The role and application features of iron fertilizers in agriculture

Why plants lack iron and how to detect a deficiency

Despite the fact that iron is among the most abundant elements in the Earth's crust, plants often experience a deficiency. Total soil reserves of the element are high, but most of them are in an unavailable form. This problem is particularly common in farms on carbonate and over-limed soils, where iron is firmly fixed.

• Total iron content in soils — 1–5%
• Content in red soils — 10–11%
• Content in light sandy soils — about 1%
• Clarke value of iron in plants — 0.02% of dry matter
• Removal of iron with the harvest — 1–10 kg/ha

The availability of mobile iron directly depends on the acidity of the soil solution. When the environment is alkalized, solubility drops: Fe³⁺ ions precipitate in the form of hydroxide or hydrate even at pH > 2.4, while Fe²⁺ ions remain mobile up to nearly neutral values. Accordingly, as soil acidity increases, the availability of iron rises.

The mobility of the element is also influenced by the redox potential (Eh) and the degree of aeration. With intensive aeration and an increase in Eh to 700–750 mV, iron is rapidly oxidized and fixed in the soil. Conversely, when Eh drops to 200–250 mV under waterlogged conditions, iron compounds are reduced and leached from the tillage layer. In rice paddies, such losses reach critical levels, worsening exchange capacity and destroying the soil microstructure, which causes it to turn into a dense monolith upon drying.

Perennial fruit crops — apple, pear, plum, peach, citrus, as well as raspberries and grapes — are most sensitive to iron deficiency. Among field and vegetable crops, the deficiency is less pronounced, but rice, oats, corn, lupine, potatoes, cabbage, and tomatoes are also susceptible to it. The main symptom of starvation is chlorosis: with a mild deficiency, leaves turn pale; with moderate deficiency, interveinal chlorosis develops; and with acute deficiency, severe chlorosis of young leaves occurs.

Assortment of iron fertilizers and application features

To compensate for the deficiency, pure water-soluble salts, frits (iron-containing glasses), and chelate compounds (complexonates) are used. Plants are able to absorb iron through roots in the form of Fe²⁺ and Fe³⁺ cations and as part of chelates, and can also assimilate the element during foliar applications. Both simple salts and modern complex forms are suitable for ensuring the needs of the crop.

Among water-soluble salts used in practice are iron (II) sulfate (ferrous sulfate) FeSO₄∙7H₂O, iron (II) chloride FeCl₂∙4H₂O, iron (III) chloride FeCl₃∙6H₂O, iron (III) hypophosphite Fe(H₂PO₂)₃, iron (II) iodide FeI₂, iron (III) nitrate Fe(NO₃)₃∙9H₂O, and iron (III) orthophosphate FePO₄∙2H₂O. Ferrous sulfate is recognized as the most effective and accessible of this group. It contains 47–53% ferrous sulfate and is produced from metallurgical industry waste — pickling solutions. Externally, the preparation is a gray fine- or coarse-crystalline powder, on which a white, yellow, or brown coating is acceptable.

Frits are glassy silicate compounds obtained by sintering and rapid cooling, which serve as a source of gradual plant nutrition. Iron is contained in them in a water-insoluble but assimilable form and dissolves in 2% citric acid. These fertilizers are not absorbed by the soil, and their digestibility increases in acidic soils compared to neutral and alkaline ones. For application, finely ground frits are introduced into the soil together with organic and mineral fertilizers during primary tillage or locally during sowing.

The use of frits is particularly promising in irrigated agriculture and rice cultivation, where water-soluble fertilizers are rapidly leached by filtration and drainage waters. In rice cultivation, frits are applied directly to the soil. The average application rate ranges from 25 to 50 kg of iron active ingredient per hectare (Fe25–50), depending on the requirements of the crop and the nutrient status of the specific field.

Chelates are water-soluble complex compounds of iron with organic substances that are poorly absorbed by the soil and easily assimilated by plants. Aminopolyacetic acids are used as the basis for these complexes. The most promising complexes for agriculture are considered to be iron complexes with diethylenetriaminepentaacetic (Fe-DTPA), ethylenediaminetetraacetic (Fe-EDTA), and polyethylenepolyaminepolyacetic (Fe-PPPA) acids. These preparations contain 7–10% iron and are supplied as dark brown, odorless solutions with a specific gravity of 1.4 g/cm³.

In industrial production, Fe-DTPA is obtained through the reaction of ferrous sulfate with diethylenetriamine. The process begins with the loading of sodium chloroacetate at a temperature of 45 °C, after which diethylenetriamine and a 40% alkali solution are added. The reaction is conducted such that the temperature of the mixture does not exceed 60 °C and the pH remains within the range of 9–11. Then, iron sulfate is gradually introduced into the mixture, which is then cooled to 18–20 °C and filtered, yielding a ready-to-ship solution with an Fe-DTPA concentration of 13–15%.

Fertilizers of the DP–DL family show high efficiency in low-volume hydroponic systems and for foliar applications. They contain iron in the chelated DTPA form and are completely devoid of phytotoxicity. The percentage of active ingredient in the preparations of this series varies depending on the brand.

Methods and specific features of iron fertilizer application

• Frits dosage for rice — Fe 25–50 kg/ha
• Concentration for seed treatment — 0.5–1 %
• Concentration for foliar top dressing — 0.1–0.5 %
• Consumption per fruit tree — 5–12 l
• Application rate for hydroponics — 0.1–0.5 kg/m³

Water-soluble salts and iron chelates are suitable for soil application, seed treatment, and foliar applications. When applied to the soil in solid form or as solutions, iron quickly transitions into a state inaccessible to plants. Treatment of planting material is much more effective, using 0.5–1% aqueous solutions of iron salts. To eliminate severe deficiency, foliar top dressing with 0.1–0.5% aqueous solutions is used; chelates act more gently on the foliage than salts and guarantee an increase in yield.

When carrying out root top dressing, it is important to ensure high soil moisture through irrigation or mulching so that the chelate reaches the root system. The consumption of the working solution per one fruit tree ranges from 5 to 12 liters, depending on the age of the plantings. When using preparations of the DP–DL family, the rates and timing of treatments are established strictly according to the manufacturer's recommendations. In hydroponics, the application rates for iron fertilizers range from 0.1 to 0.5 kg/m³.