Yeah I checked that pic... that plant is burned too.. check the brown leaf tips. Also there is a serious magnesium deficieny as you can see the pale banding between veins. In your situation with the burn I am going to say it's not def, but locked out by either overferting or poor pH control. And the miracle gro soil is a good culprit for overferting, and the 20-20-20 ferts have too much of the P and K relative to the amount of N you are using during the veg stage.
So anyway give that plant a really good flush with pH adjusted water and lay off the ferts, and you could foliar feed with 1/4tsp epsom salts in 1 gallon of water as a stopgap measure, but don't get any in the soil because you are already burning.