More on charcoal in soil

A while ago I posted briefly on the potential for using charcoal to improve soil quality. Since that time, I've been reading on and off on the topic.

As an aside: If you are interested in delving into this more, you should know that the preferred term is “Biochar” not charcoal. It is the same stuff, but biochar is used much more commonly when discussing using charcoal as a soil amendment and will give you better google results.

Native American Terraforming

The idea for using charcoal (biochar) to improve soil comes from studies of the Terra Preta or Amazonian Dark Earth soils found in the South American rain forests. The surrounding soils are typical for rainy tropical areas: highly acidic, clay, nutrient poor, containing virtually no organic matter, and are virtually impossible to farm on. The Terra Preta, in stark contrast are dark, nutrient rich, high in organic matter, and support excellent crop growth. What is amazing is it appears these rich soils were man-made by pre-Columbian Indian societies, and the key ingredient in this transformation seem to be adding charcoal.

(image from wikipedia)

Obviously excited by this, people have been studying the potential for charcoal as a soil amendment. The scientific research on the subject is still in its infancy and, like most new science findings, probably overstate the case but still look pretty cool and extremely interesting.

Organic Matter

Anyone who gardens these days has probably got the memo that organic matter is great for soil. Compost, mulch, whatever, organic matter promotes the growth of beneficial soil microrganisms, helps soil retain water and nutrients, loosens heavy clay soils, etc. But soil organic matter rots. Eventually, as soil fungi and so forth munch on it, it vanishes away back into the water and carbon dioxide that it was made out of. Because of that, most tropical rain forest soils are extremely low in organic matter – the warm, moist conditions speed up the decomposition process so much that added mulch or compost melt away seemingly overnight.

Charcoal as permanent organic matter

Charcoal in the soil, it appears, has many of the same effects as organic matter: Improving soil structure, increasing water and nutrient availability, moderating soil acidity, promoting microorganism growth. But unlike compost, charcoal decomposes very, very slowly, on the scale of millennia, not months or years. So the charcoal added to the soils in the Amazon by Indian societies are still there, hundreds of years after those societies themselves were wiped out by the arrive of Europeans bearing small pox.

Carbon Sequestration

This extremely slow decomposition provides another bonus. As plants grow, they take carbon dioxide out of the air and use it to build leaves and roots and wood. When they die, and rot or burn or whatever, all that CO2 goes right back out into the air. Which was fine, until we humans started pulling carbon that had been stored underground in the form of coal, oil, and natural gas, and burning it, thereby putting way MORE carbon into the air and causing that whole global warming thing you may have heard about. But if we take a plant, and make it into charcoal, that carbon gets locked up, safe and sequestered in the soil we've added it to. In other words, adding charcoal to your soil might not just improve your soil quality, it takes carbon out of circulation, helping reduce global warming.

Should you try it?

So, should you be adding it to your soil? I don't think the research evidence is there for it to be really universally recommended, but I do think it could be fun to play with and see what effect it has in your conditions. Based on what I've read, you've got the best chance of it being helpful if you have heavy clay soil, acidic soil, and garden in a warm climate where soil organic matter vanishes very quickly. Since charcoal will raise your soil pH, making it less acidic, it is probably not a good idea if you already have alkaline soil, or are trying to grow plants like rhododendrons or blueberries that prefer a very acidic soil.

*Addendum: I guess the pH effect can be much more variable than I thought! See the comments for more information, and consider testing the pH of the charcoal/biochar itself before you add it to your soil! *

Charcoal can also absorb a lot of nutrients, so most things I've looked at recommend that you mix it or soak it in something high in nitrogen, like composted manure or even urine before adding it to the soil. One pound per square foot is a recommended rate I've seen, but I don't think there is really the data yet to really have a firm number. Whatever you do, keep good records so we can learn from your results.

Where do you get it?

Apparently, you can use regular charcoal like you buy for your grill, but you need to make sure it isn't mixed with anything nasty. Briquettes are usually made with chemical binders that might be harmful to your soil, and certainly you don't want the stuff that has already be doused with lighter fluid. Gourmet “chunk” style charcoal is supposed to be good (but pricey), as are brands of briquettes that are bound together with corn starch. You can also make it yourself... But that looks like a bit of work.

I'm giving it a shot

I think I'm going to try using it this year. I'm planning to prepare two new beds, adding charcoal to one, and not to the other. I'll try to keep the otherwise as similar as possible so I can compare how it seems to effect the growth of my plants. I'll keep you updated on how it turns out! If you try it as well, please leave a control plot untreated for comparison, and PLEASE let me know what you think! I'm very curious.

More information:
Gardening with Biochar
International Biochar Initiative
Biochar for Environmental Management