Clay pots aren't the best for growing weed. They hold fertilizer salts in them and mess with your ph as old stuff leaches back in the soil. Lots and lots of new growers have had issues with both MG soil and their nutes. There are better alternatives out there. The MG soil has enough nutes in it to sustain those plants for a few weeks. Adding extra can cause even more problems. Yellowing leaves is a clue something is amiss, though. I use fish fert. for a quick nitrogen boost for yellowing. This is assuming that is your problem and not nute lock. The learning curve seems steep, but you will pick up good info along the way. Keep a journal, organize the topics and take notes.