Rhododendron leaves turn yellow because of sun exposure, according to the Victoria Rhododendron Society. Sunlight tends to bleach exposed leaves. When this occurs, adding nitrogen helps turn the leaves green again.
The Victoria Rhododendron Society adds that older leaves naturally turn yellow, and they are replaced every one to three years. When the leaves are yellow but the region along the veins remains dark green, the plant likely suffers from chlorosis, which is caused by a magnesium, nitrogen or iron deficiency. In this case, the soil’s pH must be checked. A very high pH hinders the plant's ability to absorb necessary trace metals. If the pH is close to 5.5, adding Epsom salts is a recommended solution. If this does not work, it helps to add a nitrogen source and chelated iron.
Chlorotic leaves are usually a sign of iron deficiency and can be dealt with by using iron sequestrene, according to the Millais Nurseries. Chlorosis is also caused by poor conditions, such as drought, soil pH or drainage problems. Over-fertilizing must be avoided, as the edge of the leaf will likely turn dry and brown. Chicken and farmyard manures are not advisable as fertilizers, because they contain too much nitrogen for rhododendrons. Similarly, bonemeal must be avoided, as it has too much calcium.