Kerex iron application made to the soil after growth has already begun will result in yellow leaves on the older leaves while the new leaves coming out after the application will be green. Q. I have a Burgundy plum tree that is 3 years old. The leaves became yellow in March so I applied Kerex iron to the soil when I first noticed the problem. Now the new leaves do not show any yellow in them but older leaves are still yellow so I do not think it is an iron problem. A. Iron does not move once inside the plant. It is still an iron issue. You said the NEW leaves do not show any yellow. That means your KeRex application in March worked for the leaves which grew after your application. Iron does not move around inside the tree once the plant takes it up, it only goes into the new growth after the application is made.We say that iron is immobile once inside the plant. It is not like nitrogen which can move from older leaves to newer leaves. Iron cannot do this. Iron can only be transported into new growth. Growth before the application will still stay yellow. Correcting the problem. The only way to correct yellow of older leaves is with iron spray applied to the leaves. This is why SOIL applications of iron are so important to make in January before growth occurs. An example of an iron product that can be used for foliar applications, or sprayed on the leaves. Spraying the leaves with an iron solution is the only way to correct yellowing once it is already started. Make sure you use a wetting agent mixed with the iron spray. EZ Wet is one example of a high quality wetting agent that does not contain any personal care products and made entirely from yucca extract. It is not too hot too late to spray iron on the leaves. It may take four or five sprays a couple of days apart to get all the yellow leaves completely green. Any iron product, labeled as a spray applied to the foliage of fruit trees will work. However, I would strongly suggest that any water mixed with this iron fertilizer should be distilled water so that the pH is close to neutral (pH of 7). Alternatively, you can adjust the tap water close to neutral using vinegar or another mild acid as well. You should add a wetting agent to the spray as well. This helps the iron penetrate the leaf surface and enter the inside the leaf. Otherwise the spray is not as effective. Some people use liquid detergents with good success. I worry a little bit about the other personal care products in liquid detergents so I would encourage you to use a commercial wetting agent. If you are still unsure how to do this, you can read more about how to do this on my blog or email me at [email protected]