Leaves of plants typically turn yellow for a variety of reasons, which may include nutritional deficiencies, poor drainage, compacted or damaged roots, overwatering, insufficient light and viral infections. In addition, leaves often turn yellow and drop off of house plants simply as a result of age. By investigating one possible cause at a time, it is possible to deduce the cause of yellowing plant leaves and, often, to correct the situation.
The first step to diagnosing the cause of yellowing leaves is to check the roots and drainage of the plant. If the plant isn't draining well, or if an inspection shows damage or compaction of the roots, then repotting, tilling the soil or mulching may be the solution. If poor drainage is evident, then cutting back on watering should also help the plant's health.
The most common nutritional deficiency causing yellow leaves in house plants is lack of iron. If the yellowing begins on younger leaves and gradually progresses to older leaves, chances are lack of iron is the cause. Yellowing that begins first on older leaves may be due to deficiencies in manganese, nitrogen or zinc. Testing the soil's pH level is a good first start to diagnosing these deficiencies.
If yellowing occurs only on the side of the leaves facing away from the sun, the cause may be a lack of sufficient light to those areas. If a virus is causing yellowing, the plant probably also shows other signs of ill health, including deformed leaves and flowers. In this case, there may be nothing that can be done to save the plant.