Iron Deficiencies: understand it to manage it

Internerval yellowing of the leaves (especially of the younger ones) and stunted growth: these are the symptoms most frequently associated (but not the only ones) with iron chlorosis, a plant disorder that affects agricultural crops suffering from iron deficiency, the meso-element that plants need in greater quantities.

Iron deficiency causes a reduced synthesis of chlorophyll, which leads to a decrease in the rate of photosynthesis and to an alteration of fundamental processes such as respiration and the synthesis of plant hormones. Ultimately, this deficiency generates a state of suffering in the plant, which also negatively affects the phytosanitary state, which in the various crops gives rise to a series of symptoms that also depress the quality and quantity of productions.

What causes iron chlorosis? How can it be countered? First of all, let’s try to understand the dynamics of this element in the soil and the mechanisms of iron absorption by plants.

In most soils, iron is present in two oxidation states – bivalent and trivalent – at relatively high concentrations, ranging from 20 to 40 mg/kg. The element is contained in several minerals (olivine, biotite, vermiculite…), but the most abundant form of iron is represented by oxides and hydroxides. The solubility of the latter in the circulating soil solution is low and depends on two main factors, pH and redox potential: neutral to alkaline pH values and oxidative conditions favour the precipitation of insoluble forms of iron, while acidic and reducing conditions promote its solubilization.

Considering that about 30% of cultivated land in the world is calcareous and therefore at an alkaline pH, the bioavailability of Iron can be very limited for plants; it has been estimated that in the pH range between 5.0 and 8.5, the concentration of Iron in the circulating solution ranges from 0.1 to 10 ^-3 µM, while the average concentration of Iron necessary for optimal plant growth is 4 to 5 orders of magnitude higher.

The form of iron preferred by plants for absorption is the bivalent form (Fe²⁺), while trivalent iron (Fe 3+) is much more difficult to absorb.

To increase iron absorption from soil, plants use two mechanisms, Strategy I (or ‘reducing’) and Strategy II (or ‘chelating’).

Plants equipped with Strategy I (dicots and monocots not grass plants) induce the reduction of trivalent to bivalent iron by producing specific enzymes and releasing protons into the rhizosphere. These lower the pH and further promote the solubilization of iron.

Plants with Strategy II (grass plants), on the other hand, release phytosiderophores, amino acid substances with a low molecular weight with a chelating action. Phytosiderophores bind trivalent iron from the rhizosphere and keep it in solution, making it available for uptake by roots.

In reality, the most recent evidence indicates that certain plants (for example rice) are able to implement both Strategies I and II as appropriate. And it has been proven that plants equipped with Strategy I release exudates, such as organic acids, phenolic compounds and flavonoids, which can perform a chelating function against iron.

Iron deficiencies can obviously be treated with specific fertilizers, with foliar or root distribution.

Applying inorganic iron salts to soils, such as iron sulphate, must always be evaluated carefully, since, depending on the chemical and physical characteristics of the soil, iron can rapidly precipitate in the form of oxides and hydroxides, thus making it unavailable for plant nutrition.

Alternatively, iron can be supplied in chelated form, using aminocarboxylated synthetic chelators in the formulation, such as HEDTA, EDTA, DTPA and EDDHA. The stability of the complexes formed by these chelating agents is variable and depends on:

• pH of the circulating solution: only the EDDHA-Fe ³⁺ chelate, containing the most expensive chelating agent, is able to keep iron in solution and transport it to the root of the plant in highly calcareous soils;
• presence of other elements (for example Calcium, Zinc and Copper), able to compete with Iron in the binding action of the above-mentioned substances and therefore of inducing the precipitation of Iron to insoluble oxides and hydroxides.

Synthetic chelators are poorly retained in the soil, with a high risk of leaching and the environmental implications that this entails.

It has also been demonstrated that some of the chelating compounds normally used (for example EDTA) are poorly biodegradable in soil and therefore very persistent in natural systems and that the accumulation of chelating compounds in agricultural soils, originating from fertilization strategies, could cause the unwanted mobilization of heavy metals that can enter the food chain, compromising the quality and safety of agricultural products.

First of all, it must be said that, as with any other plant disorder, even for iron chlorosis, good farming practices are essential to manage the problem. It is certainly essential to promote the development of the root system, in such a way as to allow the exploration of a larger volume of soil. The porosity of the soil must therefore be maintained at optimal levels and in this the supply of organic matter plays a fundamental role. Where necessary, the use of soil conditioners, organic and organo-mineral fertilizers is therefore recommended.

In addition, several experiments have shown that some useful microorganisms in the rhizosphere can play an important role in helping plants to cope with suboptimal iron concentrations in the soil, thanks to the production of natural chelators.

But we will return to this topic in a later study.

For the prevention and treatment of iron deficiencies, Agribios recommends Fumier Fer 80-20 fertilizers and, novità 2025, Agrinutrient BIO, in addition to the corrective Ferrous Sulphate.

All three products are allowed for use in organic farming.

Fumier Fer 80-20 is derived from the homogeneous mixture of humidified manure, partially hydrolyzed animal proteins and ferrous sulphate heptahydrate. The humic acids present in the humidified manure bind, through carboxylic and phenolic bonds, to bivalent iron with formations of iron humates. The completely natural chelation then makes bivalent iron available, which is highly assimilable through the complex cycle of organic matter. The presence of sulphur, in addition to acidifying the soil, determines an important contribution of this meso element useful for improving plant development and the quality of crops.

Agrinutrient Bio is a pelleted fertilizer based on ferrous sulphate with Magnesium and Sulphur, specifically designed to provide Iron, Sulphur and Magnesium to soils. Iron is necessary for photosynthesis and respiration. Magnesium plays an irreplaceable role in photosynthesis and improves the resistance of plant tissues as a substitute for calcium. Thanks to its acidic pH, Agrinutrient BIO is used in alkaline soils rich with active limestone, where it exerts a corrective action by establishing, at the root level, a microenvironment suitable for the absorption of nutrients. In addition, the acidic reaction of the product and the high sulphur content, reduce the insolubilization of iron that occurs in alkaline and limestone soils.

Solfato Ferroso is used to combat iron deficiency in the main agricultural and ornamental crops. The presence of Sulphur and Iron, together with a strongly acidic pH (2.5) make Ferrous Sulphate a highly effective corrective agent. Thanks to its acidic pH, it is used in alkaline soils, rich with active limestone. The particular fluidized formulation, obtained from specific raw materials, makes it possible to obtain a product suitable for storage and distribution.