Usually at the first sign of yellow leaves, most folks pour on the iron supplement or fertilizer. And this is understandable because 9 times out of 10 they’re doing so because of advice they got from the place they bought it. And 9 times out of 10, the diagnosis is right but the prescription and remedy is wrong.
By the time I get the service call, there are toxic levels of iron supplement and fertilizers visibly on the surface of the soil and the leaves are still turning yellow and falling off.
Since I have dealt with this same problem many times, I know the problem isn’t a lack of iron. There’s plenty of iron in our soil and most soils. Generally, the problem will be either over watering, improper Ph balance, or both.
If I can rule out overwatering by inspecting and checking the soil for mold and musty smell, I know the problem is probably improper Ph balance. The nutrients are in the soil but they’re locked up and can’t be used by the plant because the soil is out of balance. Around here it’s because of extreme high alkalinity as we live on a large Limestone shelf.
After confirming this with an inexpensive test kit, we began a slow systematic increase in soil acidity with simple garden sulfur. A little at a time spread out over several weeks. Within a few months the soil ph will become more acid, the iron will become available, and the plants will get healthy and green.
I prefer dusting sulfur, Sulfur granules, or Copperas which is a trade name for an iron/sulfer supplement that adds both at the same time.
Keep this in mind if your plants start to look sick. Don’t just start pouring the fertilizers to them. You can do your plants more harm than good. After a visual inspection for insects, disease, and over watering, check the soil Ph and adjust it to what your specific plants like. You’ll be surprised at the overall health of your plants when your soil is perfect.