Nope. It’s not. No matter what people have been saying on the radio, in their gardening columns or on gardening forums. Soot and ash are not the same thing.
Soot is black and, for the most part, carbon. But the other parts can contain all kinds of nasties. Soot is one of the earliest documented carcinogens: back in the 18th century it was associated with scrotal cancer – an unusual cancer strongly linked with working as a chimney sweep. Soot is listed by the International Agency for Research on Cancer as a known carcinogen. More specifically, it is the compounds within soot that are mutagens, but as they are intrinsic to the soot, soot as a whole is classified as carcinogenic.
Soot particles are so tiny, it’s hard to imagine. They’re classed as being around 2.5 picometers small – you could fit twenty soot particles on a cross section of human hair. The particles are so small they can travel deep into your lungs and your blood.
Soot might bring some mineral nutrients to your garden. But it might also bring all kinds of other unwanted consequences. We would recommend avoiding it.
It’s true that soot did used to be widely used in gardens, and especially for rhubarb, because rhubarb benefits from the sulphur in it. However, it also used to be common to use arsenic, mercury and lead in a wide range of pesticides. The old ways are not always necessarily the best ways.
Ash, in the way we commonly refer to it, is the residue from a fire that remains in the hearth or stove – it’s usually pale-ish in colour. Soot is the black stuff that goes up into the chimney and out in the smoke. Anybody who’s experienced a bad chimney sweep will know just how pervasive soot can be – settling for weeks after the sweep has left.