By Dr. Muhammad Imran, Technical Development Manager Micronutrients, Nouryon
Iron (Fe) is an integral component of electron transport chains and co-factor of several vital enzymes in living organisms. Except in few types of bacteria, where Fe is substituted with other metals, it is virtually essential for all life forms. Iron, as an essential element for plant growth, was discovered by Eusebe Gris in the 1940s when he demonstrated that Fe application to roots and leaves can reverse certain chlorosis in plants.
The role of iron in plants is simple: without it, plants are not green. Fe is essential in the synthesis of chlorophyl, which is the reason for yellowing (chlorosis) of leaves under iron-deficient conditions as shown in Figure 1. Furthermore, iron plays a vital role in the maintenance of choloplast structure and its functioning, the synthesis and functioning of multiple enzymes, a component of several key protiens that are important for nodule nitrogen fixation in legume crops, like soybeans
Iron is also very important for human health and development due to its vital role in the body’s product of blood hemoglobin. Anemia linked iron deficiency is a serious global public health problem that particularly affects young children and pregnant women. The WHO (World Health Organization) estimates that 42 percent of children less than five years of age and 40 percent of pregnant women worldwide are anemic (Reference). Despite an abundance of iron in the earth’s crust, its bioavailability for life is highly limited due to its presence in oxidized form. Plants serve as the primary source for iron in the food chain. Therefore, sufficient iron in plant tissues (roots, stem, leaves, etc.) and edible parts (fruits, grains, etc.) is not only necessary for optimal growth but also for human and animal nutrition.
Iron in soils Iron is one the most abundant metals in soils. Its concentrations in soils can be in the range of 0.2 percent to 55 percent (2,000 to 550,000 mg/kg) but it can be significantly variable even within localized areas due to soil types and other factors (Reference). Iron in soils is present in either ferrous (Fe2+) or ferric (Fe3+) forms under typical soil conditions. Soil pH and Eh (redox potential) determine the valence state of the Fe; form of Fe compounds is dependent on the presence of other chemicals (e.g., sulphur is needed to form FeS2 or pyrite). In soils, Fe2+ is more soluble and hence more available for plant uptake compared to Fe3+. The predominant form of Fe in soils is goethite (α-FeOOH). The divalent Fe2+ can be oxidized to the trivalent Fe3+ state and may form oxide or hydroxide precipitates which become unavailable to plants as micronutrients. For example, ferric oxide (Fe2O3) or hematite is the most abundant form of Fe in soils, which is insoluble and gives a red colour to the soil. The general rule for the fixation and mobilization of Fe in the soil is that oxidizing and alkaline conditions promote the precipitation of insoluble Fe3+ oxides, whereas a solution of Fe2+ compounds is promoted under acidic and reducing conditions. Soil water status also plays an important role in the Fe availability by moderating Fe2+ and Fe3+ forms.
For example, in water-logged or lowland soils, reducing conditions prevail and iron will be converted to Fe2+ whose availability is good (and can even cause Fe toxicity); in well-aerated or upland soils, iron will be oxidized to Fe3+ and precipitate as oxides or hydroxides, which are poorly available for plant uptake.
Fe uptake in plants and challengesThe leaf Fe concentration varies in different plant species between 50 and 250 ppm (dry biomass basis). If the Fe concentration is < 50 ppm, usually deficiency symptoms appear, and toxicity may be observed at concentration above 500 ppm. Iron uptake is strongly mediated by plants to acquire in sufficient quantities to maintain growth. There are two strategies identified in plants for Fe uptake, (strategy I) reduction of Fe3+ - complexes to Fe2+ ions at the root surface and absorb through the roots, and/or (strategy II) release of specific Fe3+ binding, low molecular weight, organic polydentate ligands known as phytosiderophores, which complex with Fe3+ ions and bring into a soluble solution form to make them available for absorption via the roots. Both these strategies for Fe uptake are used by distinctive separate plant taxonomic groups. Strategy I mechanism is restricted to the dicots and the monocots not included in the Poaceae while Strategy II uptake mechanism is restricted to grass family (Poaceae).
