Answer: This time of year, it may be natural for some of the oldest leaves to yellow and drop. Although evergreen, citrus tree leaves do age and are naturally shed. As long as your tree looks otherwise healthy and is putting out new growth, a few yellowing leaves do not indicate a problem.
However, if all the leaves, old and new, are yellowing, there may be a problem. Both over- and underwatering can result in leaf yellowing. So, making sure the plant gets sufficient water but dries out just a little between irrigation cycles is important. It could be that the plant is running low on available nutrients--magnesium in particular will result in leaf yellowing in citrus plants. Applying a liquid fertilizer on a regular basis to replace those nutrients lost through heavy cropping will help. Use a specially formulated citrus fertilizer and apply in amounts as indicated on the label. Typically this will be 3-4 times a year. Be careful not to use more than the fertilizer package recommends.
Best wishes with your orange tree!
Q&A Library Searching Tips