And, I know that Azaleas like acid soil. Should I put a cupful of sulphur around them too?

One of the bushes is mostly flowers and no leaves. Would some nitrogen around that one help?

There are several reasons for leaf drop in azalea - they can be sparse and leggy in too much shade, thrips can cause leaf loss as can drought, so fertilizer isn't always the right approach to the problem. I haven't had reason to fertilize my own in my naturally acidic soil, but Holly-Tone is the product often recommended for azaleas and rhododendrons in areas with less than perfect ph, it's 4-6-4 and contains the sulfur, rarely are larger amounts of nitrogen recommended.

Be careful too if you are thinking of cultivating around those surface feeding roots, it's easy to damage them. I find a top dressing of compost the best way to condition soil, slowly feed azaleas, conserve moisture.