Chelated iron is immediately bioavailable iron. Since it is bound to the chelating agent, it won't form an insoluble salt and it also won't be oxidized into Fe3+, which is not bioavailable. Most iron ions around are Fe3+.
It allows azaleas to take up iron even at high pH level.
There's like two or three people that did their PhD on soil pH and nutrition of rhododendron under professor Rankin in Edinburgh.
Especially the doctoral thesis of dr.Maria V. Kaisheva lays out the basic understanding of nutrition in rhododendron.
It was originally thought that calcium toxicity was the reason rhododendron azaleas didn't do well in lime-rich high pH soils. But she shows it is more likely to be iron or manganese deficiency.
Caused by the effect of pH on the solubility and uptake of both.
Link here:
I assume the same would transfer to azaleas. But this research was really mostly on lepidote large leaf traditional rhododendron.
This also explains the research where when they used only nitrate fertilizer on rhododendron, in some studies the plants became chlorotic. But this deficiency was resolved by adding chelated iron.
Often cheap fertilizer has either urea or potassium nitrate. To take up nitrate, which is a negatively charged ion, the plant has to also expel a negative ion. This could be a OH- or HCO3-. Which would both neutralize H3O+ in the root zone, increasing the pH.
However, NH4+ uptake would require the plant to expel a H3O+. So ammonium sulphate is an acidifying fertilizer. Other options to lower soil pH are iron sulphate or magnesium sulphate/Epsom salt.
You could also use aluminium sulphate, but is is not recommended at all.
If you have a plot of soil and you want to reduce the pH, then elemental sulphur is best.
This video also explains it:
Of course none of these apply to potted azaleas/rhododendron.
I believe Sequestron is chelated iron.
Also check the values of iron and manganese in your tap water.
Not aware of commercial consumer-grade chelated manganese. But it is contained in the formulation of many brand fertilizers.
Maybe it would be easier to buy manganese sulfate. But, manganese toxicity is a thing. So beware of that as well. Not good to give your plants manganese toxicity when they were iron deficient.
Doing a repot and replacing all the old soil that 'went bad' (pH too high, compaction too large, fertilizer all rained out, etc) is also good.
Manganese deficiency is very similar to iron deficiency, as they both prevent chlorophyll production. But it is said that iron deficiency yellow colouring is more regular/symmetrical and with larger veins.
If there is plenty of insoluble iron and manganese present, but not available because of a high pH, and you both lower the pH and add more iron and manganese, then you could get manganese toxicity.
Plants need very little manganese, normally take it up easily, and are able to store it.
It is much more likely you have iron deficiency.
Soil is 70/30 mix of bonsai/ ericaceous potting soil.
