Chelates in Hydroponic Solutions
It is important to note:
Microelements are supplied at low concentrations, yet have a profound effect on plant growth.
The microelements are far more critical in terms of their control and management than most of the major elements, particularly in hydroponic systems. Most micronutrient deficiencies can usually be corrected, but when dealing with excesses (toxicity), correction can be difficult, if not impossible. Therefore, care must be taken to ensure that an excess concentration of a micronutrient is not introduced into formulation.
It is important for growers to understand that nutrient and substrate pH, regardless of the chelate species used in any hydroponic nutrient formulation, plays a crucial role in micronutrient availability. That is, regardless of the type/s of chelate/s used in producing a hydroponic fertilizer (liquid or solid), the pH of nutrient solutions and substrates plays a crucial role in ensuring the availability of micronutrients to the plants.The dominant role of the pH with the uptake of micronutrients often involves that sometimes the uptake is more affected by the level of the pH than by the concentrations of the element applied.
I say this because just some nutrient manufacturers/suppliers have made claims, to effect, their products incorporate superior chelators and therefore they imply that plants grown with their formulas will thrive under a wider pH range than the same plants grown in a competitor’s formula. These types of claims are scientifically disproven and are, thus, largely misleading.
In substrate grown crops the best equilibrium for the uptake of the micronutrients (Fe, Cu, Mn, B and Zn) will be gained with values between pH 5 and 6. Molybdenum (Mo) differs somewhat where optimum uptake occurs at higher pH values.
Overview of Chelates
Soilless growing/hydroponics could not exist without the discovery that iron chelates can be used in nutrient solutions for delivering iron to plant roots.
The problem with iron in hydroponics is that iron (Fe) easily forms hydroxides and insoluble salts with other ions present in hydroponic solutions. Because of this it is important to provide Fe in a form in which it is accessible to the plant and does not fall out of solution through ‘precipitation’. Precipitation refers to the formation of an insoluble salt when two solutions containing soluble salts are combined. The insoluble salt that falls out of solution is known as the precipitate, hence the reaction’s name.
A precipitation reaction can occur when two solutions containing different salts are mixed, and a cation/anion pair in the resulting combined solution forms an insoluble salt; this salt then precipitates out of solution.
The main problem with iron in hydroponic solutions is that unlike most other transition metal ions in solution it is a hard lewis acid which readily forms insoluble salts with many of the hard lewis bases in hydroponic solutions. When iron is added to a nutrient solution as an ionic compound in its raw form (for example when adding iron (II) sulfate) the iron cation Fe2+ easily reacts with carbonate, phosphate, citrate, oxalate, acetate or hydroxide ions to form insoluble compounds that make the iron effectively unavailable to plants. In simple terms, iron has a chemical nature which is similar but opposite to that of many other constituents in hydroponic solutions, meaning that when they meet together they form a “perfect match” that does not easily separate. So, for example, iron (a cation) readily bonds with phosphate (an anion), forming iron (III) phosphate, also called ferric phosphate (FePO4), resulting in insoluble iron (and insoluble phosphate) which precipitates from (”drops out of”) solution.
Fe + PO4 —> FePO4
There is not only a problem with the higher inherent chemical match-making of iron with the anions present in solution but also iron is present at a much higher concentration than the other micronutrients. So even though other metals like copper would suffer from a similar fate they are less likely to because there is a much lower concentration in solution (i.e. iron is present in a hydroponic working solution at usually around 1.2 – 3 ppm while copper (Cu2+) is typically present at around 0.08-0.2 ppm and zinc (Zn2+) at 0.3-0.6 ppm).
Additionally, ions of metallic microelements, mainly iron and manganese and less so zinc and copper, may quickly change their valence in the presence of oxygen (metal atoms lose some of their valence electrons through oxidation, resulting in a large variety of ionic compounds including salts, sulfides and oxides), thus becoming less available for plant uptake. [1a] For example, Iron combines with oxygen to form insoluble iron oxide. Solubility in water is essential for absorption by plants. This is true of systemic chemicals as well as nutrients. The material must be soluble to pass through the root surfaces and into the cells of the plant. Insoluble mineral salts include oxides. Metal ions reacting with oxygen to form oxides is frequent in well aerated hydroponic solutions and hydroponic substrates, distinguished by high oxygen content and low cation exchange capacity (CEC).
