Replace soil in brick planter
Hennepin County Minnesota
You didn't mention what the "soil mix" and fertilizer were. Or how you cleaned up the planter before you used it. I could see several potential problems: soil not appropriate for a planter, too much fertilizer, watering issues, disease carry-over in the planter, and so forth.
I'd recommend starting over. It is plenty early to get something going. So dump the mix you've got, clean out the planter (use a 10% bleach solution), replace with a soilless potting mix (many products around), fertilize (if necessary - some potting mixes have fertilizer already in them), and replant. Read here: