Help! Several different flowering plants have brown-edged leaves in my flower boxes. What is this from?

Question

I have several different types of flowering plants including geraniums in flower boxes. Almost all of them are developing brown-edged leaves. The only thing I put in the flower boxes is water and every two weeks some liquid fertilizer in water. I'm so disappointed. I've raised them all, except the geraniums, from seeds. I've used potting soil to plant them.

Does anyone know what this could be from? Is there anything I can do about it?

Thanks, Ann

Answer 1

possibly over-watering...

Answer 2

Brown Spots on Flowers - Leaves Darken on the Edge
http://www.optimara.com/doctoroptimara/20922-21604/21404.html
There are several possibilities for the cause explore the link above.

Edit:
was your potting soil the kind that already had the pre/fertilizer in it?
Possible