By Wesley Chun Ph.D
Chief Science Officer, Grower's Secret Inc.
Iron (Fe) is an essential micronutrient for plant growth and reproduction. Fe comprises 85% and 80% of the Earth’s inner and outer core, respectively. It is the second most abundant metal in the earth’s crust after aluminum and is found in minerals such as hematite and magnetite. Fe is a member of the Group 8 transition metals. Group 8 metals also include hassium (Hs), osmium (Os), and ruthenium (Ru). The transition metals all have high density, can form different colored compounds, can conduct electricity, and have high melting and boiling points. Transition metals have incomplete inner electron subshells allowing formation of compounds with different oxidation states.
Unlike silver and gold, Fe rapidly corrodes making it difficult to find archaeological evidence. Hence, it is unknown when Fe was first discovered and identified. However, Fe has been in use by humans for more that 5000 years. The earliest known artifacts were small iron-nickel beads dated to 3200 BC which were found in ancient Egyptian burials. These beads were made from meteoric iron and shaped by careful hammering. Smelting techniques with iron appeared sporadically in the middle of the Bronze Age and were adopted around 1200 BC (Iron Age).
Iron was established as an essential element for plant growth by Eusebe Gris in 1844 when he reversed certain chlorosis in plants by treating roots and leaves with iron solutions. It is Important to all life except for lactic acid bacteria which use manganese and cobalt instead. It is a key element in hemoglobin, and other proteins in humans and animals where it is included in proteins as a catalytic center or electron carrier. Iron is important in many biological processes such as photosynthesis, respiration, tricarboxylic acid cycle, oxygen transport, gene regulation, and DNA biosynthesis.
While essential for life, very low amounts of iron are needed for normal growth. Hence, uptake, movement and incorporation of Fe in plants and animals are highly regulated. In humans most of the iron can be found in hemoglobin and only 1 to 1.5 milligrams of Fe a day are needed. Most crops need 1 to 1.5 pounds of Fe per acre. For comparison, crops need 80 to 200 pounds per acre of nitrogen for healthy growth. Iron rich foods include cereal grains, meats (beef, chicken, pork, turkey), seafood (clams, oysters, shrimp, tuna), vegetables (broccoli, lentils, peas, soy products [tofu], spinach, string beans, sweet potatoes) and molasses. Despite the variety of food sources, iron along with zinc, iodine and vitamin A, are major nutritional concerns for both developed and developing countries. Nutritional deficiencies or “hidden hunger” is widespread and annually affects a high number of children and females (see insert “Hidden Hunger”). Some of the effects of hidden hunger are anemia (iron deficiency), blindness in children and maternal mortality (Vitamin A deficiency), weakened immune systems (Vitamin A and zinc deficiencies), intellectual development (iodine and iron deficiencies), and poor growth and development (several vitamins and nutrients). Developed countries are themselves not immune to hidden hunger despite higher caloric intake. Often nutritional quality is poor leading to deficiencies.
Fighting Hidden Hunger
Hidden Hunger occurs when the quality of food does not meet the nutritional requirements to maintain good health and requirements. This chronic lack of micronutrients (vitamins and minerals) affects one in three children in the world, and around two billion people in developing countries. It is also of concern in developed countries where surveys show that Recommended Dietary Allowances are not being met for folic acid, vitamin D, vitamin E, iron and iodine. It is estimated that 85% of Americans lack essential vitamins because of a calorie rich, nutrient poor diet.
Hidden Hunger is a cyclical problem passed down through generations. Malnourished females have higher maternal mortalities. Suboptimal nutrition of the child can lead to higher mortality, reduced physical and mental development, and increased risk for chronic diseases, and stunting. Adults have impaired physical development and reduced life expectancy. Then the cycle repeats itself.
Between 1933 and 1965, studies on human malnutrition focused on protein deficiencies. It was not until the 1970’s when the world’s nutrition community realized that protein deficiency was not the major cause of malnutrition and attention then focused on energy intake. However, malnutrition still remained a problem despite efforts to supply more food and in most cases, staple crops. Diets based on staple crops such as maize, wheat, rice, and cassava are calorie rich but nutrient poor for essential vitamins and minerals. Hence, while food became available, malnutrition increased.
