Iron plays a major role in photosynthesis. That’s why a shortage directly affects the production capacity of the plant. The application of chelates has made iron much more easy to absorb. Nevertheless it’s an element that we have to keep an eye on.
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Of all the disorders caused by a deficiency, iron deficiency is the most prevalent in greenhouse production. The reasons are too little supplied, the wrong pH, cold, excessive humidity and often a combination of all these factors.

Slowing of metabolism

Iron deficiency is recognised by the yellowing of young leaves while the veins remain green. It looks similar to manganese deficiency but it’s easy to distinguish because in the case of manganese first the middle and oldest leaves turn yellow. In severe cases of iron deficiency, the leaves turn completely yellow to ivory colour and eventually die off.
But for greenhouse horticulture it’s probably more important to know that, in a mild case, you may not see any symptoms at all but damage can occur because cell metabolism is being inhibited.
The leaf yellowing during a shortage already indicates that iron has a relationship with chlorophyll, the green pigment in the plant that is essential for photosynthesis. Iron is not part of chlorophyll but it does play a role in its manufacture. This means that a deficiency immediately has an impact on production. After all, if not enough chlorophyll can be produced, the motor of the plant cannot run at full speed.
Thanks to solar energy an electron in chlorophyll is pushed to a higher energy level. It becomes involved in a complete chain of reactions and then the energy drops back to its original level. In this way the captured energy is transferred to numerous chemical substances and is eventually used to convert water and carbon dioxide into sugars. Enzymes containing iron or proteins, such as cytochrome and iron-sulphate proteins, play a role at different points in the reaction chain. Therefore we can conclude from this too that iron is essential for photosynthesis.
In addition, the element has numerous other functions. It plays a role in respiration (breathing by the cell) and it is also involved in nitrogen fixation in the root nodules.


The plant takes up iron as a dissolved ion or bound to organic compounds. In many places in the world this uptake is problematical, especially on chalky soils. This is because the pH is too high. The iron is then only available in a precipated form in the soil. This is not a problem in greenhouse cultivation as long as the grower has the pH under control. It can be a problem above 6.5 (see below). Also, the presence of too much zinc or magnesium in the nutrient solution inhibits iron absorption. Furthermore, iron can precipitate out with phosphate and can’t be absorbed at all.
Many of the problems experienced with iron in the past were solved with the advent of the chelates (EDTA, DTPA, EDDHA). These are synthetic bonds that hold iron in a solution. But the chelates are also pH sensitive. Each chelate has its own pH range. For example, DTPA is stable up to pH 6.5 but not above.
This sometimes becomes complicated because iron in the chelate can be squeezed out by another element. In this respect EDDHA is more stable than other chelates. Realising this phenomenon, the composition of potting soil – for pot plants – is important.
Other circumstances that have a negative effect on the uptake of iron are the breakdown of chelates by ultraviolet (UV) light – growers must keep the A-container covered – or a gradual rise in the pH in the A-container.
Iron deficiency is aggravated by low temperatures and stress in the root environment, such as when the greenhouse soil or potting soil is too wet. The element is quite immobile in the plant and cannot move from the older leaves to the young. Therefore it requires a constant supply.

Long list

The list of plants that are sensitive to iron deficiency is long and includes horticultural crops such as rose, azalea, hydrangea, petunia, cyclamen, strawberry and many fruit trees. The sensitivity often differs greatly by variety. It is mainly the diploid red cultivars of cyclamen that are sensitive to a deficiency. Roses are well-known for being susceptible to this problem but one variety may need two to three times as much iron as another. Even with these sensitive crops iron deficiency is easy to prevent but it requires attention to the pH (and its gradual increase), the concentration of other elements and good growing conditions (no cold and no soil/potting soil that is too wet). The fact that an iron deficiency is still relatively common indicates how close the threshold is.


Of all the diseases associated with a deficiency, iron deficiency is the most common. The young leaves become yellow between the veins because too little chlorophyll is produced. Therefore an iron deficient directly affects the plant’s production capacity. A number of other processes are also dependent on iron. Many horticultural crops – such as the rose – are sensitive to an iron deficiency. Chelates have made application easier than in the past but constant attention to pH, concentration of other elements and good growing conditions are required. Also, even when no yellow leaves are visible a loss in production can occur.

Iron is everywhere

Iron is a common element; some five per cent of the earth’s crust is made of it. It determines for a large part the colour of soil. Ground water too often contains too much, rather than too little, iron.
How is it possible then that an iron deficiency can still occur in crops grown in the soil? It’s because it is poorly available in the oxidised form. Plants have developed several mechanisms to deal with it. Grasses eject organic compounds in which the iron ions can be incorporated. In this form the grasses can take up the ejected substances plus the iron. This mechanism served as an example during the development of chelates that are widely applied nowadays. Chelates are large molecules that grab the iron like a claw. This prevents the iron precipitating out. This property also gives them their name; the Greek name for claw is chela.
Plants other than grasses have developed a simpler mechanism. They eject substances so that the ground is more acidic which allows the iron ions to dissolve better.

Text: Ep Heuvelink (Wageningen University) and Tijs Kierkels. Photos: Blgg and Wim Voogt