Following a plea for soot advice from aged tomes by Flange member VP I can provide this from The New Illustrated Gardening Encyclopedia by Richard Sudell, F.I.L.A., A.R.H.S., (the book is not dated but the inscription is 1932).
"There are 3 reasons why soot is valuable to the gardener: it contains a little nitrogen; it is a good insecticide; it darkens the soil surface, and therefore makes the soul warmer by retaining solar heat. Soot should always be stored dry and can be used in making liquid manure, or as a surface dressing along the rows of growing crops, either alone, or mixed with lime. If mixed with lime it makes the best insecticide, as the fumes are objectionable to insects of all kinds."
He also has a diagram (see pic) to show how to make soot-water - a sack full of soot, plus a brick to keep it submerged, hung from a rope in a butt of rainwater - with light excluded.
Practical Home Gardening Illustrated (1949) by the same author adds no more information but does include a very similar diagram in the month of March. The diagram bears the legend "Soot water stimulates pot plants", and I'm unclear whether March is time to make, or to apply soot water.
It does have rather lovely pictures inside the cover of Garden Friends & Foes which I’ve posted for purely gratuitous reasons.
My copy of The Complete Gardener first published 1950 (and heavily reprinted) by W.E. Sherwell-Cooper (a man with a formidable array of credentials after his name) has this -
"Soot is a nitrogenous manure which darkens soils and so enables them to absorb and retain heat better. More generally used as a top dressing in the spring. Suitable chiefly for all members of the cabbage family. Usually applied at 5ozs. to the square yard."