It is necessary that Fe is in soluble form to be absorbed by plant roots as precipitated forms of Fe cannot be taken up. There are several factors that affect the availability and uptake of Fe in plants. Some measurable soil properties like soil pH, carbonate concentration, aeration, and organic matter content are good indicators of Fe availability in the soil for plant uptake. Figure 2 shows the key soil factors and their possible share, which cause the reduced availability of Fe to plants. Soil pH is the master factor in controlling the availability of Fe in soil. In general, Fe solubility starts decreasing at pH 6.0 and as soil pH values approach 7.5, Fe is in its least soluble form, explaining the occurrence of Fe deficiency chlorosis in soils originated from calcareous parent material (e.g. limestone) and higher soil pH ranges. Under high pH soil conditions in the presence of CaCO3 and PO43- solubility of Fe is decreased due to formation of precipitates which results plant unavailable Fe forms. Accumulation of salts is a major reason for increased soil pH, according to FAO (Food and Agriculture Organization) one-third of the arable lands is salt affected. In such soils, due to high pH, management of Fe availability for crops is a great challenge.
A global soil pH map (Figure 3) reveals that mostly soils in the densely populated areas of the world are high in pH which makes it difficult to grow crops in these areas due limited Fe availability for plants. In India, for example, approximately 6.74 million ha area is salt-affected and according to an estimate every year additional 10 percent area is getting salinized, and around 50 percent of the total arable land would be salt-affected by 2050. Similar trends of soil salination are reported for other regions like Africa, Australia, and north and south America.
As mentioned, plants take up nutrients only when in soil they are presenting is in soluble form, which can be a problem with Fe in many soils. At a pH above 6.5, Fe interactions with carbonates and other ions in the soil solution cause iron precipitation and Fe becomes unavailable to plants. Although, plants have a genetic capacity to utilize soil Fe to a certain extent, in intensive and high yielding agricultural crops external/additional Fe fertilizer application is often necessary.
Chelated-Fe fertilizersTo ensure the Fe availability for plants in the soil solution, chelated Fe-based fertilizers are the best source of iron. The word chelate originated from ‘chelae’ a Greek word for a crab claw (Figure 4). A chelating agent forms stable and water-soluble metal compounds when it reacts with metal ions. The chemical structure and bonding strength of the chelating agent with metal ions determines the general stability and likelihood to bond with other compounds.
There are three common types of Fe chelate used in agriculture: Fe-EDTA (Fe-Ethylenediaminetetraacetic acid), Fe-DTPA (Fe-Diethylenetriaminepentaacetic acid), and EDDHA (Ethylenediamine di (ortho-hydroxyphenylacetic acid). There is also a special Fe chelate called Fe-HBED (Fe-di ortho-HydroxyBenzyl-Ethylenediamine – Diacetic acid). Structures of different Fe-chelates are shown in Figure 5. Red circles in both Fe-EDDHA and Fe-HBED structures represents the ortho-ortho bonding of Fe with the chelating agents.
Iron-EDTA and Fe-DTPA are commonly used in hydroponics, peat-based substrates, and in soils with neutral soil pH (6.0 – 7.0). At pH 6.0 or above, Fe-EDTA is a good Fe source but by pH 6.5, more than 50 percent iron is precipitated, and by pH 7.0, almost all the iron is precipitated and is unavailable to plants. Fe-DTPA is a good iron source up to pH 7.0. However, due to its limited stability at higher pH availability of Fe to plants its use is also restricted.
Fe-EDDHA and Fe-HBED termed as HPIC (high performance iron chelates) are the most appropriate and useful iron chelates in high pH
soils. In Fe-EDDHA, like EDTA, the Fe ion is bound by a hexadentate ligand, using two amines, two phenolate centers, and two carboxylates as the six binding sites. Depending on the molecular structure and bonding of iron, Fe-EDDHA can be named as ortho-ortho EDDHA or ortho-para EDDHA or para-para EDDHA. Fertilizer based on Fe-EDDHA contains different types of EDDHA molecules. The strength and efficiency of Fe-EDDHA depends on the presence of ortho-ortho EDDHA in the final product as ortho-para or para-para. Fe-EDDHA is extremely unstable under alkaline high pH soils conditions, hence not available for plant uptake. Iron fertilizers based on Fe-HBED usually contains 100 percent ortho-ortho bound Fe molecules which ensures 100 percent availability of the applied product. Ortho-ortho isomer of Fe-HBED has a higher stability constant (39) as compared to ortho-ortho Fe-EDDHA (35) isomer.
