I listed the soil amendment biochar
http://en.m.wikipedia.org/wiki/Biochar
So you would better understand the purpose.
As you smoke your herb your left with black carbon, especially if in an anaerobic environment. If you keep your lighter to the bowl the carbon burns away and your left with white ash..which is actually quite literally the nutrients left over that dont burn. If you get into organics it leads to sustainable agriculture..
(I dont smoke cigs)
But phenols present in ash and trichomes will remain present in the soil for a few years. This biochar is a great source of carbon (c/n ratios eventually become more important)
The ash..really its not ash..but yea, is a great amendment especially soil tilth and moisture holding qualities, but is filled with potash and calmag and does raise ph. The soil holds more water, nutes and microbes. Like the roots and bone meal there are hormones present that effect the plant.