Understanding and Applying Chelated Fertilizers Effectively Based on Soil

Plant nutrients are one of the environmental factors essential for crop growth and development. Nutrient management is crucial for optimal productivity in commercial crop production. Those nutrients in concentrations of = 100 parts per million (ppm) in plant tissues are described as micronutrients and include iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B), chlorine (Cl), molybdenum (Mo), and nickel (Ni). Micronutrients such as Fe, Mn, Zn, and Cu are easily oxidized or precipitated in soil, and their utilization is, therefore, not efficient. Chelated fertilizers have been developed to increase micronutrient utilization efficiency. This publication provides an overview of chelated fertilizers and considerations for their use to county Extension faculty, certified crop advisers (CCAs), crop consultants, growers, and students who are interested in commercial crop production.

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What is chelated fertilizer?

The word chelate is derived from the Greek word chelé, which refers to a lobster's claw. Hence, chelate refers to the pincer-like way in which a metal nutrient ion is encircled by the larger organic molecule (the claw), usually called a ligand or chelator. Table 1 lists common natural or chemical synthetic ligands (Havlin et al. ; Sekhon ). Each of the listed ligands, when combined with a micronutrient, can form a chelated fertilizer. Chelated micronutrients are protected from oxidation, precipitation, and immobilization in certain conditions because the organic molecule (the ligand) can combine and form a ring encircling the micronutrient. The pincer-like way the micronutrient is bonded to the ligand changes the micronutrient's surface property and favors the uptake efficiency of foliarly applied micronutrients.

Why is chelated fertilizer needed?

Because soil is heterogeneous and complex, traditional micronutrients are readily oxidized or precipitated. Chelation keeps a micronutrient from undesirable reactions in solution and soil. The chelated fertilizer improves the bioavailability of micronutrients such as Fe, Cu, Mn, and Zn, and in turn contributes to the productivity and profitability of commercial crop production. Chelated fertilizers have a greater potential to increase commercial yield than regular micronutrients if the crop is grown in low-micronutrient stress or soils with a pH greater than 6.5. To grow a good crop, crop nutrient requirements (CNRs), including micronutrients, must be satisfied first from the soil. If the soil cannot meet the CNR, chelated sources need to be used. This approach benefits the plant without increasing the risk of eutrophication.

Several factors reduce the bioavailability of Fe, including high soil pH, high bicarbonate content, plant species (grass species are usually more efficient than other species because they can excrete effective ligands), and abiotic stresses. Plants typically utilize iron as ferrous iron (Fe2+). Ferrous iron can be readily oxidized to the plant-unavailable ferric form (Fe3+) when soil pH is greater than 5.3 (Morgan and Lahav ). Iron deficiency often occurs if soil pH is greater than 7.4. Chelated iron can prevent this conversion from Fe2+ to Fe3+.

Applying nutrients such as Fe, Mn, Zn, and Cu directly to the soil is inefficient because in soil solution they are present as positively charged metal ions and will readily react with oxygen and/or negatively charged hydroxide ions (OH-). If they react with oxygen or hydroxide ions, they form new compounds that are not bioavailable to plants. Both oxygen and hydroxide ions are abundant in soil and soilless growth media. The ligand can protect the micronutrient from oxidization or precipitation. Figure 1 shows examples of the typical iron deficiency symptoms of lychee grown in Homestead, Florida, in which the lychee trees have yellow leaves and small, abnormal fruits. Applying chelated fertilizers is an easy and practical correction method to avoid this nutrient disorder. For example, the oxidized form of iron is ferric (Fe3+), which is not bioavailable to plants and usually forms brown ferric hydroxide precipitation (Fe(OH)3). Ferrous sulfate, which is not a chelated fertilizer, is often used as the iron source. Its solution should be green. If the solution turns brown, the bioavailable form of iron has been oxidized and Fe is therefore unavailable to plants.

In the soil, plant roots can release exudates that contain natural chelates. The nonprotein amino acid, mugineic acid, is one such natural chelate called phytosiderophore (phyto: plant; siderophore: iron carrier) produced by graminaceous (grassy) plants grown in low-iron stress conditions. The exuded chelate works as a vehicle, helping plants absorb nutrients in the root-solution-soil system (Lindsay ). A plant-excreted chelate forms a metal complex (i.e., a coordination compound) with a micronutrient ion in soil solution and approaches a root hair. In turn, the chelated micronutrient near the root hair releases the nutrient to the root hair. The chelate is then free and becomes ready to complex with another micronutrient ion in the adjacent soil solution, restarting the cycle.

