I have been growing for decades and at a certain point I was able to diagnose problems with a quick glance. Iron deficiency was always one of the first I would notice because it has a very specific look. The signs of Iron deficiency present as interveinal chlorosis or yellowing of young leaves between the veins. This can be accompanied by necrotic spotting (brown dead tissue spots) on leaves. Iron is an immobile element meaning deficiencies will always first show at the top of the plant or on the newest / youngest growth. Iron is required for photosynthesis, respiration, as well as the production of enzymes.
There are three forms of Iron chelate, FeEDTA, FeDTPA and FeEDDHA, although the most common form is FeEDTA.
With Iron nutrition, the form of Iron is very important. The three common chelated forms (Iron-EDDHA, DTPA and EDTA) differ in their ability to keep Iron soluble and available to plants as the media or hydroponic solution pH increases. Between a pH of 4.0 to 5.5, any form of Iron will work (including Iron sulfate) at supplying Iron to a plant. As pH climbs over 6.0 less than half of the Iron from FeEDTA becomes unavailable to your plants. However, as the media / fertilizer solution pH increases above 7.0, only the Iron from Iron-EDDHA will remain soluble. Research has shown that the ranking of Iron forms from most effective to least effective at supplying Iron at high media pH are:
Iron-EDDHA (pH up to 11.0)> Iron-DTPA (pH up to 7.0)> Iron-EDTA pH up to 5.5)> Iron sulfate (pH up to 5.5).
When choosing an Iron supplement make sure to check the Iron source and match it to your growing needs. I always recommend that soil and soilless growers use the Iron chelate FeDTPA because it will provide the Iron the plants need and allow them to set the pH of their irrigation solution to the ideal 6.3-6.6 pH range. The product I use myself and recommend is CALiMAGic from General Hydroponics.