This is where chelates come in. Chelates essentially surround the metal ion making them stable in solution. From a molecular/chemistry perspective, soluble trace metals in solution exist as positively charged ions. Each of these ions has a fixed number of reactive sites. Most metal ions have either four or six reactive sites. Chelates such as EDTA, DTPA, and HEDTA have six, eight, and six metal-complexing sites respectively, enabling one molecule to interact with all the reactive centers of a metal ion forming a stable complex in solution. In short, chelates maintain metals soluble in adverse chemical environments where metal ions might precipitate otherwise.
The word chelate is derived from the Greek word chelé, which refers to a lobster’s claw. Hence, chelate refers to the pincer-like manner in which a metal nutrient ion is encircled by the larger organic molecule (the claw), usually called a ligand or chelator.
Chelators, when combined with a microelement, can form a chelated fertilizer. Chelated microelements 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 manner in which the micronutrient is bonded to the ligand changes the micronutrient’s surface property and favors the uptake of microelements found in hydroponic solutions.
The common forms of chelates used in agriculture are the synthetic chelates, EDTA (ethylenediaminetetraacetic acid), DTPA (Diethylene triamine pentaacetic acid) and to a lesser extent, EDDHA. Chelates such as EDTA, DTPA and EDDHA have a high affinity for e.g. iron and generally form stable complexes with the metal across a pH range from 4 to 7 – although EDDHA has a wider pH range (past pH 9.0) and is particularly suitable for alkaline soil conditions.
The chelating agent most commonly used in soil based agriculture is EDTA (EthyleneDiamineTetraAcetic acid), which is used to produce chelated micronutrients for all the main metal trace elements.
EDTA-based iron chelate, however, is only stable under acidic conditions. This means that EDTA has limitations in alkaline conditions where about 50% of the iron becomes unavailable at above pH 6.5. Additionally, EDTA also has high affinity to calcium, so it is advised not to use it in calcium-rich soils and solutions. Notably, full spectrum hydroponic nutrient working solutions are calcium (and other metal ions) rich and for this reason EDTA is not the most suitable chelate for use in hydroponic formulations.
For example, in a study by Lucena and Chaney (2007), where cucumber plants were grown hydroponically with low concentration of Fe-chelates, the less stable Fe (III)-chelates proved more effective, exception made of those in which the displacement of the Fe by other metals takes place, as in the case of Fe-EDTA. The use of Fe (llI)-EDTA as iron supplier for plants was clearly disadvantageous compared to the rest of the compounds tested (Fe-EDDHMA: Fe-EDTA: Fe-EDDHA: Femeso- EDDHA: Fe-racemic-EDDHA; Fe-EDDHSA: Fe-PDDHA and Fe-HBED). [1b]
So, in addition to EDTA, four other chelating agents have been designed specifically for use with iron in neutral and alkaline conditions. These are DTPA (DiethyleneTriaminePentaAcetic acid), HEDTA (or HEEDTA, HydroxyEthylEthyleneDiamineTriAcetic acid), EDDHA (EthyleneDiamineDi(2-Hydroxyphenyl)Acetic acid) and EDDHMA (EthyleneDiamineDi(2-Hydroxy-Methylphenyl)Acetic acid).
While the chemistry of chelation is hydroponic solutions is extraordinarily complex (numerous factors effecting stability), in some instances it is claimed that EDDHA is the most stable of the chelates. That is, based on soil agriculture research, FeEDDHA is the most stable of all the commonly available iron chelates. This synthetic chelate is held in a bond up to 100 times tighter than DTPA because it has six molecular bonds rather than five bonds. Typically FeEDDHA is only found in premium soil fertilizers because of its higher cost. FeEDDHA is stable past pH 9.0 (pH range = 4 to as high as 11). This said, FeEDDHA is shown to be inherently unstable in acidic concentrated/saline solutions[1c] due to the fact that (to simplify somewhat) the EDDHA compound is composed of several different isomers, some of which are not very stable. Additionally, the theory that FeEDDHA is more available across a wider pH range often doesn’t translate into practice where hydroponics is concerned. For example, the interaction between Fe-chelate types (EDDHA and DTPA) and pH was studied with roses cv. ‘Kiss’ and ‘Escimo’ on rockwool substrate. pH levels compared were about 7, 5.8 and 4.5. The treatments resulted in significant chlorosis caused through iron deficiency and consequently yield reduction at high pH with both cultivars and both chelate types
This is an important point for growers to understand. That is, regardless of the type/s of chelate/s used in producing a hydroponic fertilizer (liquid or solid), the pH of nutrient solutions and substrates plays a crucial role in ensuring the availability of micronutrients to the plants. It is important to note that in substrate grown crops the best equilibrium for the uptake of the micronutrients (Fe, Cu, Mn, B and Zn) will be gained with values between pH 5 and 6. Molybdenum (Mo) differs somewhat where optimum uptake occurs at higher pH values.[2b]
This also means that while EDDHA as a standalone chelate may prove to be the most efficient Fe source in soil based growing, this isn’t necessarily the case in hydroponics.