There are different types of Fe chelates available; therefore, to ensure sustainable use of Fe for crop production 4R approach of nutrients stewardship by considering the Right source, Right rate, Right timing, and Right placement of iron fertilization can compensate for Fe deficiency in crops. pH stability (Reference) of different Fe chelates must always be considered before selecting the most suitable Fe chelate.
For soil application, Fe-EDDHA and Fe-HBED are the best source of iron fertilizer but as mentioned, quality of Fe-EDDHA varies on the presence of ortho-ortho Fe bonding. For example, a six percent iron fertilizer with 3.2 percent ortho-ortho Fe-EDDHA have only 3.2 percent plant available stable Fe-chelate under alkaline conditions, similar is true for 4.8 percent ortho-ortho Fe-EDDHA containing fertilizer where only 4.8 percent of the applied Fe is available for plant uptake. Unlike Fe-EDDHA, fertilizer based on Fe-HBED usually provides 100 percent plant available Fe.
Due to high ortho-ortho bound Fe, even at higher soil pH conditions, Fe-HBED is able to keep Fe in the solution for longer periods of time as compared to Fe-EDDHA. This character of Fe-HBED helps to reduce the dose rate and increase intervals between the application timing. In laboratory and greenhouse experiments, Clara Martin-Fernandez et al. (2016), Fe-HBED demonstrated a high durability in soils and soil materials, maintaining more than 80 percent of Fe chelated after 70 days, and its application at an early physiological stage resulted in a high Fe accumulation in soybean and soil after 36 days. In contrast, the stability of Fe-EDDHA decreased because of the retention of its lowest stable isomers. Different trials in soybean have shown that amount of ortho-ortho Fe plays an important role in the iron fertilizers efficiency. Soybean plants reflected a better plant growth and had greener leaves when supplied with higher ortho-ortho Fe fertilizers (Figure 6a). Goos (2021) has also shown (Figure 6b) that increased levels of o-o Fe in fertilizer have positive effect on soybean growth. In other reported trials Fe-HBED increased chlorophyll contents up to 19 percent in cucumber and 11 percent in tomato leaves (Medawara et al., 2016).
Considering the stability and efficiency of different Fe chelates,
Fe-EDDHA and Fe-HBED are the most suitable source of Fe fertilizer in high pH and alkaline soils. Furthermore, not the total amount of Fe mentioned or present in the fertilizer is important rather percentage of ortho-ortho Fe determines the fertilizer quality. Therefore, Fe-HBED is not only the best source of Fe for soil application but also in fertilizers and biostimulant mixtures intended for soil application. ●
Supporting literature:
Bin, L. M., Weng, L., & Bugter, M. H. J. (2016). Effectiveness of FeEDDHA, FeEDDHMA, and FeHBED in preventing iron-deficiency chlorosis in soybean. Journal of Agricultural and Food Chemistry, 64, 8273–8281.
Clara M. F., S. L-Rayo, L. H- Apaolaza and J. J. Lucena. (2017). Timing for a sustainable fertilization of Glycine max by using HBED/Fe3+ and EDDHA/Fe3+ chelates. Journal of Science of Food and Agriculture. 97 (9), 2773 – 2781.
Goos. R. J. (2021). Laboratory and greenhouse evaluation of four iron fertilizer sources. Agric Environ Lett. 6: e20052.
Medawara G., G. Srourb and D. El Azzia. (2016). Comparison of chlorophyll content in greenhouse tomato and cucumber leaves after HBED-Fe and EDDHA-Fe applications. Frontiers in Life Science. 9 (3), 182 - 189.
