What is Chelation?

The word chelate (pronounced: “key-late”) is derived from the Greek word “chele” which literally means “claw”, a rather fitting association because chelation is a process somewhat like grasping and holding something with a claw. Chelation occurs when certain large molecules form multiple bonds with a micronutrient, protecting it from reacting with other elements in the nutrient solution and increasing its availability to the plant.

Imagine a lobster’s claw made of carbon and hydrogen atoms holding an ion. The more bonds that form between the ion and the carbon atoms, the stronger the ion is held within the chelate. The strength of the chelate’s hold on the ion determines, as pH increases, how long the element will continue to be available to plants.
Many micronutrients are unavailable to plants in their basic forms. This is typically due to the fact that these metals, such as iron, are positively charged. The pores or openings on the roots of the plant are negatively charged. As a result, the element can’t enter the plant due to the difference in charges. However, if a chelate is added, it surrounds the metal/mineral ion and changes the charge into a neutral or slightly negative charge, allowing the element to easily pass across the cell membrane and travel into the plant.
There are several chelating agents that are commonly used in commercial fertilizers. The chelating agent can be identified on the label beside the trace element it serves to make available to plants. If the label has EDTA written next to a trace element, the fertilizer contains ethylene-diamine-tetra-acetate, the most commonly used chelating agent. Higher quality grades of fertilizers may also contain DTPA (diethyle-netriamine-penta-acetate). EDDHA (ethylene-diamine-dihydroxy-phenylacetic-acid) represents the highest quality of synthetic chelating agents available and are the most effective across all growing environments.
EDTA is the most common chelating agent and is used for both soil and foliar applied nutrients. It has four points of connection. Like other synthetic chelates, EDTA is a foreign compound and is therefore not absorbed by the plant. When the chelated element is required, the plant will remove the element, for example iron, from the chelate and absorb the element. However, since the chelating agent is foreign to the plant, it will give up the chelating agent (EDTA) back into solution where it is free to chelate other elements. EDTA is better suited to slightly lower than neutral pH levels, as high pH conditions will cause it to release the element back into the nutrient solution instead.
DTPA is a chelating agent better suited for high pH conditions. It has five points of connection to the element it chelates, allowing it to hold the element more tightly. DTPA is more costly than EDTA and is less soluble so it is found in smaller quantities than EDTA in most synthetic fertilizer formulations.

Types of Chelating Agents and their Stability at Different pH Levels

The graph to the right shows the level of iron stability with different chelating agents at different pH levels. As you can see, EDDHA is the strongest chelate of any of the commonly used materials and maintains iron availability to plants past pH 9.0. However, other types of chelating agents can be used as long as pH conditions are suitable. EDTA is better suited to slightly lower than neutral pH levels. DTPA is most effective at slightly higher pH levels.
The strongest and therefore most effective of the synthetic chelating agents is EDDHA. It is found only in select fertilization formulations because of its relatively high cost. House & Garden Nutrients uses EDDHA chelates to enable maximum absorption of their nutrients, making them the most unique and effective nutrient line on the market. Plants supplied with adequate levels of EDDHA are able to absorb more zinc than plants supplied with EDTA, which is very important as zinc is often locked out in the later part of flowering due to excessive phosphorous levels, causing plants to yellow.
There are also natural chelating agents available, such as fulvic acid, which is derived from the decomposition of organic matter. Unlike synthetic agents, fulvic acid can be absorbed into the plant, adding to the mobility of the nutrients within the plant. Fulvic acids can be most effective when the growing environment in the rhizosphere is above or below optimal. Even under adverse conditions, plants supplied with fulvic acid have been found to be remarkably free of signs of stress and deficiency.
For best results, use only high quality fulvic acid products such as Mad Farmer’s Nutrient Uptake Solution (N.U.T.S.). N.U.T.S. is sourced from ancient, organic humic shale, which is then extracted using only pure, cold water. This extraction process avoids the use of chemicals, heat, and pressure, which can destroy the quality of the fulvic compounds. N.U.T.S. is also UV filtered before bottling to ensure quality. N.U.T.S. can be added to the nutrient solution and/or used as a weekly foliar spray until one week prior to harvest.
To optimize growth in your garden, use several sources of chelation in your nutrient solution. This allows for more efficient nutrient absorption, transport, and other biochemical reactions. Look for nutrients that offer a range of chelating compounds, so that nutrients will be available through a wide range of conditions, including those above or below optimal. For increased results you may consider the addition of fulvic acid. When used properly, chelating agents can maximize nutrient uptake and increase photosynthetic response, greatly increasing plant performance and yields.