Chemical reactions between micronutrient chelates and soil can be avoided by using a foliar application. Chelated nutrients also facilitate nutrient uptake efficiency for foliar application because crop leaves are naturally coated with wax that repels water and charged substances, such as ferrous ions. The organic ligand around the chelated micronutrient can penetrate the wax layer, thus increasing iron uptake (Figure 2). Compared to traditional iron fertilization, chelated iron fertilization is significantly more effective and efficient (Figure 3) than non-chelated fertilizer sources.

Therefore, chelated fertilization can improve micronutrient use efficiency and make micronutrient fertilization more cost effective. The images in Figure 3 show the difference in three treatments with lychee: chelated Fe(II) is greener than FeSO4 plus sulfuric acid, and FeSO4 plus sulfuric acid is greener than no iron fertilization (Schaffer et al. ).

Which crops often need chelated fertilizers?

Vegetable and fruit crop susceptibility to micronutrients differs significantly (Table 2). For those in the highly or moderately susceptible categories, chelated fertilizers are often needed. For those with low susceptibility, no chelated fertilizers are needed unless the soil is low in micronutrient bioavailability, as demonstrated by a soil test. Soil pH is a major factor influencing micronutrient bioavailability; therefore, if soil pH is greater than 6.5, then the soil may have limited micronutrient bioavailability, and chelated fertilizers may be needed.

Which chelated fertilizer should be used?

Each of the ligands (Table 1) can form a chelated fertilizer with one or more micronutrients. The effectiveness and efficiency of a particular chelated fertilizer depends on the pH of the plant growth medium.

The ligands EDTA, DTPA, and EDDHA are often used in chelated fertilizers (Table 4). Their effectiveness differs significantly. EDDHA chelated Fe is most stable at soil pH greater than 7 (Figure 4, A and B). Chelated fertilizer stability is desired because it means the chelated micronutrient will remain in a bioavailable form for a much longer time period, thus increasing micronutrient use efficiency in vegetable and fruit production. The stability of three typical chelated Fe fertilizers varies at different pH conditions (Figure 4, A). The Y-axis represents the ratio of chelated Fe to total chelate and ranges from 0 to 1.0. A value of 1.0 means the chelate is stable. The X-axis represents soil pH. At 6.0, the ratios for all three chelated Fe fertilizers are 1.0 (stable), but at pH 7.5, only the ratio of EDDTA chelated Fe is 1.0. That of DTPA chelated Fe is only 0.5, and that of EDTA chelated Fe is only 0.025. So, in practice, EDDTA chelated Fe fertilizer is most effective when pH is greater than 7 but most costly. Accordingly, crop yields of these three chelated fertilizers are in this order: FeEDDHA > FeDTPA > FeEDTA (Figure 4, B). See Micronutrient Deficiencies in Citrus: Iron, Zinc, and Manganese (https://edis.ifas.ufl.edu/publication/ss423) for effective pH ranges of iron chelates. Table 3 shows the relationship between soil pH and chelated fertilizer requirement.

Correction of Fe deficiency depends on individual crop response and many other factors. For instance, for vegetables, the rate is usually 0.4–1 lb. chelated Fe in 100 gal. of water per acre. Deciduous fruits need 0.1–0.2 lb. chelated Fe in 25 gal. of water per acre (Table 5). Foliar application is more effective than soil application. For foliar application, either inorganic or chelated Fe is effective, but for fertigation, chelated Fe should be used. In high pH soil, crops are also vulnerable to Cu deficiency stresses. Chelated Cu is significantly more effective than inorganic Cu. A commonly used copper chelate is Na2CuEDTA, which contains 13% Cu. Natural organic materials have approximately 0.5% Cu (Table 5).

In addition to soil pH, Mn is also influenced by aeration, moisture, and organic matter content. Chelated Mn can improve Mn bioavailability. Mn deficiency occurs more often in high pH and dry soil. Similar to other micronutrients, foliar spray is much more effective than soil application. For commercial vegetable production, 0.2–0.5 lb. MnEDTA in 200 gal. of water per acre can effectively correct Mn deficiency (Table 5). Zinc is another micronutrient whose bioavailability is closely associated with soil pH. Crops may be susceptible to Zn deficiency in soil with pH > 7.3. Spraying 0.10–0.14 lb. chelated Zn in 100 gal. of water per acre is effective (Poh et al. ). Animal waste and municipal waste also contain Cu, Mn, and Zn micronutrients (Table 5). For more information about micronutrient deficiency in crops, see Plant Tissue Analysis and Interpretation for Vegetable Crops in Florida (https://edis.ifas.ufl.edu/publication/ep081), Micronutrient Deficiencies in Citrus: Iron, Zinc, and Manganese (https://edis.ifas.ufl.edu/publication/ss423), and Iron (Fe) Nutrition of Plants (https://edis.ifas.ufl.edu/publication/ss555).

