It is always a good idea to have a soil test done before determining whether or not fertilizing is necessary. Soil acidity can be altered to some degree without the need for ferts. Clay soils do bind nutients but not necessarily to the extent the nutrients become unavailable - rather the binding prevents nutrients from being leached out or washed away with rainfall or heavy irrigation. A clay soil is not necessarily an infertile soil. If you do determine the need to fertilize, a single acidifying fert is fine for all those plants. These tend to to be only mildly acidifying and since the vast majority of plants prefer slightly to moderately acidic soils anyway, they work well for virtually any broadleaf evergreen or coniferous shrub/tree and most deciduous shrubs as well. As long as you select a low nitrogen fertilizer (2nd and 3rd numbers higher than the first) a late season/early fall fertilization application is the most advantageous. The roots of woody plants are in their most active growth stage at this time and nutrients are most easily absorbed and metabolised. What you don't want to do is promote a lot of leafy or stem growth that a high N fert would encourage and that would not have sufficient time to harden off before winter. The attached link has a very good synopsis of why it is often necessary to fertilize woody landscape plants and the best time to do so. FWIW, fall fertilizing of woody plants is a pretty well-accepted horticultural practice. Here is a link that might be useful: fall fertilization

The Encores aren't continuously blooming, it's more like three times per year. Like traditional azaleas, they begin by blooming each spring. Once this initial blooming ends, new shoots begin to grow and set buds. Then blooms emerge again in mid-summer, then early Fall until first frost. Yours may not bloom to their full potential with just 2 1/2 hours of sun. Both drought and soggy roots could cause yellowing leaves and leaf drop. You'll need to check the soil near your plants, when it's dry to a depth of a few inches - provide water. I have a clay base too, wind, but much less heat and I water slowly and thoroughly every 7 - 10 days in months we don't have rain, for established plants.

My first impression is that the dying one is dry. If the root ball dries out before planting, sometimes it is almost impossible to get them to absorb water. One root ball may have dried out and the other didn't. Even though you water a dry root ball it may not be getting any of the water. Rhododendron and azalea guru Harold Greer noted: "Quite often a plant will get completely dry and then no matter how much water you apply, the rootball will just keep shedding it. The top of the soil may seem wet, and the soil around the plant may even be very wet, but the actual rootball of the plant is bone dry. This is especially true for newly planted rhododendrons, and it is the major reason for failure, or at least less than great success with that new plant. It is hard to believe that a plant can be within mere inches of a sprinkler that has been running for hours and still be dry, yet it can be SO TRUE!" Unfortunately the other possibility is too much water. It will kill a plant.

You, too, and good luck with your project. I've certainly thought about soil testing, and have always been stymied by not knowing who could do it. And the answer is idiotically simple: go to our local nursery and ask. Instead I'd rather do it the hard way (and it is hard). I never claimed to be a smart gardener. About my soil amending and fertilizing practices: I was much influenced by a book, 'Teaming with Microbes', that was recommended to me by my horticulturalist sister. Basically it talked about the importance of establishing a thriving soil microlife that then nourishes the garden plants. The gardener establishes a virtuous (as opposed to vicious) cycle in which the microlife supplies nutrients to the plants, which grow, die, and the detritus of which feeds the soil microorganisms which recycle the nutrients. Getting the process started is the hard part, in land like ours which is poor in organic matter--hence all the holes and amending--but which, once the cycle gets going well, can pretty much be left to itself. We have so much land to turn into garden that the work is enormous. But I look out my living room window right now at the garden outside (it used to be the farm courtyard), and I think, this works. I don't fertilize my two-story tall 'Jaune Desprez', the substantial shrubs of pittosporum, sarcococca, myrtle, the hardy palms and Tea roses. The bed gets pruning clippings and plant debris, and everything grows fine. Of course we have our clay soil. But if you look at Florida, for example, much of it has very poor sandy soil. And yet jungles grow there. The nutrition isn't coming from the mineral-rich soil, and not from any added fertilizer. It's due to the recycling of nutrients.