In fact, since iron DTPA is stable under both acidic and neutral conditions it has become the standard in hydroponics worldwide, especially in Europe, with dose rates between 1 to 2 ppm, occasionally 3 ppm in the nutrient solution. DTPA is stable up to a pH of about 7.2 and unlike EDTA, has a low affinity to calcium. This makes it the ideal standalone choice as a chelator in hydroponic solutions. Studies have shown that DTPA as a Fe source contributes to a better status with iron, manganese and copper relative to the use of EDTA Fe.[2c ]
In some countries FeEDTA is still in use, but the dose rates need to be higher due to the lower stability; often 3 to 5 ppm is stated as about optimum ppm in solution. In alkaline solutions/conditions iron chelates derived from EDDHA and EDDHMA are typically used.
In some cases combinations of chelating agents may improve stability and broaden effectiveness. That is, a mix/blend of EDTA, DTPA, EDDHA or EDTA and DTPA in formulation may best ensure nutrient availability over a wide range of conditions, including those above or below optimal. For example, one study with fertigation solutions which compared the efficiency of EDTA and EDDHA at pH 6.0 and 7.5 showed that a combination of the two chelates provided the best iron status in solution. The authors of this study concluding, “it appears that FeEDTA may be used as a carrier of Fe during the reformation of the FeEDDHA.”  What this really comes down to is that chelate chemistry is extremely complex and the efficiency of any given chelate species, among other things, depends on their reactivity with macronutrient fertilizers in solution, including nitric and phosphoric acids. Acid conditions in solution (pH < 3.0) can displace Fe from FeEDDHA chelates. On the other hand, FeEDTA loses its effectiveness in high pH and where calcium is present at high levels in solution and substrates, since EDTA complexes with other metals, namely Ca. A mixture of FeEDTA and FeEDDHA seems to partially overcome the problems associated to the use of each chelate separately.
For this reason, even in hydroponic growing environments where optimum pH (water temperatures etc) can be monitored and maintained, there are potentially benefits gained from using a blend of chelated elements in solution.
While differing ‘stability’ pH ranges in hydroponic solutions are specified by various authors, as a guide:
EDTA pH range = 4- 6.2
DTPA pH range = 4- 7.2
EDDHA pH range = 4 -9
Keep in mind that this is a less than perfect guide because the chemistry of chelates in hydroponic solutions is complex and various factors will influence proceedings, not the least of which is substrate and solution pH, re the availability of iron. Ultimately, regardless of the Fe chelate species used in any formulation, hydroponic growers should seek to maintain pH within the optimal range of 5.5 – 6.0 to help ensure adequate Fe and other nutrient ions are available for plant uptake. I say this because just some nutrient manufacturers/suppliers have made claims, to effect, their products incorporate superior chelators and therefore they imply that plants grown with their formulas will thrive under a wider pH range than the same plants grown in a competitor’s formula However, in understanding the science of mineral nutrition and plants we can see that these sorts of claims are somewhat misleading.
To place some perspective on the importance of pH re iron availability, below is a graph that demonstrates the availability of iron to cucumber plants grown in hydroponics at pH 6.0 and pH 7.5 where various chelators have been compared. You will note in all instances iron is more available at pH 6.0 than at pH 7.5.
Source: Lucena, J. J. and Chaney, R. L. (2007) Response of Cucumber Plants to Low Doses of Different Synthetic Iron Chelates in Hydroponics
Now compare this graph (above) to one (a soil based graph) that is often bandied around by commercial interests (and others) as to the stability ranges of the various chelates. What you will find is the graph following tells us that 100% of Fe is available in soils as FeEDDHA within the pH ranges of 4 – 10. I’m not going to debate this (far too complex to explain, but it completely fails to take into account the quality of the chelate and the physical and chemical properties of various soils) other than to say that these types of graphs (when used out of context) can be highly misleading to novice growers who are unable to extrapolate the data in a qualitative way.
