IRON Role in plants

Iron (Fe) is classified as a micronutrient, meaning it is required by plants in lesser amounts than primary or secondary macronutrients. Do not let the classification create confusion as iron is very important to the health and growth of plants. Of the micronutrients, iron is needed in the greatest quantity and its availability is dependent on the pH of the growing medium. All micronutrients, except molybdenum, become less available as the growing medium's pH increases, but become more available as the growing medium's pH decreases. The ideal pH range for crops is determined primarily by their ability to acquire micronutrients.
  Function of Iron: Iron is a constituent of several enzymes and some pigments, and assists in nitrate and sulphate reduction and energy production within the plant. Although iron is not used in the synthesis of chlorophyll (the green pigment in leaves), it is essential for its formation. This explains why plants deficient in iron show chlorosis in the new leaves.
  Iron For Plants: Why Do Plants Need Iron? Every living thing needs food for fuel to grow and survive, and plants are just like animals in this regard. Scientists have determined 16 different elements that are crucial to healthy plant life, and iron is a small but important item on that list. Let’s learn more about the function of iron in plants.
What is Iron and its Function? The role of iron in plants is as basic as it can get: without iron a plant can’t produce chlorophyll, can’t get oxygen and won’t be green. So what is iron? The function of iron is to act much like it does in a human bloodstream — helping to carry important elements through a plant’s circulatory system.
Where to Find Iron for Plants: Iron for plants can come from a number of sources. Ferric oxide is a chemical present in soil that gives dirt a distinctive red color, and plants can absorb iron from this chemical. Iron is also present in decomposing plant matter, so adding compost to your soil or even allowing dead leaves to collect on the surface can help to add iron to your plants’ diet.
  Why Do Plants Need Iron? As previously stated, it’s mostly to help the plant move oxygen through its system. Plants only need a tiny amount of iron to be healthy, but that small amount is crucial. First of all, iron is involved when a plant produces chlorophyll, which gives the plant oxygen as well as its healthy green color. This is why plants with an iron deficiency, or chlorosis, show a sickly yellow color to their leaves. Iron is also necessary for some enzyme functions in many plants. Soil that is alkaline or has had too much lime added often causes an iron deficiency in the plants in the area. You can correct it easily by adding an iron fertilizer, or evening out the pH balance in the soil by adding garden sulfur. Use a soil test kit and speak with your local extension service for testing if the problem persists.
IRON AVAILABILITY TO PLANTS: Although most of the iron on the earth crust is in the form of Fe3+, the Fe2+ form is physiologically more significant for plants. This form is relatively soluble, but is readily oxidized to Fe3+, which then precipitates. Fe3+ is insoluble in neutral and high pH, making iron unavailable to plants in alkaline and in calcareous soils. Furthermore, in these types of soil, iron readily combines with phosphates, carbonates, calcium, magnesium and hydroxide ions. In such types of soils, it is recommended to use iron chelates. 
IRON UPTAKE BY PLANTS: Plants uptake iron in its oxidized forms, Fe2+ (ferrous form) or Fe3+ (ferric form). Plants use various iron uptake mechanisms. One of these is the chelation mechanism => the plant releases compounds called siderophores which bind iron and enhance its solubility. This mechanism also involves bacteria. Another mechanism involves the release of protons (H+) and reductants by the plant roots, to lower pH levels in root zone. The result is increased iron solubility. In this respect, choice of the form of nitrogen fertilizer is significant. Ammonium nitrogen increases proton release by roots, thus lowering pH and facilitating iron uptake. Nitrate nitrogen enhances the release of hydroxide ions that increase pH in the root zone and counteract efficient iron uptake. New roots and root hairs are more active in iron uptake, therefore it is imperative to maintain a healthy active root system. Any factor interfering with root development interferes with iron uptake.
