In Iran's calcareous orchards, iron is not scarce in the soil; it is absent from the soil solution. pH, the type of chelate, the ortho-ortho isomer and spring timing decide whether the yellowing of young leaves reverses.
If, in the orchards of Najafabad, Shahreza, Neyshabur, Marvdasht or the calcareous plains of Kerman, the young leaves of a tree have turned yellow while the veins are still green, the first diagnosis is almost always iron chlorosis. The common mistake that has made feeding programmes useless for years is the grower's assumption that the soil is poor in iron. The reality of Iran's carbonate soils is the opposite. Iron is the fourth most abundant element in the earth's crust, and in most of the country's agricultural land the total quantity is more than sufficient. What is missing is the soluble form the roots can take up. A root takes only the ion that is free in the soil solution; the ferric oxides and hydroxides fixed on the lime particles effectively do not exist for the tree.
Why iron does not reach the roots in Iran's calcareous soils
Calcareous and carbonate soils cover a large share of Iran's orchard land. Their pH is usually between 7.6 and 8.6, and the active lime percentage runs into double figures in many plots in Isfahan, Fars, Razavi Khorasan, Yazd and Kerman. In that environment, the ferrous ion oxidises quickly and turns into the insoluble ferric form. The higher the pH climbs, the more the concentration of free iron in the soil solution falls, logarithmically. At a pH near 8, the amount of soluble iron is so low that even the active roots of stone and pome fruit trees cannot meet the demand of spring growth. The result is the familiar pattern: interveinal yellowing of the young leaves while the older leaves are still relatively green, because iron is poorly mobile within the plant.
Lime does not only raise pH. Calcium carbonate coats the surface of the particles and occupies the exchange sites. Irrigating with hard water, which is common in many wells on the central plateau, adds bicarbonate to the root environment and intensifies the fixation. In plots where drainage is poor, or flood irrigation is done on a long interval, root aeration also declines and taking up even that small amount of soluble iron becomes harder. This is why spreading iron sulphate on calcareous soil usually becomes useless within days — sometimes hours. The sulphate is fixed rapidly and stays where it is in the soil without ever reaching the active root zone.
EDTA, DTPA and EDDHA: three completely different behaviours
A chelate is a molecule that holds a metal ion and stops it precipitating. But not all chelates are the same. Every chelating agent is stable only within a defined pH range, and outside it releases the iron to be fixed again. In the Iranian fertiliser market these three names come up more than any others, and choosing wrongly between them is equivalent to throwing money away.
- EDTA: stable up to about pH 6. It has a use in neutral to slightly acidic soil. In Iran's calcareous soils it is effectively useless and loses its iron very quickly.
- DTPA: its stability extends to about pH 7 to 7.5. In soils with little lime, or in amended plots, it may cover part of the requirement, but at pH 8 and above it cannot be relied on.
- EDDHA: it holds its structure up to a pH near 9 and is the right choice for the carbonate soils of the Iranian plateau, provided the quality of its isomer is stated.
In practice this difference is very large. Applying EDTA iron chelate in a soil at pH 8.2 is close to applying nothing at all, however far you raise the rate. Increasing the dose does not compensate for fixation; it only raises the cost. Many growers who say iron did not work have in fact used a chelate that was not stable at their plot's pH. Before buying, look at the pH and the active lime percentage of that plot, not at the product's generic label.
Ask for the ortho-ortho isomer on the analysis sheet
In EDDHA products, the total iron figure on the sack does not on its own establish quality. What holds iron at high pH is the ortho-ortho isomer. A product with 6 per cent iron and only 2 per cent ortho-ortho performs far less well in calcareous soil than one with the same 6 per cent iron and 4.8 per cent ortho-ortho. The ortho-para and para-para isomers are weaker in an alkaline environment and release much of the iron early. If the seller does not know the ortho-ortho percentage or will not produce an analysis sheet, do not choose that consignment for a calcareous orchard. This is not a luxury laboratory detail; it is the difference between young leaves greening up in May and the yellowing returning in June.
Before buying an iron chelate, put three numbers on the table: soil pH, active lime percentage, and the ortho-ortho isomer percentage. If one of the three is missing, your decision is a guess, not a plan.
Spring timing and the soil route, not foliar spraying alone
The best application window for iron chelate in most Iranian orchards is early spring, coinciding with the start of vegetative growth: when the roots become active and the demand for iron to build chlorophyll in new leaves is high. The application must go with irrigation so the product reaches the depth of the active roots rather than sitting on a crusted surface. In a drip system, injecting during irrigation and delivering the solution into the wetting bulb is the most effective method. Surface application without enough water throws away much of the effect. In flood-irrigated plots, placing it in the band under the canopy and irrigating immediately afterwards is essential.
- Measure the pH and active lime of that plot; do not rely on a figure for the orchard as a whole.
- Choose a chelate whose stability matches the pH; in Iran's calcareous soils, usually EDDHA with a stated ortho-ortho content.
- Confine the application to the start of spring growth and to irrigation, not to every round of fertilising.
- Treat foliar spraying as a supporting tool, not a replacement for soil application. Its effect is temporary and only greens the leaves already there.
- If severe chlorosis persists after one correct season, examine drainage, the bicarbonate in the irrigation water and root health separately.
Foliar iron has its place, but its effect is temporary. The leaf absorbs only the surface that was wetted, and the leaves that emerge after the spray turn yellow again, because iron reserves do not move around within the plant. Those who green the leaves every fortnight and see the yellowing return are paying to maintain a symptom, not to treat a cause. For old trees or urban landscaping where access to the roots is limited, trunk injection of iron alongside a corrected soil programme is worth considering, provided the trunk environment and the condition of the vascular tissue are assessed. Iron chlorosis in Iran is a matter of root-zone chemistry far more than of a missing element. Farin Kesht Taban works on exactly this logic in Isfahan: identify the fixation, choose the chelate, and deliver the element to where the tree can actually take it up.
