Lower leaf discoloration: mag deficiency??

Hi! Yes it looks like a magnesium deficiency but if you are growing in soil your pH of 5.9 is slightly low and it might cause lockout of nutrients unless you are using chelated nutes 'cos I've read that they are usable no matter the pH. One way to deal with this is to give some epsom salt (Magnesium and sulfur) as a foliar spray. This way you don't have to change your nute mix or anything and it is much faster way than to add magnesium to soil and wait for the roots to suck it up and transfer to leafs. Of course if your nutes are constantly lacking magnesium you can add that epsom salt to your watering routine too. Epsom salt is slightly acidic so adding it tothe soil might raise the pH a littlebit which is a good thing in your case