what's the ph? looks like you have a ph prob to me. do you ad dolimite lime to the soil mix? an absolute must imo! the way the yellowing leaves have brown spots in them is one of the things that make me think it's ph issues. test the ph and post the results. without that info noone can make a definitive dx./rx. imo. keep us posted, i'm curious to see if my suspisions are correct.