First slow, down and dont try to fix everything. Go slow and you will have less mistakes. If you try to fix everything that comes at you, this all will over power you.
I use to feed Epsom salt to my veggies in the back, but for MJ plants I prefer to use MAGCAL & I mix it 25 ML per 20 GAL or 50ML per 20 Gal when you can see that it needs it.
Remember LESS IS MORE.
I'm a hydro kinda guy, but your doing soil so I would suggest getting some milk jugs and just mix the mixture per gal. Yes C02 can help but I dont personally think you'll need to use it right now.
Also what strain are you growing? Each Strain is different, each requires a different amount of attention -- but what I'd suggest doing is every 3-6 days feed with a mid dose of magcal and if you start to see your plants are wanting more, slowly adjust and give more...Its easier to give then to take back, once you feed it, no way to take it back.