The local chimney sweeps here in Hastings leave bags of soot at the gates to our allotments. This is usually in the autumn as that’s when people get their chimneys cleared before the winter sets in.
The longstanding tradition is to leave the soot to ‘weather’ for about one year. This ensures that polyaromatic hydrocarbons and other substances that could harm plants have time to dissipate.
The UK Biochar Research Centre are doing trials with biochar (horticultural charcoal) and so adjust for the presence of black carbon as soot in order to validate their test results. Soot, once it has weathered, is a fine particle size version of biochar and another way to help build the carbon framework in soil that encourages microbial activity and better soil structure.
http://biocharm.wordpress.com/08-other-sources-of-biochar-in-the-soil/
As founder of Carbon Gold I am self-confessedly a biochar enthusiast but I have been using soot since the early 90s, without really understanding until a few years ago why it had functional benefits until I learned more about biochar http://www.carbongold.com