Just read this article on www.madaboutgardening.com
There are many uses to which soot can be put in the garden which were widely known decades ago, but which have passed out of use lately. The soot that is good to use is from wood fires. Never use oil soot in the garden.
Soot is the most valuable fertilizer for many kinds of plants. It imparts a dark color to the soil, which assists in the absorption of heat and so renders it more suitable for early crops. When applied to the soil in spring it is changed by bacteria into nitrates, in which form it is available as a plant food. Nitrates increase growth so that soot may be applied to ground on which lawns, shrubs or flowers are to be grown. These plants require large amounts of nitrates to enable them to grow quickly and healthily.
While many of the uses to which soot can be put are widely known, there are one or two points not so familiar to the average gardener. Soot, fresh from the chimney, contains about 12 percent water, 35-50 percent ash, and the remainder various volatile substances which are rich in ammonia and which form the chief fertilizing properties. The ash contains calcium, iron, magnesium, potassium and sodium, the first four of which are valuable plant foods, as they are combined with phosphoric and sulfuric acids, while silicates are also present.
If kept dry and allowed to stand for about three months, soot becomes mellowed, when it safely can be used as nitrogenous manure, in powder or liquid form.
Soot can be used to scatter along the rows of onions, carrots, turnips and radish to prevent root worms or maggots.