Heathers generally require an acid, ericaceous, base, with little other feed, surviving happily on poor acid based soils, and will even withstand some dryness, though in Scotland it rains and rains, but the soil is very well-drained!! If your heathers are looking sparce and weak, then it is possible they need some ericaceous base, and you could water-in some sequestered iron, available from your local garden centre.