I have a friend who recently invited me to his house for dinner. Sometime in between, he had lit up a cigarette whilst we were standing on his balcony where he's kept a lot of potted plants. I noticed that he was throwing the ash off the cigarette onto on of the pots he was standing next to. At first i thought he was just trying to keep it off the floor but upon asking i found that this is something that he does often deliberately believing that it's beneficial for the plants. This is the first time im hearing of this. Is there any actual evidence of this being beneficial? For all i know, if ciggerate smoke is harmful then so would be the ash.
Ash is never a viable soil amendment. Agriculture used to burn fields of corn stalks, sugar cane or whatever plant material was left after harvesting. That is no longer a practice today.
Burning the plant material is a waste. If there was any nitrogen tucked away in the newly dead plant material such as bark chips, leaves...that nitrogen is being used up by the decomposers. Decomposers are always first on the scene and they need nitrogen to do their work.
Allowing plant material to decompose versus burning adds to the life of the soil. Soil macro and micro organisms need DECOMPOSED organic matter to use for fuel, energy. The decomposers are an entirely different group of organisms.
Ash does nothing for the soil. If anything it makes clay harder to manage. The only way to improve ANY type of soil is by the dumping of DECOMPOSED organic material on the surface (after a one time double digging to make a plant bed different than the surfaces we walk on).
The major problem with tobacco, the smoke, the ash, the residue on one's hands is virus. Mosaic Tobacco Virus.
This virus is horrible for many plants such as; Tomato plants, all solanaceae, nicotiana, Marijuana plants...no one should touch plants after smoking, no smoking allowed nearby and absolutely no smoking in greenhouses! No smoking tobacco in the home with house plants. Virulent virus. –
Abstract from one of the links I attached: Tobacco mosaic virus (TMV) is resistance to high temperature and able to survive over 10 years on dried leaves, and plant debris is considered as source of inoculums of TMV in the field. In order to inactivate TMV, TMV-infected cigar tobacco debris was composted at starting temperature of 50 ºC for two to three days; however, TMV was still infective in the extract compost. If a half leaf cigar tobacco 'H877' was inoculated with compost extract, the symptoms appeared as a necrotic local lesion (NLL) and did not develop systemic lesions. The dilution end point of TMV in extract compost was 10-3. The number of lesion was higher in the glasshouse with average daylight temperature of 32 ºC than in the field with average daylight temperature of 29-30 ºC. The number NLL was lower and NLL size seemed to be smaller on the first and second inoculated leaves with extract than that of on the first and second inoculated leaves with TMV inoculums. There was a delay of time about 58-106 hours after inoculation of NLL from extract compost inoculums to appear than those of from TMV inoculums. These could be happened because of mineral nutrients of compost and also the temperature of maintaining tobacco plant which inhibited the infections, and of a thermal composting process which destroyed some TMV particles, particularly degraded it’s coat protein.
You'll be an expert on this virus! You'll be able to explain to your smoker friends why they have to wash their hand and no smoking near your plants. Using your plants for a cigarette ash tray is not at all good for your plants. Humans have become a vector for this virus because of cigarettes and cigars.
Whether it's helpful depends a lot on what kind of plant you're growing.
Plant ashes in general have a lot of benefits to plants (which I have seen firsthand). They contain water soluble minerals that plants can use rather quickly. Wood ash contains a lot of calcium, an appreciable (but not huge) amounts of potassium, phosphorus and trace minerals; it also contains a lot of carbon (which some people think is good for the soil). Wood ash tends to contain so much calcium, in fact, that people use it to raise the pH of soil. Wood ash contains no nitrogen.
As stormy pointed out, Tobacco can carry tobacco mosaic virus. Various plants in the Nightshade family can contract the virus (e.g. tomatoes, peppers, tobacco, etc.); so, it's not a good idea to put cigarette ashes on any soil with these plants, and you probably don't want to put it on soil where you may plant those plants in the future, either. However, if the plants aren't in the Nightshade family, I don't see the harm in giving them the ash. If your friend was just putting it on a house plant, I doubt that it matters.
Tobacco ash is likely to contain a decent amount of calcium and other minerals, but I don't really know the exact profile of nutrients. I'm pretty sure only infected tobacco plants carry the virus; so, some cigarettes may carry it and some may not (I don't know what percent of commercial tobacco has the virus). Either way, I'd keep it away from tomatoes, peppers, and other stuff in the Nightshade family.
Plant ashes were used to help people survive when the British began to colonize Australia, since the soil was so poor.
I personally find wood ash useful for seedlings, tomatoes, transplants, and a variety of other crops. For seedlings, it helps the plants to be quite strong and it helps to keep obscure nutrient deficiencies away (of course, you don't want to use very much wood ash for that). I have used it on pre-transplant plants with weird deficiencies, and they did go away as a result (the plants greened up a lot, too).
I personally would not put cigarette ash on my plants, however, since I do grow a lot of Nightshade plants (but I don't smoke, either). I would also be concerned that the tobacco might be treated with other substances that may be harmful to plants.
One of the issues about wood ash, however (probably not so much about other plant ashes) is that they tend to contain a certain amount of heavy metals (the levels aren't particularly alarming, but if you use a lot of wood ash, the heavy metals may accumulate over time). Now, dynamic accumulators of heavy metals (e.g. sunflowers) will probably also have higher levels of heavy metals in their ashes.
I've met some people that are pretty biased against wood ash. They have their reasons, but for me, I find it to be a nice soil amendment, if used judiciously. One of the main arguments against it seems to be that there's no guaranteed level of certain minerals in it; so, using it is more of an art than a science. Some people think it can mess up your soil by raising the pH too high, and while it can raise the pH, it takes a fair amount of the stuff to make it so you can't grow anything in it (you don't need to use very much to use it as a soil amendment).
Other than wood ash, I haven't seen a lot of information online about plant ashes being used as soil amendments. Suffice it to say, though, they contain nutrients that were at one time absorbed by plants (granted they may be in different forms than they once were, due to the burning process, and some of the nutrients may be absent, like nitrogen).
I don't know whether nicotine survives the burning process, but that's something you may want to know.
I don't believe that smoke bothers most plants, in moderation (humans are a different story); obviously, if they're covered in ash, that may block the sunlight. However, I don't know if tobacco mosaic virus can be transported on the smoke (but I kind of doubt it). I know smokers with tomatoes, peppers, etc. who have never had the virus in their garden. (But I don't endorse smoking.)
Wood ash, and/or charcoal, is the only ash I'm aware of being used as an amendment. The ash from a wood burning stove can be used to adjust PH of soil and add potassium / potash. Charcoal is similar to biochar, which is purported to support beneficial microbes and bacteria. But both should be used only if they come from clean wood, and sparingly.
Cigarette ash is likely to also act as PH modifier, but may contain trace chemicals since cigarettes contain many ingredients other than just tobacco. It probably isn't anything you'd want to add to edible plants. Ornamental plants may not mind it, and it might offer a small amount of potassium--but likely less than wood ash, and extremely little compared to most any plant fertilizer or compost.
So overall it could provide some benefits, but toxicity is a valid concern. Cigar ash might provide similar benefits with less risks, due to fewer additives. However, relative to nearly all other options tobacco ash is likely to be a poor amendment.