Automatic Fertilizer Dosing (Under $15)
 

For fairly apparent reasons, the soap bottle may not be the best option... that said, I'm sure you can easily find another option.

http://hackaday.com/2011/01/21/ferti...ener-hardware/

Intriguing! I would have never thought to use an air freshener.

What kind of timer/delay functions do those air fresheners have?

That's very interesting!

That hack a day post links back here: I just cant get enough of autodosers