i think i had started to write this up, but i don't find the draft. so...
first of all some background. the soil i normally use for one garden is mostly clay that i've been reconditioning over the years by growing cosmos and then burying the leftover dried stems and any other remaining organic materials i can find in a long trench. from year to year i keep moving the trench back and forth digging it up and chopping the stems as i shovel. the stems don't fully rot for several years.
this past year after reading about terra preta and biochar i thought i would try to make some charcoal and then use it in areas to see what the effect would be on seed sprouting (and later growth as the season advances) for several species of plants.
first of all i used the worst part (the area with the most clay) of the trench from last season to dig down further into the clay (about 3ft down total). making a space about 10ft long and 3-4ft across. i put all the super dry cosmo stems and other easily burning stuff in the bottom and stomped it down good before taking some dried up shrubs and trimmed off all the small leaves and stems and put that down next with the trunks on top then stomped all this down too.
then to start building a canopy i put some long branches across and other longer pieces of wood and old pallet pieces that we would normally just bury and let the fungi/mushrooms work on. on top of these i put some cardboard and damp earth to seal things up on top. i left both ends open a little so i could get a fire going.
with the wind shifting constantly it was a real PITA. i walked miles around that trench and the smoke kept shifting to follow me. with my lungs it wasn't the best, but the smoke actually was mostly steam coming out.
as the ends burned and dried the covering soil it tended to cave in and self limit the amount of oxygen, but i also wanted to make sure it was getting hot enough to char as much of the materials as possible. using such a primitive thing as a trench, cardboard and dirt i wasn't going to get perfect results. i was curious to see how well it would do anyways. a more controlled setup like a double burner made of metal would probably give better results but that is beyond my current skills to build/operate.
after several hours of steaming and burning i finally buried the largest chunks remaining and called it a day. heavy rains that night and the next day put the trench under water and i was busy for a few days letting it dry out a bit before curiousity and time became available to do a little digging. after a few minutes i could tell there was a lot of change in how the clay and charcoal with plenty of water added was sticking to the shovel. actually, it wasn't. that was an immediate nice thing when it comes to clay. i normally spend a fair amount of time just knocking or scraping the stuff off the shovel. this made digging it all up much easier and faster.
i ended up with about 3cu feet of charcoal and as i was digging it out it mixed somewhat with the baked clay and clay around it. there was standing water at the bottom of the trench so that gives you an idea of how wet it was, but i didn't have much of a sticking mess problem at all. for that reason alone i would consider this a good thing... what it will do longer term we shall see, and if i keep making more and adding it that will also be interesting to see what happens.
with the charcoal/clay mix i made two test areas a few feet long and a little over a foot across separated by normal soil of about the same size and another part on the other side with some extra sand in it. then i ran four lines of different seeds through the whole area crossing the test patches, regular soil and added sand soil. after about a week it appears to not have negatively impacted seed germination for birdsfoot trefoil, alfalfa, green beans or buckwheat. i expected some change because it was so newly burned and it wasn't mellowed at all by adding any compost or aged or anything other than watered.
further through the season i will keep an eye on all of them and see how they do.
i'm unsure what i will do in the future as the burning took a lot of effort. a better means of doing it (covering with metal instead of cardboard) and then sealing it up with dirt would let me fire it, get it going and then leave it alone so i wouldn't have to spend so much time monitoring and fiddling. of course an oven with temperature and oxygen controls would be much better but that's unlikely to happen any time soon here.
the carbon in the charcoal should stick around for many years longer than the stems and sticks otherwise would have so this is a sure soil building tactic. adding compost and green stuff to it and then getting some worms planted into it would certainly make it even better. the charcoal itself will lighten the clay, i've already seen the direct evidence in that from digging it out of the trench.
grinding it up some would probably help too as there are enough larger pieces in there, but i'll let that go for now and let them break down as i spade and turn under materials as i normally would when done with a plot. the stripes of birdsfoot trefoil and alfalfa along the edge will be left alone for a multiyear observation. other than weeded i have no plans to turn or add materials to those plantings other than what grows there itself. unfortunately i didn't have enough material to set aside larger areas to use as controls and test plots or to have more variables to test. some other time perhaps.
i thought for sure that the charcoal and clay mix would be too harsh for worms but as i was smoothing out the areas to get them ready for seeding i did notice several worms in there that seemed ok. it being so wet and the clay probably well buffered or neutralized any harsh chemicals. if it was dryer and there was dry ash i wouldn't have expected to see worms that is for sure.
so it is possible to make charcoal in a low tech way and to sprout seeds in it within a few days. nothing really earth shattering in any of this, but interesting that the effect on clay was immediate. i could see why an ancient rain forest dweller with clay subsoil (as is common in rain forests) would notice it and want to use it for a garden/soil ammendment.
songbird