Rhododendrons belong to a group of plants known as ericaceous, which will suffer if there is any lime present in their soil – it prevents them from taking up certain important nutrients such as iron. The main symptoms are yellow leaves and poor growth. Give your plant an immediate tonic with a dose of J Arthur Bower's Sulphate of Iron then feed every six weeks during the growing season with J Arthur Bower's Azalea and Heather Plant Food. If your soil is limey, you would be better to grow your rhododendron in a container of ericaceous compost. Other ericaceous plants include azaleas, camellias, blueberries and heathers.