Potting soil can be a dangerous fire hazard.

Did you know your potting soil is made of a lot more than dirt? The things that make up the contents can dry out and create heat. Potting soil can be made of shredded wood, bark or peat moss mixed with minimal amounts of actual dirt. Styrofoam pellets, perlite and vermiculite are some of the other items that are often added to the soil mixture for aeration and water retention. In addition, many mixtures also include different types of fertilizers. Some fertilizers are oxidizers, which will make a fire grow rapidly.
Many people plant in the spring and put the dead plants and pots pile in their garage come fall. What people don’t realize is that bacterial decomposition occurs within the mix that creates heat. What makes things worse is people think they can put out their cigarettes in a planter. If this happens, you can likely count on that pot catching fire. If that pot is close to your house, you can likely count on your house catching fire too.
So to be safe don’t ever allow anyone to put their cigarette out in your potted plants. In the fall, go around and discard your old pots properly. You can spread them in your yard or garden. If you do keep them in your garage do not leave them by any combustible material and get clay pots instead of plastic.