Answer: I don't believe that iron is the problem. Your boxwoods are suffering from a root-related stress. I could be that a drought stress or soggy soil has brought on the problem. However the cause is probably one of two root attacking organisms: nematode damage to the roots, or a fungal root wilt disease. Neither is something that you can control or cure. You should not replant boxwood in that location if it is one of these two culprits.
To find out for sure which it is, dig a plant in the last stages of decline (don't wait until it is completely dead) and examine the stem and roots. The fungal disease appears as dark brown/black streaks between the bark and inner wood when you slice down the stem lengthwise with a knife. Nematodes appear as swollen areas of the roots.
Depending on what you find you can make a good choice in selecting replacement shrub species for that area.
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