 

Solutions of Zn, Cu and Mn chelates of EDTA, DTPA and EDDHA were reacted separately with a calcareous soil for periods up to 28 days. DTPA was an effective chelate for Zn and Cu; more than 77 and 55% of the added Zn and Cu, respectively, remained soluble after 28 days of reaction with the soil. The stability of Zn-EDTA and Cu-EDTA was relatively less than those of the respective DTPA chelates, whereas Zn-EDDHA and Cu-EDDHA were highly unstable in the soil. The loss of soluble Mn from Mn-EDTA, Mn-DTPA and Mn-EDDHA additions to soil was very rapid and completed in about one week.

It was found that adsorption of Zn-EDDHA, Cu-EDDHA and Mn-EDDHA molecules by the soil was the main process removing Zn, Cu and Mn from solution. Whereas, replacement of the metal in the metal-chelate molecule by Ca ion from the soil was a more serious factor affecting the stability of DTPA and EDTA chelates of Zn, Cu, and Mn.

Along with Boron (B) and Molybdenum (Mo) the six essential micronutrients include the metals Copper (Cu), Iron (Fe), Manganese (Mn) and Zinc (Zn). Growers know that soluble iron (Fe) will chemically react with soluble phosphorus (P) supplied as part of the fertilizer. The reaction forms an insoluble precipitate of iron phosphate (FeO4P) that settles out as a deposit in the bottom of the mixing tank. Being insoluble, this material cannot be taken up by the plant, and so the plants quickly become iron deficient. For this reason the iron (Fe) is provided in “chelated” form, usually as Fe EDTA chelate. The chelate binds to the iron and stops it reacting with phosphorus, or anything else, but the chelated iron remains in solution, and it remains available to the plant.
But what about the other metals, Copper (Cu), Manganese (Mn) and Zinc (Zn). Should they be chelated too…? These metals do not react with phosphorus, so on the face of it there is no reason to chelate these metals.
However, within the fertilizer solution, the EDTA chelate is not irreversibly attached to the iron (Fe). The chelate “prefers” to bond with the other soluble metals – a process called “preferential chelation”. The chelate will attach to the Copper (Cu), Manganese (Mn) and Zinc (Zn) in preference to the Iron (Fe). Once the iron (Fe) is no longer bonded to the EDTA, it is free to react with the phosphorus (P) and is lost from the solution.
The process of “preferential chelation” is illustrated in the diagram below. This shows all the components of the nutrient solution and the interactions which take place . The iron (Fe) from the FeEDTA is released into the solution which then reacts with phosphorus (P) to form an insoluble iron phosphate precipitate.

The photographs below show the “preferential chelation” process recreated in the laboratory. Precipitation of iron phosphate first results in a solution with a cloudy appearance which on standing for a short time, develops into a visible layer.

By contrast the beaker on the right shows a clear solution by using fully EDTA chelated sources of copper (Cu), manganese (Mn) and zinc (Zn) along with the usual FeEDTA. No precipitation means no loss of iron.
This process can be readily observed in the fertilizer stock tank solution, where an insoluble deposit will soon form.. The concentrated solution of nutrients react together very quickly. At final dilution rates, the concentration of nutrients is low, and the reaction it therefore much slower – although it still occurs. For this reason it is possible to get away with only chelating the iron (Fe) – and rely on cheap sulphate formulations for the copper (Cu), manganese (Mn) and zinc (Zn) – in situations where the components are held apart and are only mixed together at the low rates of final dilution e.g. by dosing directly into the irrigation system. This only works if, for example, the iron (Fe) chelate is held separate from the other metals – perhaps in mix with the calcium nitrate and so is not applicable for NPK blends where all the trace elements are included together and the iron cannot be separated.
For a robust product that will remain fully in solution in the stock tank, the answer is to chelate all the metals, not just the iron (Fe). This is a little more costly, but for peace of mind that all the micronutrients remain fully available, it is excellent value.
Below, Solufeed Strawberry Special water soluble fertilizer blend with label expanded to show the trace element content and form. Note that the copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn) are all fully chelated with EDTA.