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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. However, limestone soils are an exception. Naturally, these soils contain very little iron, and the small amounts they do contain are easily immobilised by excess calcium. Iron fertilization should be considered based on the crop type, and are either applied as small doses 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. Severe deficiency leads to chlorosis, for example in 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 solubilisation and oxidation cycles. The high abundance explains the red colour of ferriferous soils. Acidity and a lack of oxygen, which creates reducing conditions improves the solubility of iron. The pH of calcareous soils is high and Fe is nearly insoluble while soluble calcium is abundant. Acid and reducing soils feature ferrous iron (Fe2+) but the roots lack oxygen. Inversely, when the soil is sufficiently aerated, the roots are active but the iron undergoes oxidation. The ferric state (Fe3+) is less availability if it is not chelated by organic molecules.
INTERACTIONS AND SPECIAL FEATURES
The absorbed amounts are greatly influenced by the quantity available in the soil solution. Furthermore, other mechanisms are involved, such as the secretion of substances (siderophores) by the roots of grasses in order to collect the iron. There are also bacteria that transport iron across cell membranes by the excretion of these iron-chelating compounds.
Iron is the most abundant trace element in soils. It represents approximately 5% of the weight of the earth’s crust. Primary minerals made of iron are essentially mafic silicates. They are decomposed through hydrolysis and oxidation. The solubility of iron is higher in acidic environments, whereas in alkaline environments with a high calcium content, the portion of Fe2+ is reduced or missing.
Sensibility table & Symptomes
As an important component of many enzymes it plays and important role for nitrogen reduction and fixation. Iron deficiency is frequently observed on crops growing on calcareous soils and causes significant reductions in yield and quality. Fe-deficiency is causing chlorosis, which is manifested by a yellowing of the interveinal areas of young leaves. In very severe cases, the entire leaf turns yellow and even white. Iron deficiency can easily be mistaken for nitrogen deficiency, which however affects the older leaves first.
Excess & Needs
Plants prone to iron toxicity include tomatoes and basil but iron toxicity is not very common. The symptoms include bronzing and stippling of leaves. Although iron is needed for chlorophyll production, too much iron can affect the functioning of chlorophyll. An excess of iron in the soil also impairs the uptake of other nutrients from the soil.
SOIL CONTENT AND ORGANIC MATTER CONTENT
Analysing 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 antagonistic effects. The regular input of organic matter feeds the soil with iron and by combining with it, it reduces the chemical fixation or precipitation of iron as ferric hydroxide. On the other hand, rapid microbial respiration may produce sufficient carbon dioxide to form bicarbonate ions which immobilize iron within plants, resulting in deficiency.
A high soil pH and excess of Ca ions or bicarbonates in the soil solution, can induce Fe deficiencies.
Iron deficiency occurs most frequently in cool and wet soil early in the growing season. 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.