Practical Take-Home Message

• High pH soil (pH > 6.5) often has low bioavailability in micronutrients such as Fe, Mn, Zn, and Cu, and micronutrient fertilizers are needed for commercial crop production.
• Crop susceptibility to the above micronutrient deficiencies depends on the plant species and cultivar. Commercial crops can be categorized into three susceptibility groups: high, medium, and low. The first two groups often need chelated fertilizers.
• Inorganic water-soluble micronutrient application to the soil is often ineffective for correcting micronutrient disorders.
• Chelated fertilizers are less reactive to soil conditions and can significantly enhance nutrient uptake and utilization efficiencies.
• Chelate fertilization rates range from 0.2 to 1 lb. micronutrient per acre for vegetable production and 0.1–0.5 lb. micronutrient per acre for fruit production.
• Foliar application of chelated fertilizers is often more effective than soil application.

References

Alloway, B. J. . Micronutrient Deficiencies in Global Crop Production. Heidelberg, Germany: Springer Science + Business Media, B. V. Berlin.

Fullerton, T. . "Chelated Micronutrients." Agro Services International Inc. http://www.agroservicesinternational.com/Articles/Chelates.pdf.

Havlin, J. L., J. D. Beaton, S. L. Tisdale, and W. L. Nelson. . Soil Fertility and Fertilizers: An Introduction to Nutrient Management (7th ed.). Upper Saddle River, NJ: Pearson Education.

Lindsay, W. L. . "Role of Chelation in Micronutrient Availability." In The Plant Root and Its Environment, edited by E. E. Carson, 507-524. Charlottesville: University Press of Virginia.

Morgan, B., and O. Lahav. . "The Effect of pH on the Kinetics of Spontaneous Fe (II) Oxidation by O2 in Aqueous Solution – Basic Principles and a Simple Heuristic Description." Chemosphere 68(11): –.

Norvell, W. A. . "Equilibria of Metal Chelates in Soil Solution." In Micronutrients in Agriculture, edited by J. J. Mortvedt, P. M. Giordano, and W. L. Lindsay, 115–136. Madison, WI: Soil Science Association of America.

Sekhon, B. S. . "Chelates for Micronutrient Nutrition among Crops." Resonance 8(7): 46–53. https://doi.org/10./BF

Schaffer, B., J. H. Crane, C. Li, Y. C. Li and E. A. Evans. . "Re-Greening of Lychee (Litchi chinensis Sonn.) Leaves with Foliar Applications of Iron Sulfate and Weak Acids." Journal of Plant Nutrition 34(9): –.

Common synthetic and natural chelate compounds (ligands).

Selected vegetable and fruit crop species' relative susceptibility* to some micronutrient deficiencies.

Cu

If you are looking for more details, kindly visit chelated micro fertilizer.

Fe

Mn

Zn

Soil pH and chelated fertilizer requirements in commercial crop production.

Chelated fertilizers, formula, and nutrient content (%).

Source

Formula

Nutrient (w/w, %)

Examples of chelated fertilization rates for selected commercial vegetable and fruit crops.

Crop

Nutrient Rate

Source

By Dr. Wes Chun Ph.D. 
Grower's Secret, Chief Science Officer Emeritus

April 4,

Introduction
Supplying micronutrients to plants can be problematic. Many N, P, or K agricultural fertilizers lack sufficient quantities of essential micronutrients to meet plant needs. Micronutrients are often applied pre-plant but additional in season applications may be needed to address plant micronutrient deficiencies that may occur due to high or low soil pH, plant or environmental issues. Synthetic micronutrient fertilizers in the ionic form are available to plants within a limited pH range (pH 6.5 to 7.5).  High soil pH (pH>7.5) decreases the availability of copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn). When the soil pH is low, the macronutrients calcium (Ca),  magnesium (Mg), and molybdenum (Mo) may be limited. Thus, soil applications of synthetic micronutrients are particularly challenging and can be more problematic especially in sandy soils or when growing crops that have high micronutrient demands. Synthetic and organic chelated micronutrients were developed that are less reactive to soil conditions. These fertilizers can significantly enhance nutrient uptake and improve efficiency of utilization.  Application rates of most chelated micronutrients are 0.2 to 1 pound per acre for vegetables, and 0.1 to 0.5 pound per acre for fruits.  Foliar application is often more effective.

