Citrus trees thrive in warm areas where they are not affected by frost or freezing temperatures. In addition to keeping citrus trees warm, gardeners must also keep them healthy. Citrus trees that show signs of disease or damage need a gardener to diagnose and treat their problems to encourage a healthy harvest.
Nitrogen deficiency is one of the main causes of yellow leaves on a citrus tree. Nitrogen is a nutrient used in almost every process of life for citrus and fruit trees, from growth to the production of fruit. A lack of nitrogen in the soil will affect the tree's health, beginning with the leaves. Add a nitrogen-rich fertilizer to the soil early in the spring to prevent nitrogen deficiency later in the growing season.
Like nitrogen, iron is a vital nutrient for citrus trees. Citrus trees that lack iron in the soil will produce stunted, small and yellowish leaves that weaken the tree due to their lowered capacity for photosynthesis. Adding iron-rich fertilizers or compost to the soil around the citrus tree helps increase the soil's nitrogen content and improve the health of the tree. Compost and fertilizer should be applied in early spring.
Drought or a lack of water turns the leaves of citrus trees yellow. Citrus trees grow best in moist and well-drained soil, as well as full sun; direct sunlight dries out the soil more quickly, which means the trees need more frequent watering to remain healthy. Water citrus trees once a week, or more frequently during dry spells and summer, to keep the soil damp and the tree's leaves green.
Pests like aphids, spider mites and scale insects feed on the leaves of citrus trees, draining them of sap and nutrients and turning them yellow. Additionally, pest insects often spread disease or leave citrus trees vulnerable to fungal and bacterial infection. Regular pesticide sprays throughout the growing season of citrus trees prevents pests from damaging the leaves and turning them yellow, and it lowers the likelihood that the tree will contract a disease from the pests.