Soot used to be something most people had access to and was usually used on the garden. Up until the 1970’s coal was the most common fuel for heating in town houses and burning coal produced soot.
The chimney sweep would normally offer the householder the soot from the chimney and any from earlier calls to use the soot in the garden. The coal man and the chimney sweep were part of the fabric of town life, just like the milkman.
As the cities grew in size all the coal fires and their smoke became a problem. As early as 1853 an act of parliament was passed to address the problem – Smoke Nuisance Abatement (Metropolis) Act 1853. 100 years later in 1952/53 the ‘Great Smog’ hit London, claiming 12,000 lives due to the coal burning smoke.
This was followed by various clean air acts making the burning of smokeless fuel compulsory in urban areas which increased the price of coal based fuels.
The writing was on the wall for coal and by the mid-1970s cheap North Sea gas started coming in and people were installing gas central heating which was far cheaper, cleaner and less trouble than coal.
As coal became less popular, the availability of soot fell away and its use on the garden faded.
Benefits of Soot on the Garden
My grandfather, who was a coal miner and had subsidised coal as a benefit of the job, used to use the ashes on garden paths and the soot on the vegetable plot.
The main constituent of soot is carbon along with various chemicals from the coal burning process. It contains no major nutrients (NPK) and, as far as I know, no useful micro-nutrients of help to plants.
What the soot does do is to darken the soil. It was stored away until early spring and, on a day without wind, spread onto the soil. Being darker, the soil would absorb more heat from the sun and so its temperature would be a little higher. This would help bring the season forward a bit.
Lawrence D Hills in his Grow Your Own Fruit & Vegetables recommended the use of soot and salt on the onion bed. The soot presumably to warm the soil.
Hazard Warning – Coal Soot
It was noticed that chimney sweeps, usually boys who would be forced to climb inside the chimneys from as young an age as four developed a particular form of cancer on the scrotum. This usually presented post-puberty to their twenties.
Remember, these children were exposed every day, even sleeping under sacking covered with soot and washing was rare.
The risk to the gardener using coal soot is most probably minute. Soot from wood burning fires does not carry the same risk.