Help with Yellowing and Brown Spots - No Clue

Can anyone help me diagnose what might be wrong with my plant? The plant has been outside since birth on April 1st and potted in Fox Farm soil. Within the past week I have begun seeing the brown spots on the middle leaves and the yellowing of the bottom leaves. The growth on the top and the new growth seems pretty healthy. I had been giving 15-30-15 nutes every other week with watering, which I have been doing 1/2 gallon once a week. You can see the top of the plant seems fairly healthy and as you work your way down its worse. Any help would be much appreciated. Also, am I watering too little? 1/2 gallon a week, plus whatever rain we get? Thanks.

Few more pics, Thanks again for any help.

looks like you got nutrient burn, make sure when you add your fertalizer that you flush it through so you get a lot of run-off. if you dont the nutrients the plant doesnt use will build up in the soil and change your pH and burn your plants as well. if you get a PPM meter you can test the water run-off so you dont have to guess. you could have your ppm at around 600-800
good luck