It's not so much that the soil pH has dropped as that it was probably low to start with. Did the label on the soil bag say if it contained any lime? If not, that is what is needed. Dolomite lime mixed into the soil before planting will help to buffer and stabilize the pH. Since they are already planted the best way to raise it quickly is using hydrated lime. Just be cautious as hydrated lime can cause root damage if too much is used. If you can find some, let me know and I'll direct you on how best to use it.
Regarding the nutrients, that depends on how much "Organic Growth Stimulant" is in that soil mix. If it's only some starter nutrients, then you may need to feed when they are about 2 weeks old. If it's heavy with things like worm castings, guano or manure, then you probably won't need any nutrients for 3-4 weeks.
