Each year as the wet weather sets in, there is a narrow window where it is possible to burn off the accumulated branches from the preceding year, to keep the ground clear and mitigate against bushfires. This year, one pile was made with a lot of sand in it. And it rained soon after it was lit. So the result was a pile of charcoal under a crust of wet ash, that continued to burn slowly.
In this situation, the charcoal is formed under a condition of low oxygen potential. This situation should provide for a high surface area charcoal and it bears some similarity to the environment 0f the pit kilns that appear to have been responsible for the pottery chard-filled biochar of antiquity.
In the meantime, I had built two above-ground garden beds that I had filled with horse manure/hay. After a few days, these beds were beginning to show weed seed germination and one was infested with fly larvae. The beds had been boosted in phosphorous by the application of blood and bone, but there was no source of potassium in the mix.
So I decided to combine the two problems. My thoughts were that the hot charcoal/ash mix would kill the fly larvae and the surface weed seeds as well as providing some potash for the mix.
I headed off for the fire heap in the tractor. Splutter, splutter. Oh no, ran out of diesel. Added diesel but the battery ran flat without a start. Took out the battery and have it on charge. Fingers crossed that I will not have to bleed the fuel system.
So, I grabbed the wheel barrow, headed for the pile and the dead tractor and starting filling the barrow with hot charcoal.
I then wheeled the barrow up to the raised garden beds, which were made from rings cut from an old water tank, and added the hot charcoal to the top of the beds. I did a few loads like this. Then I dug the hot material into the top of the horse manure for a depth of about 200 mm. This produced a lot of steam and in effect sterilized the top layer of the beds.
My intention is to plant potatoes into the bed shown above. I will put a layer of sand in first and will top it up as the plants grow with further sand, as sand is freely available here. Last year, I used sawdust, and all my potatoes turned yellow and died! So, no more sawdust.
Before I plant anything, I will water the beds well though, as the initial watering will produce a very high pH liquid, potassium carbonate, that will need to be neutralised by the natural acids in the manure before the beds can be planted. That high pH liquid will be a further source of sterlisation for the horse manure.
Update, November 2010
The potato plants have all died off and I have begun to harvest them. They are all beautifully clean as they come out of the sand and just need a light wash to clean them up for cooking.