Marschner, P. (2011) Mineral Nutrition of Higher Plants. 3rd Edition, Academic Press, London
Sun A. K. and Mary L. G. (2003). Mining Iron: Iron transport and uptake in plants. FEBS Letters. 581, 2273 - 2280.
Figure 1. Fe deficiency symptoms in long beans in Mexico
Sufficient iron in plant tissues (roots, stem, leaves)and edible parts (fruits, grains) is necessary for optimal growth
Figure 2. Factors affecting the solubility of Fe in soils
Figure 3. Global soil pH map
Source: soilgrid.org
Figure 4. Analogical diagram of chelation
Figure 5. Different types of Fe-chelates
Figure 6 a & b. Effect of different levels of o-o Fe in fertilizer on soybean growth in two different trials- (a) leaf greening and (b) plant dry biomass per pot.
pH stability of different Fe chelates must always be considered before selecting the most suitable Fe chelate.
Vanguard Award Winner
Honouring those who have made significant contributions and worked hard to make an impact during their career is an important practice in any industry. It celebrates great work and accomplishments, while encouraging and inspiring the next generation. Each year, the Irrigation Association (IA) recognizes excellence in irrigation by presenting awards to deserving individuals and organizations.
This year’s award recipients represent some of the most innovative and dedicated individuals in our industry. These winners will be officially recognized during the General Session at the 2022 Irrigation Show and Education Week in Las Vegas Dec. 5-9.
Vanguard Award
The IA’s Vanguard Award honours an innovative project in the irrigation industry that exemplifies the IA’s mission of promoting efficient irrigation. These projects are a collaborative effort, executed by a team of individuals, companies, organizations or other group entities.
The SDI-E: Subsurface Drip Irrigation for Dairy Effluent Water Application, a project between Netafim USA, De Jager Dairy Farms, Sustainable Conservation, Western United Dairies and McRee Farms, received the 2022 Vanguard Award. The SDI-E project is an innovative irrigation system that uses patented blending control technology to mix dairy effluent with freshwater to create a natural fertilizer blend for farmers. Using subsurface drip irrigation, the nutrient-rich water is delivered directly to the crop root zone.
“The inspiration [for the project] came directly from our dairy farmers during a time of drought, who saw all their wastewater from their dairy processing as a potentially untapped resource,” said Domonic Rossini, team leader agronomy west at Netafim USA. “With water conservation a critical issue right in our backyard, we saw the urgency of creating a solution with a near-immediate impact and went to work.”
The SDI-E makes it economically possible for dairies to manage their manure in ways that boost water quality, conserve water supplies, keep their farms in line with regulatory mandates and protect their bottom line.
Excellence in Education AwardEach year, the IA recognizes an outstanding educator who teaches irrigation, water management and/or water conservation at a two- or four-year institution with the Excellence in Education Award. These recipients demonstrate a commitment to not only elevate the level of education but also promote the field of irrigation as a viable and sustainable career to students.
Charlie Abee, CAIS, is a 2022 recipient of the Excellence in Education Award. Abee is an agriculture technology and irrigation professor, and he developed an irrigation training program from the ground up at the College of the Sequoias in Tulare, California. Starting in 2017 with a basic water management course, he worked with local industry and California State University partners to create a program that meets the need for an entry-level technician, while also preparing and encouraging students to continue on to higher levels of education. Abee embeds career exploration and awareness into his classes on a daily basis.
When asked about the role he plays in promoting smart irrigation and water use efficiency, Abee said, “As an educator, my role in smart irrigation and water use efficiency is to learn what the research is telling us and put that knowledge into practical skills that I teach my students. Water is our most precious resource, and I convey that to the students with enthusiasm.”
In addition to these award recipients representing the agriculture sector of the industry, the IA is also honouring the following award recipients representing the landscape irrigation sector:
The Industry Achievement Award was presented to Jack Buzzard. Jack Buzzard was the vice president and director of the Rain Bird International Strategic Business Unit until he retired in 2006. During his career Buzzard lived and worked in France, introducing Europe, and later parts of South America, to the concept of total, complete automatic irrigation systems as we know them today.
