Is it possible for you to post a picture? It would be easier for someone with knowledge (not me, sorry) to help you.
In the meantime, I wouldn't take desperate measures like adding anything, instead, check the soil around the roots for moisture or lack of it, if there's a bug, etc.
Best luck!
Carol

Could it be spider mites? I'm having the same problem w/ one tomato--the one getting a little shade. The other peppers and tomatoes are fighting off the invader, apparently.

How long has it been since you transplanted? If recently and relatively few lower leaves are involved, I would call it pretty common.

It has been about three weeks since I transplanted. I tried from seed, but they did not take, so I bought these plants at a reputable nursery, which I have been going to for years.

Could it be that the soil is draining too fast?

I would think the plants should have taken hold in three weeks. Assuming you see no insect pests and the plants are getting sufficient water, I guess it could be draining too fast and taking nutirents with it. I have seen that happen with really sandy soil but why aren't the beans and cukes affected. Maybe the problem is that my experience is with the dirt that came with my house and I know squat about licorice root. You might try to get some nitrogen to the plants with the recipe you mentioned or some water soluable Miracle Grow.