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  • Iron (Fe)
  • Forme ionique
    Iron (Fe) ionic formula image
  • Anion/Cation
  • Iron (Fe) influance image
  • Iron (Fe) origin image
    Origine: Volcanic
  • Iron (Fe) mobility image
    4-6mm around the root



The risk of iron deficiency is generally reduced as most of the bedrock, as it undergoes alteration, provides iron in sufficient amounts to meet crops needs. There is, however, a clearly identifiable exception: limestone soils. Naturally, these soils contain very little amount of iron, and the small amounts they do contain are easily immobilised by excess calcium. Iron inputs should be considered based on the crop type, and are not always easy to control: they can either be small doses administered through foliar application, or yearly inputs of chelated iron via the soil, in particular for perennial crops.
Iron is primarily used by chlorophyll for photosynthesis. A severe deficiency leads to chlorosis (vines). In leguminous crops, iron plays a role in protein synthesis and in nitrogen fixation. Finally, iron participates in numerous enzymatic reactions and in plant respiration.
Iron is generally relatively abundant in soils. All magmatic rocks bring it up to the surface from the earth’s core. Silicates release iron through the cycle of solubilisations and oxidations. This explains the red colour of ferriferous soils. Acidity improves the solubility of iron, it is the same with lack of oxygen, which creates reducing conditions. However, limestone contains very little acidity, and its availability is further reduced due to the fixation with excess calcium. In fact, acid and reducing soils feature ferrous iron (Fe 2+ ) but the roots lack oxygen. Inversely, when the soil is sufficiently aerated, the roots are active but the iron undergoes oxidation and is transformed to a ferric state (Fe 3+ ), which reduces its availability if it is not chelated by organic molecules.
The absorbed amounts are nonetheless greatly influenced by the quantity available in the soil solution. Furthermore, other mechanisms are involved, such as the secretion of “siderophore” substances by the roots of grasses in order to collect the iron. There are also “siderophore” bacteria that can interfere in the assimilation process.
Iron is the most abundant trace element in soils. It represents approximately 5% of the weight of the earth’s crust, right after oxygen, silicon and aluminium. Primary minerals made of iron are essentially mafic silicates. They are decomposed through leaching and chemical reactions (hydrolysis and oxidation). The solubility of iron is higher in acid environments, whereas in alkaline environments, with high calcium content, the portion of Fe2+ is reduced or missing.
Iron expelled by volcanoes is oxidised by the oxygen present in the water, which leads to its precipitation. The iron used in fertilizers originates from iron mines. In order to ensure sufficient assimilation, the use of chelates is unavoidable in the context of reasoned iron complementation, applied directly onto/into the soil or on the leaf. Several types of chelates are available: EDTA/DTPA/EDDHA... In terms of soil application, iron sulphate bears a risk of burning and blocking.
The analysis of the soil’s iron content is a good method for identifying deficiencies. There are various extractives, in particular EDTA and DTPA chelate extraction, which are both reliable indicators. It should be noted that in limestone-rich soils, the required content is higher than in neutral to acid soils.
Organic matter plays an important role in the availability of iron but it also has antagonist effects. In a sense, the regular input of organic matter enables to feed the soil with iron; however, the respiration of micro-organisms causes an increase in CO2, and therefore a reduction of the amount of absorbable iron.
Humid and compacting conditions favour the reduction of iron from Fe3+ to Fe2+, along with reduction of stresses. However, in viticulture it has been observed that during rainy years iron deficiency increases.
Most cases of iron deficiencies are induced deficiencies, resulting from poor assimilation caused by other factors: high soil pH, excess of Ca ions or of bicarbonates in the soil solution, interaction with other excess elements such as Cu, Ni, Co. Regarding the single effect of soil pH, the higher the pH, the greater the risk of deficiencies.