Plants such as azaleas, rhododendrons and camellias love soil which is acid-rich and will struggle to grow if your soil is more alkaline.
You can get around this issue by using an ericaceous compost, which is designed to introduce an acidic pH level into your soil.
Gro-Sure Ericaceous Compost is perfect for acid-loving plants, contains iron for greener foliage and 4 months feed.
You could also use Westland John Innes Ericaceous Compost.