I would like to use charcoal made from hardwood tree trimmings to open up my heavy clay soil. How much should I add?
There’s a lot of interest around charcoal in soils, particularly terra preta, the black earth of the Amazon Basin: soil made by humans way back when. It’s a mixture of charcoal made at low temperatures, plus bone and manure, and is highly fertile. This sort of charcoal is called biochar. It is supposed to do miracles for your soil; improving water capacity and plant growth, and reducing soil acidity. It should be added at anywhere from 1-5kg/sq m.
My allotment has its fair share of bonfires, so I thought I’d add it to my soil. I ended up with a huge magnesium deficiency. I can’t directly blame it on the charcoal; it may have been wood ash mixed in it. I assume it altered the pH of the beds, causing nutrient lockup. Wood ashes are known to increase alkalinity.
The simplest solution is to add charcoal or wood ashes to your compost heap in batches. This way you don’t end up adding too much to your soil, but still get the benefits. Good homemade compost will improve clay soil and darken it, meaning it warms up faster in spring.
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