新叶发黄几乎总是固定问题，而非绝对缺乏。为什么选对螯合剂比加大用量更重要。
If the young leaves on your trees have turned yellow while the veins stay green, you are almost certainly looking at iron chlorosis. But here is the point that usually gets missed: your soil probably is not short of iron.
The problem is availability, not quantity
Iron is the fourth most abundant element in the Earth's crust and is present in adequate amounts in most Iranian soils. The problem lies elsewhere: above pH 7.5 — routine in calcareous soils — iron converts to ferric oxides and hydroxides that are effectively insoluble. A root cannot take up what is not in the soil solution.
That is why broadcasting iron sulphate onto calcareous soil usually achieves nothing. The sulphate fixes within days — sometimes hours — and stays there in the soil without ever reaching the root.
What the chelating agent does
A chelate is a molecule that traps the metal ion and prevents it forming an insoluble compound. But chelates are not interchangeable; each agent is stable across a specific pH range:
- EDTA: stable to roughly pH 6. Practically useless on calcareous soil.
- DTPA: works to about pH 7–7.5. Suitable where lime content is low.
- EDDHA: holds stability to a pH close to 9. The right choice for Iran's calcareous soils.
The practical difference is very large. Applying EDTA iron chelate to a soil at pH 8.2 is close to applying nothing at all, however high you push the rate.
Take the ortho-ortho isomer seriously
In EDDHA products, the ortho-ortho isomer percentage is what determines real quality. Only that isomer holds iron at high pH. A product labelled 6% iron with 2% ortho-ortho performs far worse on calcareous soil than one with the same 6% iron and 4.8% ortho-ortho.
Ask for the certificate of analysis before you buy and check the ortho-ortho figure. If the seller does not know that number, do not buy the product.
When to apply
The best timing for iron chelate is early spring, coinciding with the start of vegetative growth. Application should go in with irrigation so the product reaches the depth of the active root zone. Surface application without adequate watering wastes much of the effect.
Foliar iron is also possible, but its effect is temporary and only improves leaves that already exist at the time of spraying. Solving the problem at the root requires soil application.