I water these potted plants almost every day, since the soil seems to dry out quickly. I use Miracle Gro about once a week.
Tips on the Leaves of My Petunias Are Turning Brown
You're using too much fertilizer; plants in pots need to be treated very differently from plants in the ground.. Fertilizer should be used at 1/2 or 1/4 strength, and no more than once a month. You can run water through the pot (5 times the volume of soil) to wash out excess salt, which comes from a build up of unused fertilizer, and don't fertilize again for 2 or 3 months. Also it would probably be good to increase the light if you can. Potted petunias need several hours of sun a day, if ;possible.