Thursday, February 17, 2011

Helping rhizobia thrive in the soil with biochar

Continuing the idea of helping nitrogen fixing bacteria (rhizobia) to flourish, I have been researching using biochar (terra preta) and how it helps the foundation of the soil. This approach has been used by natives of the Amazon for thousands of years with great success in charging their soil with carbon. I don't have a rain forest of trees handy, so I have been screening the ashes from our wood stove this winter and saving the charcoal.

With my research, I have found that adding biochar not only increases the pH but offers microscopic locations where this type of bacteria like to set up residence. But before you dump your bucket of ashes into your garden, note that you need to take a few steps to avoid negatively impacting your garden seeds with this strong, alkaline additive.
  1. Not all charcoal is created equal. Charcoal briquettes have a lot of additives that would not be good in your garden. You can actually put waste material in a metal can in your fireplace and the high temperature baking without much oxygen will turn that waste to charcoal. Because pine needles, coffee grounds, and manure is acidic, the charcoal produced from these products will be more acidic than the charcoal made from traditional firewood. For soils which are already alkaline, the more acidic charcoal is obviously a better choice.
  2. Crushing the charcoal will increase the surface area and make it more effective for the bacteria. It also makes it easier to distribute evenly using traditional fertilizer spreading equipment. The caveat with crushing is that larger pieces will obviously take longer to break down in the soil and provide better tilth than charcoal powder which may compound hard pan in clay soils. (That is the last thing that I need in my soil which becomes nearly hard as concrete once it dries out.)  The ashes should be separated from the charcoal by screening it and handling separately. (Ashes are highly alkaline and care must be taken to know if its natural liming action is needed for your soil type.)
  3. Charcoal needs to be "activated" by soaking in water and a high nitrogen source. Otherwise it will pull existing nitrogen from the soil and away from growing seedlings. Nitrogen sources may include chemical fertilizer (if you must), a compost tea, etc. (I have heard some people use urine for this purpose, but I'm not ready to jump on the humanure bandwagon quite yet.) Compost tea is created by putting several scoops of composted manure into a 5 gallon bucket of water, and letting it sit for several days before adding the biochar. The smell of the tea is bound to be strong (I haven't tried this myself) but is wonderful for charging the charcoal with enough nitrogen and beneficial organisms. Another warning: compost tea which sits too long without sufficient oxygen can actually encourage the harmful types of bacteria to develop. Some gardeners will counter this risk by using an old fish tank aerator to bubble the water while it is sitting, then use it right away to prevent the harmful bacteria from getting established. Adding an extra food source like molasses will further encourage the beneficial microorganisms to reproduce quickly. (This reminds me of adding sugar to yeast when preparing dough for bread.) Any extra compost tea makes a great fertilizer and may even be spread with a sprayer if it is filtered.

I haven't been saving enough firewood charcoal to make a big difference in the garden, so I think that I may try making some out of manure in an outdoor fireplace. (I'm not brave enough to risk cooking a can of manure in the house!) Our alkaline soil will benefit from a less alkaline charcoal.

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