On the soil:

It is the most abundant microelement found in the soil in the form of different salts and mineral compounds.

Washing gives Fe 2+, in solution, stable under non-oxidizing conditions and acidic pH; but in other conditions and with the logical presence of oxygen there is a precipitation in the form of oxides.

As the pH increases, the formation of insoluble Fe compounds, mainly ferric clay minerals, is accentuated.

In relation to Fe, the problem arises because the soluble Fe content represents a very unimportant part in relation to the total Fe present in the soil. These soluble inorganic forms are Fe 2+, Fe 3+, Fe (OH) 2+, Fe (OH) ++, Fe (OH) 3, Fe (OH) -.

We also find chelation phenomena, finding soluble organic complexes that play a very important role in plants, particularly chelates.

Fe chelates may come from humic acids, microorganisms, root excretions or what remains after synthesis processes. Fe chelates, for their stability and the concentration that they can reach in the soil, play an important role.

It should be noted that the stability offered by EDTA at pH <6 and the good performance of DTPA in the range of pH 6 to 7.5 (very common in agricultural soils). Ca competes or displaces the Fe chelates at pH> 6.5.

The stability of the chelate in the soil is particularly important.

Thus, it should be highlighted that in aerated soils, iron is mostly in the form of oxide, thus, if these are soluble, the Fe will also be in its soluble form (depending on the pH).

The low availability of inorganic Fe in soils with normal pH conditions confers a special importance to the role of chelated Fe, either in organic form or added specifically. In the following graph you can see the low importance of soluble Fe of inorganic origin compared to the average needs of the plant.

The Fe in the plant:

It is available to roots in the form of Fe 2+, Fe 3+ as well as chelated, but in order for it to be absorbed by the root, Fe 3+ will have to reduce this cation charge to Fe 2+. The importance of root exudates and their reducing effects in scarce Fe conditions has been demonstrated.

Fe chelates are soluble but must also transform into Fe 2+ and separate from the chelated complex at the root surface before being absorbed.

Different plant species show different abilities in terms of Fe absorption, especially in deficit conditions. In general, as the plants reducing capacity increases, the greater the predisposition to withstand poor levels of Fe.

It is important to note the sensitivity to the influence of other macro cations (Ca, K, Mg) and micro (Mn, Zn, Cu …), establishing a competition at higher absorption level in Cu> Zn> Mn.


It occurs under natural conditions in the form of citrate (organic acids); The presence of this compound is transcendental.

There is a significant inhibition of Fe transport caused by Zn, especially important in some cases such as in soy.

The mobility of Fe from the leaves to the roots or from the grain to the seedling is not easy. In fact, the chlorotic symptoms are manifested earlier in young leaves and buds, so the meristems and

leaf sprouting must receive xylem-iron or from external inputs.


Fe is a component of numerous enzymes. In greater quantity we find Fe as ferric phosphoprotein, the photo-ferritin, which functions as an Fe reservoir for plastids in their photosynthetic functions. Up to 80% of the total Fe contained in the plant has been found in chloroplasts.

Ferredoxin is also found in chloroplasts, a protein that participates in redox processes. Ferredoxin is an Fe-S protein where the S comes from cysteine and inorganic S. Ferredoxin intervenes in photosynthetic redox processes for the reduction of nitrite, sulphates and atmospheric N fixation.

In general, the main functions of Fe is to participate in photosynthesis, respiration, chlorophyll synthesis, and atmospheric N fixation.

On respiration:

They participate in the mitochondria. In extreme ferric deficit condition, the plants present respiratory inhibition.

Chlorophyll formation. The route of radioactive Fe in chlorotic tomato has been traced observing a correlation between the green areas and the Fe received.

In photosynthesis:

Photosynthesis is based on a process in which the radiation captured by the chloroplasts’ photosensitive molecules (chlorophyll and cytochromes) cause an electronic transfer that leads to the generation of chemical energy (synthesis of ATP and reduction of NADP + ).

In the process of electronic transfer between photosystems 1 and 2, ferredoxin is the main electron receptor.

In protein metabolism

It participates in the synthesis of nucleic acids, particularly of the RNA present in nuclei, cytoplasm and ribosomes.

In nitrogen-fixing process: The process of fixing the legumes of the soil, the Rhizobium has a nitrogenase with a complex enzymatic system in which both have Fe in different proportions. Ferredoxin also intervenes here by transferring electrons.

Fe and nitrate reduction: Both ferredoxin and nitrite-reductase participate in the transfer of electrons; first in the chloroplast and then in the cells’ cytoplasm.

The accumulation of P in the soil is one of the main reasons for the appearance of iron chlorosis symptoms. The competition established at the root level between these two elements has been observed in soybeans. In the case of tomato, the negative interaction with this element has also been attributed to the formation of Fe phosphates in the root. Just as remarkable, is the reduction of mobility that the Fe presents in high amounts of P.