Traditionally, the recommendation is NOT to put wood ashes in the garden. Ashes have very minimal nutrient value, no nitrogen and only small amounts of phosphorus and potassium, which in our soils we do not need. They are very alkaline and may benefit compost piles if used sparingly.
So what's the deal with this new burnt wood thing called BioChar? First glance tells us it's just ashes. No, it's more than that, and for better reasons.
A thousand-plus years ago, pre-Columbian inhabitants of South America were making their otherwise relatively infertile Amazonian soil very viable. This soil, "Terra Preta de Indio" (black earth of the Indian), is rich in carbon, organic material and very stable, making it ideal for crops. The key is the carbon in the form of what we commonly call charcoal or, if used as a soil amendment, BioChar.
BioChar is basically organic material (wood, nut hulls, manure) that has burnt incompletely or been heated to a temperature to drive off all the gasses (hydrogen, oxygen, methane) its contents contained. This process is called pyrolysis. What's left is a black, hard, flakey, inert, carbon particle with a very large surface area and microscopic open pore space to hold nutrients, moisture, microbes, fungi and air. All these things support biological growth, and the stuff lasts for, as was discovered, thousands of years.
We have seen and done this many times as we doused the campfire with water or covered the smoker to shut it down and were left with hunks of these black, crumbly things.
BioChar is produced in a slow, low-heat, oxygen-starved burn. Part of the material is burned to produce the heat to break down the rest of the material. The heat can be used, and in sophisticated systems, the released gases are collected. The resulting material is free of the gases but retains its carbon. This carbon sequestration makes this a carbon-negative process. It is not a silver bullet to solve our greenhouse gas emission issue but certainly moves us in the right direction.
Grants from SARE (Sustainable Agriculture Research and Education) are being used to test this process here in Douglas County. Initial results were inconclusive because the grasshoppers ate the crop. Methods of small farm BioChar production were, however, successfully developed and methods of application and combinations with other materials and nutrients investigated. The growers involved will try again. In other research-based tests, combining this BioChar with new organic material and/or nutrients is showing positive results. Studies are also underway to evaluate what material makes the best BioChar. This product is also being developed for commercial use.
As home gardeners, we can possibly benefit from these findings by making our own. The fireplace ashes can be screened for the large particles remaining; in an airtight stove we can close all the dampers toward the end of the burn, or possibly dedicate an old grill to first having a strong, hot fire of wood, leaves, etc., and then closing off all air intakes, allowing it to cool on its own. One note of caution to our enthusiasm: Many tests show that it could take years for noticeable results.
-- Stan Ring is the Horticulture Program Assistant for K-State Research and Extension in Douglas County. Extension Master Gardeners can help with your gardening questions at 843-7058 or firstname.lastname@example.org.