IMPORTANCE FOR PLANT LIFE

METABOLISM

Iron is an element primarily involved in chlorophyll functioning and plays a vital role in photosynthesis. Severe iron deficiency shows up especially in species such as grapevine, with pronounced chlorosis (yellowing) on young leaves.

In legumes, iron plays an important role in protein synthesis and nitrogen fixation. In general, iron is part of the structure of many enzymes and is involved in reactions that take place in plant respiration.

UPTAKE MECHANISMS

Iron is generally a relatively abundant element in soils. All magmatic rocks bring iron from deeper layers to the surface during the formation of the Earth’s crust. Silicate minerals release iron throughout cycles of dissolution and oxidation; this accumulation is also what gives iron-rich soils their red color.

Reducing conditions created by acidity and oxygen deficiency increase iron solubility. In calcareous (calcium carbonate–rich) soils, the pH is high, so Fe becomes almost insoluble, while soluble calcium is abundant. In acidic and reducing environments, Fe is mostly in the Fe²⁺ form; however, under these conditions the root zone also tends to suffer from oxygen deficiency.

Conversely, if the soil is well aerated, roots are highly active; but iron is oxidized to the Fe³⁺ form. Ferric iron (Fe³⁺) is much less accessible to plants unless it is complexed with suitable organic molecules (i.e., chelated).

INTERACTIONS & DISTINCTIVE FEATURES

The amount of iron taken up by the plant is strongly influenced by the amount of available Fe in the soil solution. In addition, grasses such as cereals can secrete iron-binding substances called siderophores into the rhizosphere to mobilize and capture iron.

Some soil bacteria also release such iron-chelating compounds to transport iron across cell membranes. In this way, iron can be shuttled between microorganisms and plant roots, becoming more efficiently utilized.

IRON IN SOIL

Iron is the most abundant of the trace elements in soils and makes up about 5% of the Earth’s crust. Primary iron-bearing minerals are mainly mafic silicates, which break down through hydrolysis and oxidation processes.

Iron solubility is higher under acidic conditions. By contrast, in calcium-rich and alkaline (high-pH) environments, the amount of dissolved Fe²⁺ decreases or almost disappears. For this reason, iron deficiency is common in calcareous, high-pH soils.

SENSITIVITY TABLE & SYMPTOMS

Iron is an important component of many enzymes and plays a critical role in nitrogen reduction and fixation. Iron deficiency is frequently observed in crops grown on calcareous soils and can cause significant losses in both yield and quality parameters.

Fe deficiency manifests as interveinal chlorosis (yellowing between the veins) on young leaves. In very severe cases, the entire leaf may turn yellow and even become almost white. Iron deficiency can easily be confused with nitrogen deficiency; however, nitrogen deficiency affects older leaves first, whereas iron deficiency appears primarily on young leaves.

EXCESS & REQUIREMENT

Plants prone to iron toxicity include tomato and basil, although iron toxicity is not very common. Under excess conditions, bronzing and pinpoint spotting (stippling) may be observed on leaves.

While iron is necessary for chlorophyll production, excessive iron can negatively affect chlorophyll functioning. In addition, excess Fe in the soil can hinder root uptake of other nutrients, leading to nutritional imbalances.

ORIGIN & SOIL FACTORS

SOIL IRON STATUS & ORGANIC MATTER

Analyzing the iron status of soil is a good way to identify potential deficiencies. Different extraction methods—such as EDTA and DTPA chelate extraction—are considered reliable indicators for assessing plant-available Fe. In lime-rich soils, the Fe level considered adequate must be higher than in neutral or acidic soils.

Organic matter plays an important role in iron availability, but it can also cause antagonistic effects. Regular additions of organic matter can supply iron and, by forming complexes with organic molecules, reduce Fe chemical fixation or precipitation as ferric hydroxide.

On the other hand, rapid microbial respiration of organic matter can produce enough carbon dioxide to promote bicarbonate ion formation. These bicarbonates can immobilize iron within plant tissues and trigger iron deficiency.

pH

High soil pH and an excess of calcium ions or bicarbonate in the soil solution can trigger Fe deficiency. Especially under calcareous, alkaline conditions, the Fe³⁺ form dominates and, if not chelated, has low availability to plants.

CLIMATE

Iron deficiency often appears at the beginning of the growing season under cool and wet soil conditions. Moist and compaction-prone conditions can promote the reduction of Fe³⁺ to Fe²⁺ and alleviate certain stresses.

However, as observed in viticulture, iron deficiency symptoms may increase in rainy years—highlighting how critical the delicate balance is between pH, lime content, water regime, and root-zone oxygen status.

KEY FACTORS