Younger leaves: Yellowing between the veins

Plants can tell you a lot, especially when they are not happy. There are some tell tale signs when you read the leaves.

Plants with the younger leaves exhibiting interveinal chlorosis usually denote that iron is unavailable. A little investigation is needed to make sure the exact cause. Interveinal chlorosis can occur due to many factors.

The substrate pH is too high, generally above pH 6.4. Check the substrate pH to confirm. (High probability)
Iron is not being provided in adequate quantities. A tissue analysis is required to confirm. (Low probability because most fertilizers provide ample iron)
The plants are being over-irrigated, which makes it more challenging for the plant to take up iron. Look at the substrate and irrigation practices. Hydrogen and oxygen (H₂O) toxicity can lead to iron deficiency! (High probability)
Root rot, such as Pythium, inhibit iron uptake. Let's face it, without roots the plant cannot access many nutrients at all. Inspect the root system for discoloration. Send off a sample to a diagnostic clinic to confirm. (High probability)

So interveinal chlorosis is not just about the lack of iron being available to the plant. A little investigation will help you determine the real cause of the problem.

Source: eGroBlog (Brian Whipker)