At the same time, many countries were also aware that iron, vitamin A, and iodine deficiencies were more widespread than protein deficiencies. Micronutrient deficiencies started receive serious attention in the mid 1980’s. A pivotal study by the Johns Hopkins University showed a 34% reduction in preschooler mortality with vitamin A supplementation. Micronutrients gained further attention when iodine deficiency disorders (cretinism and goiter) could be remediated by use of simple iodized salt on a global scale.
International agencies such as UNICEF were strong champions to eliminate IDD and vitamin A deficiency. At the 1990 World Summit for Children, supported by UICEF, the World Bank, WHO, FAO, UNDP, CIDA, and USAID supported elimination or reduction of iodine, vitamin A, and iron deficiencies by the year 2000. In 1991, a Conference on Ending Hidden Hunger was held in Montreal. Convened by UNICEF and WHO, attendees agreed on the formation of the Micronutrient Initiative. This led to health agencies programming and implementation to provide training, field work, improve laboratory capacity, and data dissemination. The goal was to plant the seeds to address vitamin A deficiencies. This included promotion of breast-feeding babies, periodic supply of high dose vitamin A, and increase dietary diversification to include more fruits and vegetables. Despite dramatic program scale-up, the problem still exists and consistent delivery has been a major challenge. Currently 1/3 of children in need are not receiving vitamin A supplementation.
Solutions for malnutrition varies with the micronutrient. Iodized salt was a simple solution. Supplementation has been used with some success to address Vitamins A and D deficiencies. Diversification of diet to include micronutrient rich foods is problematic for developing countries. Fortification of commercial foods has been used with success. A newer approach is biofortification of foods either by traditional breeding techniques or with genetic technology. Currently there are several genetically biofortified foods Vitamin A (orange, sweet potato, maize, cassava), beta-carotene (rice), iron (beans, pearl millet), and zinc (rice, wheat). There are proponents that are in support of a food-based approach. However, general support for genetically modified foods are still mixed. Certain crops such as rice would contribute to a food-based approach to address malnutrition. Rice is an energy source for half of the world’s population. There are currently several back crossed cultivars available. However, despite the effort to make it freely assessible, carry familiar traits, and cost is free, it still has not been generally accepted. Obviously, education needs to be expanded to plant the seeds for genetic change.
Iron in Plants
Iron, while essential for plants, is needed in very low amounts. An average crop will need one to one and a half pounds of Fe per acre while 80 to 200 pounds per acre of nitrogen is needed for healthy growth.
Most plants need 1 to 1.5 pounds per acre of iron compared to 80 to 200 pounds per acre of nitrogen. Iron is taken up by the plant as chemical compounds or organic complexes via roots and leaves. Root growth and expansion of the root systems are very important to the plant in obtaining Fe. When near roots, iron can be taken up along with the water uptake by the plant. Iron is taken up in the greatest amount in the zone of the root that is between cell elongation and maturation (about 1 to 4 cm) behind the root tip. This is an active region where plants release protons and reductants to reduce Fe3+to Fe2+, or release chelated Fefrom complexes, for plant absorption. Hence, active root growth and expansion of the root system are very important for efficient Fe uptake. It is important to ensure proper fertility to support strong root growth.
Once reduced, Fe is actively moved across the cell membrane via active transport systems. This process can be interfered by other cations such as Mn2+and Ca2+producing iron deficiency symptoms.
Grasses produce and release phytosiderophores into the rhizosphere. These organic acids form complexes with iron before Fe is moved into the root. The complexes are absorbed into epidermal and endodermal cells of the root. Iron is then moved into the pericycle cell, then to the xylem, and transported as Fe-citrate complexes to the shoot ends and the shoot apoplast. It then moves into the cytoplasm and into organelles such as chloroplasts. Once incorporated it can but is not normally translocated to other shoots. Hence, it is considered a non-mobile plant nutrient and symptoms of and iron deficiency occurs first in new shoots.
