, Calcium, Magnesium, and Potassium).  Secondly, charcoal contains pyrogenic compounds (complex organic molecules that are produced by destructive distillation during the charcoal-manufacturing process), some of which seem to stimulate the growth of plants.  The first factor is well understood by scientists; the latter is poorly understood, and not well categorized.

If you are interested in using charcoal as a soil amendment, it is important to realize that, if your soil is already rich, you will perceive little benefit from the addition of charcoal.  The Amazonian regions where Terra Preta is found contain mostly Oxisols, which are very old, highly weathered, infertile soils.  Oxisols have experienced so much weathering that the more complex silicate clays that we would normally find in soils of temperate regions are, for the most part, gone, and what remains are amorphous (non-crystalline) clays, composed mostly of hydrated oxides and hydroxides of Aluminum and Iron ("Sesquioxides"). Sesquioxides do not behave like silicate clays.  One important characteristic of Sesquioxides is that they have little, or no, cation exchange capacity, and thus no ability to supply cations to plant roots.  Indeed, in the tropical rainforest, almost all nutrients are locked up in the organic fraction (duff layer) on the forest floor; when that disappears, the soil is effectively sterilized.  Furthermore, most sesquioxides, at the ambient soil pH values encountered (~3.8 to 4.5), have a net anion exchange capacity, so they effectively retain (and fix, in a manner making them unavailable to plants) important anions such as the Phosphate anion.  Thus, crop production in Oxisols is problematic.

Charcoal, when added to an Oxisol, provides much needed cation exchange capacity--and lots of it.  This additional CEC means that, when wood ashes--which contain needed plant nutrient cations, such as Calcium, Magnesium, and Potassium--are added to the soil, those cations don't just wash down through the soil with the next rain.  Instead, many will be retained, in exchangeable (and available) form, by the charge sites on the organic colloids of the charcoal.  Thus a previously infertile soil is rendered fertile.  And, since charcoal is relatively resistant to decomposition, this benefit remains for centuries after the initial addition of the charcoal.

Most loamy or clayey soils in temperate regions have adequate cation exchange capacity for fruit tree production.  Sandy soils, and the loamy Ultisols found on the coastal plain of the deep South, often have relatively low CEC values, and would probably benefit from the addition of charcoal.  In other regions of the country, any benefit would probably stem from the aforementioned pyrogenic compounds, which stimulate the general soil biotic community (mostly by providing food for soil bacteria); this pryogenic stimulus lasts only a few years, until the compounds are metabolized by the soil microflora. 

I would not recommend using ashes from other people's stoves or fireplaces; if they have burned coal in them, the ashes will contain high concentrations of heavy metals, such as Cadmium, Lead, and Mercury.  Instead, if you are determined to use charcoal, either make it yourself, or go to a charcoal plant, and collect some dust from their conveyors or hoppers (charcoal factories always have a lot of dust, and it's just a waste product to them, so they are generally glad to give it away).  That's what the farmers in the Amazon do.

I cannot advise you on a specific amount to use in a potting soil mix.  If you are adding some other organic substance, such as compost or peat, you will already have significantly higher CEC than would be available in a straight mineral soil, and charcoal would then be superfluous.  I have seen photos of Brazilian plantings of Avocados and Citrus that had what appeared to be perhaps a gallon or so of charcoal spread around the base of each young plant, but again, those plantings were in Oxisols.  I am not aware of any research in charcoal addition that has been done here in the U.S., but an internet search might turn up something.

Eddie Earles



  • Re: [NAFEX] Charcoal in Soil (and Terra Preta), jas39a, 07/31/2008

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