Question
We have a young (7 in. dia. trunk) live oak in our yard (Florida) and have noticed it's leaves are slowly turning more yellow than all our neighbors trees. Does it need more/less water or a specific fertilizer?

Answer
Yellow leaves and green veins are often the result of a common problem called iron chlorosis. This is the result of either an iron deficiency or iron unavailable for plant uptake.


Iron plays an important role in producing chlorophyll, a green pigment involved in the absorption of light needed for plant growth. A deficiency in iron results in a reduction in chlorophyll producing a yellowing of the leaves. This is an early sign of iron chlorosis. On deciduous trees, the leaf veins remain green while the area between the veins turns yellow-green to yellow. Leaves that emerge in early spring are frequently normal size, while those that emerge later in the growing season are often smaller and more yellow. In severe cases, the leaf color may change from yellow to white and finally brown. In advanced stages, twig dieback may be observed, especially toward the top of the tree and the ends of long branches. After several years, the tree may die if the condition is not treated.

Iron chlorosis is most often caused by iron not being readily available for plant uptake. Iron levels are usually adequate in most Kansas City soils. However, high soil pH makes it unavailable for plant uptake. In alkaline soils with a pH above 7.0, most plants are not able to take up iron. To check the pH of your soil, contact your local extension office for instructions on soil testing.

Once iron chlorosis is confirmed, several methods may be used to provide iron to affected plants. Here are several ways to treat for iron chlorosis.

Foliar application - If a rapid response is needed to correct a chlorotic condition, a foliar spray with iron sulfate or iron chelate solution may be applied when the tree is in full leaf. This treatment is not very effective however. It is only temporary. New growth that emerges will be chlorotic.


Soil treatment - Since iron chlorosis is often the result of high pH or alkaline soils, lowering the pH would be the most reasonable solution to prevent chlorosis. It is difficult however to lower soil pH at depths of 18 to 24 inches. The best approach is to add iron to the soil under the tree canopy. This treatment remains effective for two to three years.


I would then follow this with a application of fertilizer (10-10-10) about 1 lb per inch of trunk diameter scattered under the tree and watered into the soil.