It's normal for some leaves of citrus trees to turn yellow in the winter and then drop to the ground. If your citrus leaves are turning yellow and dropping during the active growing season (March through October), this condition can indicate poor nutrition or disease---take care of this situation as soon as you can. One of the most common causes of yellowing leaves is a condition known as yellow leaf chlorosis. You can treat this condition easily and return your citrus tree to vibrant health.
Making Citrus Trees Green
Reduce the amount of water your tree receives in the winter if yellowing leaves are a concern during that time of year. Citrus trees need only about one-tenth as much water in the winter as they do in summer. If your tree is planted in the ground and you receive a large amount of winter rain, you can set up an umbrella type of situation to keep the excess water off your tree.
Give your tree the correct conditions it needs to remain healthy. These include regular watering, good drainage, proper soil pH and pest control. Citrus trees favor a pH between 6 and 8: to increase pH, dig in hydrated lime; to lower it, add organic sulfur.
Fertilize your tree regularly with a balanced citrus fertilizer beginning in March and then repeat the application every other month through summer.
Prune dead branches to encourage new growth, which normally comes in several "flushes" during the tree's active growing season.
Treat yellow vein chlorosis with chelated iron, available at nurseries. If your tree has this iron deficiency, the leaves will appear yellow with dark green veins. This condition is common where soil is alkaline. Follow label instructions by spraying the entire plant---expect to see improvement as soon as three or four days after you spray your tree.