Ashes contain a lot of Calcium, but also some Phosphorus and Potassium, and other elements depending on the vegetal that was burned.
It is not recommended for plants that don't like alcaline soils, but "
slash-and-burn" agriculture has been practised for ages all over the world as a means of fertilizing soils.
A much more efficient method seems to be including not ashes, but charcoal to the soil: see for instance "
terra preta".
Well, if pipe tobacco is ejected from the pipe, it isn't usually totally burnt like cigarette ashes as far as I can remember: there's ashes, but also some partially burnt tobacco, and even some tobacco that hasn't been burnt at the bottom of the pipe, but should contain a lot of nicotine.
Nicotine is a natural, organic insecticide.
Ashes will provide mineral salts.
"Charcoal" will help assimilate nutriments and fight fungus.
So, smoking slowly kills, OK, but I'm not in a hurry, and I'm pretty sure "
pipe tobacco ash" is a mild, organic, beneficial addition to the cultivation of potted plants.