CHELATED MINERALS VERSUS COMPLEXED MINERALS
Not all nutrients can be chelated. The positively charged cations, Iron, zinc, copper, manganese, calcium, potassium and magnesium can be chelated while other nutrients (the negatively charged anions) cannot.
Some nutrient elements only have the ability to be partially surrounded by a chelator/chelating agent and are referred to as a “complexes”, while those that are capable of being completely surrounded are termed chelates.
Confusion and often contradictory information exists surrounding chelated and complexed minerals. Terms such as amino acid complexes, amino acid chelates, polysaccharide complexes, lignosulfonate complexes, and amino proteinates abound. In some cases chelates are referred to as “complexed” or “chelate complexes” while in other cases fertilizer producers wrongly promote complexed minerals as “chelated”. This only serves to confuse further. However, the difference between “chelated” and “complexed” can be understood via some basic principles.
ALL CHELATES ARE COMPLEXES – NOT ALL COMPLEXES ARE CHELATES
In order for a compound to be called a true chelating agent, it must have certain chemical characteristics. This chelating compound must consist of at least two sites capable of donating electrons (coordinate covalent bond) to the metal it chelates. For true chelation to occur the donating atom(s) must also be in a position within the chelating molecule so that a formation of a ring with the metal ion can occur.
The term “complexed” originates from combinations of minerals and organic compounds that do not meet the guidelines of a true chelate.
The key difference between a chelated mineral and complexed mineral is that chelates are relatively more stable under adverse conditions while complexes are less thermostable and release the atom quickly under adverse conditions.
Not all nutrients can be chelated. The positively charged cations iron, zinc, copper, cobalt, nickel, manganese, calcium, magnesium, and potassium can be chelated while the negatively charged anions such as phosphorous cannot.
While the negatively charged anions cannot be chelated they can be complexed via the use of donor atoms such as e.g. oxygen, nitrogen or sulphur. Amino acids such as glycine and/or lysergine are often used to complex the anions. Similarly, fulvic acid can be used to complex the negatively charged anions.
Both chelated and complexed minerals are more bioavailable than non-chelated and non-complexed minerals. This makes the use of additional organic (e.g. amino acid, fulvic acid) and inorganic chelators/complexers potentially beneficial in hydroponic solutions.
More to be added shortly re chelate chemistry, quality, optimal ppm of the micronutrients in solution etc. For now, read more about the micronutrients/microelements in hydroponic nutrient formulation here…..
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[1a] HÖFNER W. 1992. Aufhname und sorption von zwei und dreiwerigen Eisen durch Sonneblumenplanzen
(Helianthus annuus L.). Pflanzen Bodenkunde., 131:130-138.
[1b] Lucena, J. J. and Chaney, R. L. (2007) Response of Cucumber Plants to Low Doses of Different Synthetic Iron Chelates in Hydroponics
[1c] J.D. Jordà, M.D. Bermúdez, M. Juárez, M. Cerdán, J. Sánchez-Andréu (2001) Behaviour of FeEDDHA-Isomers in Nutrient Solutions
 Voogt, W. and Sonneveld, C (2007) The Effects of Fe-Chelate Type and pH on Substrate Grown Roses, ISHS International Symposium on Growing Media 2007
[2b] Sonneveld, C. and Voogt, W. (2009) Plant Nutrition of Greenhouse Crops pp. 289. Springer Netherlands
[2c] Kolota, E. Chohura, P. Komosa, A (2013) Efficiency of Chelate Forms of Micronutrients in Nutrition of Greenhouse Tomato Grown in Rockwool
 Bugter, M. H. J. and Reichwein, A. M. (2005) pH Stability of Fe – Chelates in Soilless Culture
 Lucena, J. J. Jimenez De Aberasturi, M. A. and Garate, A. (1991) Stability of chelates in nutrient solutions for drip irrigation
 Iron Nutrition in Soils and Plants: Proceedings of the Seventh International Symposium on Iron Nutrition and Interactions in Plants, June 27–July 2, 1993, Ed. Abadía, Javier pp. 157 – 158