Horticulturists have finally realized that 16 different elements are crucial for good plant nutrition. Seven of these essential plant nutrients are micro nutrients.
Iron is one of them. Our plants require iron when they are manufacturing chlorophyll. The lack of iron is the reason we see the yellowing in leaves. The amount of iron needed is extremely small. That amount is slowly increasing thith our new flowering plants. We are starting to see increased iron use, because the lower leaves on many of our new plants can not move iron up the stem to feed the young upper leaves.
The plants that have problems moving iron up to the leaves are bacopa, brachycome, calibrachopa, diascia, pansies, petunias, scaevola, snapdragons, and vina.
The lesson is simple, if you see yellowing on any of the above plants, check the water quality, and the fertilizer ejector settings.
--- The best approach is read the info, on the fertilizer package!