Bradley Jakubowski was named as a second recipient of the 2022 Excellence in Education Award. Jakubowski is an instructor of plant and turfgrass at The Pennsylvania State University. While at Penn State, he has transformed theory-based turf science and landscape irrigation courses by redeveloping them, nearly from scratch, to incorporate both instructional theory and hands-on troubleshooting and design.
The IA congratulates these award recipients and looks forward to honouring them in person at the 2022 Irrigation Show. Nominations for the 2023 IA awards open in early 2023. Go to irrigation.org/awards for more information about these awards, to view past winners and for announcements about the 2023 awards process. ●
Charlie Abee, CAIS
The Zimbabwe government is launching a USD$20 million Smallholder Irrigation Infrastructure Development Fund to finance irrigation projects in eight provinces of the country. The aim is to strengthen the resilience of smallholder farmers to the drought that is causing food insecurity.
A total of 18 irrigation projects will be funded, with the aim to improve agricultural yields in eight drought-affected provinces. The future schemes will cover a total area of 2,700 hectares, increasing the amount of irrigated land in the country to 252,700 hectares.
Currently, Zimbabwe has 250,000 hectares of irrigated land. The Zimbabwean government’s ambition is to irrigate 350,000 hectares of land, in line with its horticultural growth and recovery plan. ●
DXAS Agricultural Technology, a joint venture between Kagome Co., Ltd. And NEC Corporation, is enhancing NEC's AI-enabled agricultural ICT platform, CropScope, with the addition of AI farming management advice and an automated irrigation control function for pulse drip irrigation.
Conventionally, pulse drip irrigation is commonly known as a cultivation method that maintains optimal soil water content and reduces water consumption. However, this cultivation method has not been widely used because it is difficult to determine the optimal amount of water, which can change constantly, implementation of the method by producers who manage large and multiple fields is complicated, and it requires a heavy workload.
In order to solve these difficulties, Kagome and NEC conducted demonstration tests in Portugal this year, using AI farming advice on irrigation and fertilization provided by CropScope, in order to realize the automation of pulse drip irrigation. The companies state they succeeded in increasing the yield by about 20 percent, with about 15 percent less irrigation when compared to a field that did not utilize CropScope.
Based on the results of the demonstration tests, DXAS will accelerate farming support by promoting the introduction and spread of the new services in the global tomato market, mainly in Europe, the Americas and Australia. The platform’s new functions are expected to be deployed starting in April 2023. ●
Utah, U.S.-based Aqua-Yield, a nanoliquid company for agricultural production, closed its Series A investment round of $23 million.
The round was led by the Larry H. Miller Company with participation from Penny Newman Grain Company and San Leonardo. The company will leverage the funding to accelerate the global adoption of its nanoliquid solutions.
Aqua-Yield’s nanoliquid solutions leverage nanoparticles to make liquid agricultural products more effective at plant absorption, and increase crop yield and profits for farmers while enhancing the sustainability of the soil.
According to Clark T. Bell, Aqua-Yield CEO and co-founder, since 2014, the Aqua-Yield team sustainably grow profits by solving plant absorption problems and have helped thousands of growers reduce their synthetic fertilizer and crop protection inputs by 25-50 percent.
“The patented nano-based technology acts as a catalyst to traditional liquid agricultural fertilizers and crop protection products,” said Bell. “This tech works with all traditional ag inputs by enhancing plant absorption of nutrients and efficacy of crop protectants. With our suite of 16 commercially available products we have an answer for nearly every crop.”
Clark added that its nanoliquid technology enables growers to stretch their ag inputs. “This has been key to growth as the agribusiness industry faces supply chain struggles, product availability, and the current environmental strains that will continue to play a key role in the future.”
The company’s solutions work for farms of all sizes and in all regions of the United States, and is currently used on four million acres of farms, and in more than nine countries. Between 2014 and 2021, the company stated it conducted more than 750 field trials that resulted in an average 3:1 return on investment for growers. “Since 2014, Aqua-Yield’s solution has reduced farmers’ use of micronutrients by up to 80 percent and use of macronutrients by up to 50 percent by minimizing fertilizer and chemical applications and increasing nutrient uptake, germination rates, crop yields and growth cycles.” ●