What are Chelated Micronutrients?
Chelated micronutrients are fertilizers where the micronutrient ion (for example Fe or iron) is surrounded by a larger molecule called a ligand or chelator. Ligands can be natural or synthetic chemicals.  These compounds combined with a micronutrient forms a chelated micronutrient. Chelated micronutrients are protected from oxidation, precipitation, and immobilization in certain conditions. A few examples of ligands are in Table 1. These chelates have different effective pH ranges.  The effective pH range for Fe-EDTA is 4 to 6.5, Fe-DTPA is 4 to 7.5, and Fe-EDDHA is 4 to 9.  Fe-EDDTA is effective when pH is greater than 7 but it is costlier. There are also many naturally occurring chelating agents such as amino acids, organic acids, humic and fluvic acids, ligninosulfonates, ligninipolycarboxylates, sugar acids, phenols, polyphosphates, flavonoids, and siderophores. These are generally less expensive, functional over a wider pH range, and less toxic to plants.  Both synthetic and non-synthetic chelates are OMRI permitted.

Are Chelated Micronutrients Needed?
Soil is heterogenous and complex and applied traditional micronutrients may become unavailable to the plant because of oxidation or precipitation. Use of chelated micronutrients improves the bioavailability of micronutrients and can contribute to crop quality and yield. Chelated micronutrients should be considered if plants display micronutrient stress, in alkaline soils that limit micronutrient availability, or when soil micronutrient supplementation is insufficient.

Where am I Least Likely to Need Chelated Micronutrients? 
Soil that has igneous parent material or other rocks high in nutrients can supply the necessary micronutrients. Some igneous rocks are high in zinc. Clay soils with high Cation Exchange Capacity (CEC) or micronutrients in the mineral structure may not need additional micronutrients. High CEC soils can “capture” micronutrients and maintain them in ionic form for the plants. Micronutrients in the mineral structure are released over time.  Soils with high organic matter also tends to maintain micronutrients in bioavailable form.

Where am I Most Likely to Need Chelated Micronutrients?
US and Canadian soils may be deficient in boron (B). Boron is not influenced by organic matter, leaches easily, and can be locked up by fresh lime. Boron is higher in alluvial than igneous soils. It is low in subsoils and dry weather may create boron deficiency symptoms when plant roots go deeper in search of water.

The type of soil can affect micronutrient availability. Leached sandy soils have low CEC are low in B, Cu, Mn, Mo, and Zn. Quartz is low in Zn. Alluvial soils, weathered acid soils that are subject to leaching, and low organic matter soils all tend to be deficient in micronutrients. High pH soils are low in Fe, Mn, Zn.  Acidic soil may be deficient in Mo.

Does my Crop Need Chelated Micronutrients?
The answer is an obvious yes if you are seeing micronutrient deficiency symptoms in your crop. The good news is that foliar application of a micronutrient can usually remediate the problem in a few days.  You should consider additional micronutrients if you are growing a particularly susceptible crop such as citrus which is susceptible to low levels of Cu, Fe, Mn, and Zn and especially if the soil is alkaline. (See Table 2 for a list of vegetables and fruits, and their susceptibilities to micronutrient deficiencies).

It is important to address micronutrient deficiencies especially in crops that are highly susceptible. In these instances, significant improvement can be obtained in crop quality and yields. In situations where no micronutrient deficiency exists, application of additional micronutrients may have some value. For example, total yield in pear was higher (but not statistically significant) with amino acid chelated Fe compared to controls and ranged from 7 to 64% increase between years of the study. Shoot length increased and there were no changes in fruit firmness. In other crops, Fe and Zn content in leaves were higher when Fe and Zn chelates were used. Since amino acid chelated micronutrients are relatively new to the market, additional research is needed to determine if there are consistent benefits for additional micronutrient supplementation.

Take-Home Message

• High pH soil often has low bioavailability in micronutrients such Cu, Fe, Mn, and Zn.
• Crop susceptibility to micronutrient deficiencies depends on the plant species and cultivar, and environmental conditions.
• Inorganic water-soluble micronutrient soil application is often ineffective for correcting micronutrient disorders (foliar application is recommended).
• Chelated micronutrients are less reactive to soil conditions and can enhance nutrient uptake and utilization.

Note:  Check with your local recommendations and regulations as some states require documentation of a micronutrient deficiency before use of these types of products.

For more information, please visit agricultural biostimulant.