Some of the lower leaves on my Hydrangea plant are going very pale, a kind of yellowy white. I've read this could be due to a nitrogen deficiency. How do I rectify this?? My soil is very, very quick draining and sandy, so with all this hot weather and the Hydrangea being in the full sun until around early afternoon, I have been watering it daily because it has been wilting. So, I'm guessing this has depleted any nitrogen there was in the soil, which has resulted in the paling of the leaves? Should I add something to boost the nitrogen??