Hydrangeas need very little fertilizer and your soil seems to be providing them with what they need! That is great!

But I would add some fertilizer in June and then leave it at that. You can add a cup of manure, cottonseed meal or use a slow-release general-purpose chemical fertilizer. See the link below for fertilizer info; scroll down to the section titled Fertilizing Hydrangeas. Do not fertlize in any way beginning in September so the plants will go dormant on time.

Here is a link that might be useful: Fertilizing Hydrangeas

Thanks for the info.