If those MiracleGro boxes have anything to do with it, I'm going to guess your soil's nutrient balances are a little off... It looks more like a nutrient burn issue to me than a deficiency. Typically, when my leaves burn at the tips like that it is due to a nitrogen burn. Cal-mag deficiency, for me, typically looks more like leaf-spotting than necrosis at the tips and edges. This is just my opinion, but I think your soil is too "hot"--i.e. it has too much nitrogen.
How often are you watering? What type of soil are you in? These things all matter!!
Happy growing to you!