Your hydrangea is lovely! I was wondering how it's doing now. On April 8th you wrote:
I usually water my hydrangea from the bottom, by leaving a film of water in the tray. The water "disappears" within minutes, although we have very high humidity. I'm a little concerned as the soil on top is starting to feel dry & sandy, while when I first brought the plant home, it is soft & moist. Hence, my question about the watering. Should I spray water from the top periodically to keep it moist?
I would water from the top down and let the water run through to the saucer. I wouldn't let it sit in water for more then 15 to 30 minutes. Then I'd dump any remaining water. I don't think there is enough water for it to all rise to the top. The entire rootball may not be getting enough water.
What is the reason for the fast drying, and how moist should I keep the soil?
I don't think there is enough water in the saucer and the top is not getting any. Hence may be the reason you are having yellowing leaves. I find the best way to check if there is enough moisture in the soil is to stick my finger in to the first knuckle. If it's dry, time to water.
I also have a question regarding fertiliser... is a spray once a week good?
When you say "a spray" I'm thinking you mean that you just spray the leaves. That could be done from time to time, but I would think that fertilizing the roots every two weeks during the growing season would be fine with the fertilizer mixed in the water.