Iron Sources: A few water sources provide sufficient iron for most crops, but this is unusual. Iron is typically provided by a fertilizer and most plants prefer a constant iron application rate of 1 ppm. Plants such as calibrachoa, diaschia, petunia, scaevola, snapdragon, etc.
MANAGING IRON DEFICIENCIES:
Iron Deficiency: Iron deficiency is expressed as an interveinal chlorosis of the new leaves (leaves are yellow with green veins). To determine the cause of the deficiency, first examine the roots. Plant roots that are diseased or stressed from overwatering do not take up nutrients efficiently, causing chlorosis. It is important to allow the growing medium to dry out between waterings to reduce plant stress, and to apply an appropriate fungicide drench if roots are diseased. Iron deficiency in Calibrachoa.Iron deficiency in Petunia.Iron deficiency in zonal Geranium If the roots are healthy, send a sample of the growing medium and plant tissue from several plants to a lab for verification. The pH of the growing medium directly affects the uptake of iron by plants. If the pH of the growing medium exceeds 6.5, iron is converted to a form that is unavailable to the plant, causing deficiency. The pH of the growing medium can be reduced by acidifying the irrigation water and/or using a fertilizer with a higher potential acidity. Since this may take up to a few weeks to correct the problem, chelated iron can be used to quickly green up the plants. The most effective chelating agent is iron-EDDHA. However, iron-DTPA is almost as good. If testing shows iron is deficient in the growing medium and tissue, but that the growing medium pH is normal, look at the fertilizer application rate. Fertilizing at low nitrogen rates means that iron is also being applied at low rates. Increasing the fertilizer application rate may take care of the problem. However, calibrachoa, diaschia, petunia, scaevola, snapdragon, etc. require additional iron over and above what most fertilizers supply. Therefore, iron chelates may need to be added to your fertilizer program. Iron deficiency is a limiting factor of plant growth. Iron is present at high quantities in soils, but its availability to plants is usually very low, and therefore iron deficiency is a common problem. When iron deficiency is identified, it can be treated in the short term by applying a foliar spray of iron, but the best course of action is prevention. Therefore, the grower should identify the real cause of the deficiency and treat it, in order to prevent the problem from occurring in the future.
Often, iron deficiency does not indicate insufficient iron supply. It may also be related to various conditions that may affect iron availability. For example: carbonate levels in the soil, salinity, soil moisture, low temperature, concentration of other elements (e.g. competitive microelements, phosphorus, calcium) etc. Evaluating these factors and correcting them can save a great deal of money spent on ineffective and unnecessary iron applications. The reason for testing is to check the levels of other micronutrients in the growing medium and tissue. Often, the deficiency symptoms of manganese and other micronutrients look like an iron deficiency. Correcting an iron deficiency will not help if another micronutrient is deficient.
Iron Toxicity: Iron toxicity occurs due to a low growing medium pH or from an excessive application of iron. Iron-manganese toxicity, as it is commonly referred to, is more common in zonal geraniums, African marigolds, lisianthus, New Guinea impatiens, pentas, or other crops that prefer the growing medium's pH to be 5.8-6.6. Again, have the growing medium and tissue tested to confirm the problem. If the pH of the growing medium is a problem, but less than 0.5 pH unit below the normal range for the plant, alternate fertilizer applications with a potentially basic fertilizer (15-0-15, 14-0-14, 13-2-13, etc.) and, if applicable, refrain from injecting acid.
  Iron-Manganese toxicity in Geranium. Chloloris affecting leaves. If the pH of the growing medium is more than 0.5 unit below the normal range, drench with potassium bicarbonate or liquid limestone. With either product, rinse the foliage with clear water to remove residues and avoid phytotoxicity. Potassium bicarbonate (2 lb/100 gallons of water) adjusts a growing medium's pH quickly, but provides 933 ppm potassium and increases soluble salt levels in the growing medium. Liquid limestone does not increase EC and has a longer staying power, but takes one week to fully adjust pH. Keep in mind that it is abrasive to injectors and requires agitation in stock solutions.