Most studies have shown that Fe movement in plants from the roots to shoots and seeds is a highly regulated process. In leaves it appears to be mostly immobile. Eusebe Gris first showed that foliar application of iron would eliminate iron chlorosis symptoms. Since then foliar applications of Fe has been for remedial use. Recently it was shown that Fe chelated or amino acid chelated iron is taken up by leaves better than ferrous sulfate. In another study, when the tip half of peach tree and sugar beet leaves were treated with Fe, the treated portion of the leaf showed regreening, increased Fe concentrations, and increased photosynthetic pigments. The non-treated half of the same leaf showed slight Fe increase but no regreening. Thus, there may be slight movement of Fe distal from the application site. However, Fe sprays can be used to remediate Fe chlorosis but results may vary depending on the leaf type and Fe fertilizer used. Synthetic Fechelates are available but each have certain conditions that affect their performance, are more costly, and may have some environmental concerns. Amino Fe chelates are also efficient for foliar application and are more environmentally friendly. One study on corn showed that FeSO4 and amino acid chelated iron was translocated to the stems and older leaves and that the amino acid chelated iron was absorbed better. However, soil applications of Fe may be needed to address chronic deficiencies since iron generally is fixed and incomplete foliar coverage would not help the whole plant.
Iron in Soils
Iron is abundant in soils (7,000 to 500,000 mg per kilogram of soil). The bulk of it is in the insoluble ferric (Fe3+) form of iron which is not available for plant uptake. Most of it is present in rock, minerals, and soils in ferrous (Fe2+) and ferric (Fe3+) forms. The most abundant form of iron in soils is ferric oxide (Fe2O3) or hematite, which is extremely insoluble and gives a red color to the soil. The oxide form is commonly hydrated. In aerobic soils, the oxide, hydroxide, and phosphate forms control the concentration of Fe in solution and availability to plants.
In a typical aerated soil with a pH of about 6, the concentrations of Fe3+and Fe2+are around 10-15molar. As pH increases, Fe3+activity decreases and insoluble Fe2+hydroxide is formed. Thus, alkaline soils decrease iron availability and causes iron deficiency in plants. Addition of reducing agents with H+or other reductants increases Fe solubility and Fe can be adsorbed on soil as an exchangeable ion.
In waterlogged saturations, Fe3+is reducedto Fe2+. Abundant sulfates in the soil can become oxygen sources for bacteria. The result is formation of black-colored ferrous sulfide.
Organic matter helps increase Fe solubility and availability in the soil. Iron exists as soluble Fe2+iron in the soil solution or is absorbed onto the soil particle surfaces. Organic acids (aliphatic acids, amino acids), and complex polymers (humic and fulvic acids) form soluble complexes with Fe or act as chelating agents that hold Fe in more soluble forms. Iron chelates in soil with organic matter can reach 10-4to 10-3M while aerated low organic matter soils 10-8to 10-7M, too low for adequate growth of most plants.
Iron can move in the soil from regions of high concentrations to locations of lower concentrations, i.e., the area at the root surface where iron is being taken up by the plant.
Iron Deficiency Symptoms
Iron deficiency manifests itself as an interveinal chlorosis of young leaves and in some plants, stunting may occur. Most plants with Fe deficiency will restrict root extension and increase root hair production. Correspondingly increase secretion of protons and reductants to acidify the environment and increase Fe availability and uptake. Grasses on the other hand will expand the root system and secrete phytosiderophores to capture more iron.
It is important to note that other nutrient deficiencies cause chlorosis in plants. A nitrogen deficiency in grasses causes yellowing in older leaves first and tends to occur over a broad area while Fe deficiency causes striping and occurs in spots or patches. Magnesium deficiency also has interveinal chlorosis but on older leaves. Sulfur deficiency causes pale green chlorosis over the whole plant. Chlorosis can also be caused by pathogens, flooding and herbicide damage so these must be eliminated before diagnosing an Fe deficiency exists. Corn and sugarcane may show iron deficiency symptoms when there is a K deficiency since Fe becomes immobilized in stem nodes.
