Yellowing leaves can be a manifestation of different plant-related problems. Determine the cause before treating the plant with the first thing you can get your hands on. Check for the presence of bugs on leaves, branches, and roots. See if the soil needs some iron-rich fertilizer. Assess if the plant is receiving adequate sunlight. Check the soil pH, water alkalinity, and if water is draining correctly. After pinpointing the cause, act accordingly.