Sorry to hear of this problem, Susie VanEmburgh.
Many plant nurseries sell some cheap soil pH kits that will help you determine if the soil is acid or alkaline but, if it is, it would be in the extreme if it is killing everything in there. Cement leeches lime which makes the soil alkaline. Hydrangeas tolerate some alkalinity but you have not mentioned anything regarding symptoms of alkalinity so perhaps, a formal soil test would be even more useful as it would also look for issues with others minerals or with chemicals that others have drenches into the soil... I too recommend it that test (plant nurseries or your local Agric Extension Service might be able to direct you).
And sorry for all these questions but, I was wondering if you could shed some light in a few items that I was thinking of... Answers might help me or others get ideas...
What kind of hydrangea is this? There are several types around: H macrophylla, H quercifolia (aka, oakleaf hydrangea), H. arborescens, H. paniculata, H. serrata, etc. It would also help if you know the exact variety such as H. macrophylla Endless Summer for example. If you need help with this question, browse the information on this link:
http://hydrangeashydrangeas.co...
Can you post a picture of the plant or a link to a picture of the plant?
Can you describe what symptoms make you say that it is 'dying faster and faster every day'?
You said that it gets 'plenty of sun and water'. How much water does it get per watering and how many often is it watered? If you insert a finger into the nearby soil to a depth of 4", how does the soil feel? dry, almost dry, moist, wet? Is the plant mulched? How often do you fertilize and what fertilizer do you use?
Do you have nearby a Walnut Tree (specifically Black Walnut or Juglans nigra L.) or a Butternut (Juglans cinerea L.)? These trees/shrubs produce a substance called juglone that is toxic to other plants. Juglone can be found in the all or some of the parts of the trees: leaves, roots, etc.
When was the hydrangea planted in this bed? How long have you owned this house? How long have you been unable to grow plants in this bed?
