Improving Soil Fertility via Burning

Slash and burn may not be the devil it’s been portrayed to be. Like many gardening/farming practices, there are upsides and downsides.

If you don’t simply burn everything to ash and till the area, it could improve the soil for an extended period of time. It’s going to depend on soil, climate, rainfall, and who knows what else, but there are times that fire makes things better.

As I wrote in my post from October 9th:

Burned areas often grow back very nicely as the ashes and charcoal feed the next crops. I visited one couple who had cleared the woods around their house and replaced it with a large lawn and pasture. The homeowner pointed out multiple dark green areas in the grass where he had burned piles of trees years before. Even after almost a decade, they were still maintaining fertility (…) Seeing such long-term improvements in fertility does tend to support our biochar/terra preta experiments.

Beneath that post, Ashkan comments:

V.A. Tiedjen’s book mentions a story about some Mennonite(??) settlers who moved into an area and found that plants grew way better where the natives had kept fire pits. I think he mentions that the natives tended to toss their shellfish remains into the fire, too. In any case, the point was that this was evidence for the effectiveness of calcium at improving fertility. Since calcium is the principle element in wood ash, that, plus the char (a resilient organic matter) might be why they had such good fertility after burning brush in Florida.

This is the book referenced:

It’s out of print, but I just bought a copy on ebay. I was also able to download a .pdf, and found the anecdote related by Ashkan:

Wood ashes have been used by plant-growing people since the eighteenth century. The story is told about the Mennonite scout who was sent out to look over a site for a settlement. When he inquired about an area adjacent to a river in one of the southern states, he was told the land was worn out. But he took a second look and found small round areas covering five to ten square yards where weeds grew in abundance. On examination he found that these plants had deep roots, while those nearby were very shallow. As he studied these areas, he surmised that they were the campfire sites of Indian tribes who had frequented the area many years before. Wood ashes and burned oyster shells had been left. He encouraged his people to buy this “poor” land and apply liming materials to grow their crops. Today this is a highly productive community of farmers.

I don’t know why he claims “since the eighteenth century,” when the use of ashes for fertilization goes back much farther than that. However, he also claims in the introduction that Jesus fed the “Philistines” with 5 loaves and two fishes, so there are some obvious errors in the text. This doesn’t make it devoid of useful information, I am sure, but it’s something to keep in mind if you read the book.

The long-term effect of burning is where my interest lies. One Grenadian told me that he knew a farmer who fed his land with “nothing but ashes,” and he did great.

Calcium is part of the key to successful yields. But will carbon in the form of biochar/charcoal help keep it around?

It seems to make a difference. We’ll keep studying and experimenting.

And now I have another book to read.