How do I determine the cause of chlorosis in my Savannah Holly?

When we see chlorosis here, it's not so likely that there isn't iron in the soil, it's that the plant can't pull the iron up.

One of the best remedies for this is to loosen the soil at its feet, add a 2" (or more) layer of good screened compost and a cup of cornmeal and water it in. A nice layer of mulch would be a great finishing touch to protect against the soil getting too cool. You should see a marked improvement within a few days. This is also a good measure now as fall is one of the best times to put compost in the soil.

Best wishes!
Kathe

I agree but all the rest of the ingredients in the Sick Tree Treatment, especially greensand will help even more.