Iron deficiency in soils is caused by lack of soluble or available iron, not the lack of iron in the soil. Soils with a pH >7.2 can result in the appearance of iron deficiency symptoms since iron is in the unavailable oxide form. Over liming soils raises the pH and Fe becomes insoluble. The calcium also becomes a competing cation that interferes with Fe uptake. Copper from copper containing pesticides is also a competing cation. Presence of cadmium and cobalt causes iron to accumulate in the roots causing Fe deficiency symptoms in the shoots.
Iron deficiency can also occur in well weatheredacid soils where Fe can bind to phosphate. Acidic soils can also lead to iron deficiency symptoms due competition with manganese uptake. Low soil temperatures can slow root extension and reduce Fe uptake. Overly wet, compacted soils, and low organic matter soils are susceptible Fe deficient soils.
Fertilizing with Iron
There are several forms and methods for applying iron as a fertilizer. Inorganic iron fertilizers include ferric sulfate, ferrous ammonium phosphate, ferrous ammonium sulfate, and oxides of iron. However, improper soil conditions can rapidly oxidize the iron to insoluble forms and thus provide minimal benefit to the plant. Chelated iron fertilizers such as Fe-EDDHA, Fe-EDTA, citrate-Fe, amino acid – Fe, and humic-fulvic FE are more effective in providing iron to the plants via soil and foliar applications. However, some of these fertilizers can be more expensive and cost prohibitive for large acreage crops especially for foliar applications where repeated applications may be needed to correct iron deficiencies. There are other things to consider when choosing and Fe-chelate. EDTA chelatesare not stable in alkaline soils and have a high affinity for calcium (ties up calcium). DTPA requires pH levels below 7 but has a lower affinity for calcium. EDDHA chelates are not affected by pH but can be very expensive.
The newer amino chelated Fe fertilizers generally result in higher nutrient uptake efficiency (use less to achieve same result) and are highly effective for plant growth. These have been used for leaf recovery, improving mineral composition, higher chlorophyll concentration, delay of leaf senescence, stimulation of cell growth, and improve plant quality.
Crop Responses to Iron
Soil and foliar applications of Fe fertilizers have been used to remediate chlorosis symptoms. Most crops with that are deficient in Fe will show yield increase with proper levels of Fe. Crops that are slightly tolerant of low Fe levels will show slight improvement. Used alone or in combination with other micronutrients, Fe can have positive effects on growth, yield, iron concentration in plants (biofortification), and fruit quality is improved.
Iron as a Pesticide
Iron phosphate slug and snail baits were originally used in Europe and has been registered in the United States since 1977. Determined by the EPA to be relatively non-toxic (compared to metaldehyde slug and snail baits), these are sold as pelleted bait, applied to the ground around plants, and when properly used (About 4-8 pellets/sq. ft). In 2013, iron phosphate products were approved by OMRI. However, not all iron phosphate products are the same. Some contain spinosads which kills insects, sow bugs, and pill bugs. Others use sodium ferric EDTA, a chelated form of iron. While more effective at doses lower than those for iron phosphate, it can be toxic if accidentally consumed in large quantities by animals.
Iron Toxicity Symptoms
Iron toxicity is not normally a problem but can cause leaf bronzing. In waterlogged systems Fe3+is converted to Fe2+by microorganisms and can become toxic under acid conditions.
Interveinal chlorosis in young leaves cause by an iron deficiency is common in citrus crops. In general, plants that prefer moderately and slightly acidic soils will perform better when Fe is available. Blueberries and cranberries prefer lots of iron. Strawberries, raspberries and grapes will also benefit from higher iron content. Although the total amount needed is low compared to other nutrients, iron plays an important role in photosynthesis. While iron is abundant is most soils, environmental conditions greatly affect its availability. Iron can be actively taken up by roots and passively via leaves. Chelated iron appears to be best for foliar applications when the leaves show deficiency symptoms.