old brushfire coals

[bower]

There is an endless amount of brush here in the woods. Every year whether you cut wood or not, there are blowdowns and a new supply of brush that has to go somewhere. So we used to at least try to burn some of it every year, but that has stopped being a fun and reasonable thing to do. In the past I had tried digging the ash and residues into garden beds or incorporated them into new beds, with very poor results. While fine ash seemed to be useful, the coals were a waste material that seemed toxic to plants.

In any case, some friends were helping to clear up the garden seven years ago, and raked all the old brushfire coals into a mound. It sat there and became colonized eventually by tansy. I've been wanting rid of it for years now, but finally with an early bit of spring weather I got at it with the pick before the tansy was up. It looks like the coals have broken down a fair bit, as I was mostly finding pieces no bigger than chipped rock and plenty smaller.

Having read so much recently about 'biochar', I thought that instead of carting it off to dispose in the woods somewhere, I would consider incorporating or making it the subsoil material for a new raised bed. I have several compost piles in various stages of finished that I want to use as the main material for the bed, and hoped to plant my leeks and onions there.

Was hoping that someone here had experience with 'biochar' and some helpful comments about how to go about building this bed. In case it makes a difference I will note: these are conifer coals, not hardwoods. Some of the questions running through my mind now:

- Should I leave this stuff as a subsoil or dig it into the main material? (not only concerned about past performance, but tansy roots are allelopathic as well, so their residues might be a problem)
- Should I lime it? This is a cold, wet, very acid environment, we constantly have to lime.
- I have some not-very-finished compost from last year - maybe I should mix this with the coals as a "charger" and make that the subsoil layer?
- Should I forget about building a bed over the coals where is, and instead dig them into that not-so-finished compost and leave for a year?

TIA for your thoughts.

[Nan_PA_6b]

My worry would be if you got tansy roots in there: tansy is impossible to eradicate and takes over in my yard. For that reason, I'd leave the charcoal underneath all the other materials in the bed.

Nan

[bower]

Thanks Nan. Yeah tansy is invasive alright. I'm planning to pull all those roots out but would also use a cardboard barrier under the bed if I think there are any left. Their density in the coals is not as bad as in regular soil or even gravel.

Just reading about biochar last night, and it seems it is recommended in very small amounts - 2% is 'enough'. So although it's old stuff, maybe should shovel some out of there and mix with compost, let it sit. Or maybe I should go back to the original plan - ditch beyond the garden.