yeah , basically, my mate grew some potent skunk last year and he was ashing in the plants soil. so i have been doing this aswell, from when the plant was a seedling. even cigarette ash goes in their and it doesnt mind! i think this is because ash holds certain periodic nutrients and/or minerals that break down in the soil. it is a natural thing (from burnt trees in the forset which give nutrients to the soil) and ash from a joint also happens to be good for my plant! you should try it if you cant find your ash tray, the pot is an ashtray itself!