Understanding Chelation in Plants

Chelation is a process that protects the integrity of nutrients. This is true for human nutrition and plant nutrition. Chelates, the actors in the chelation process, help increase the mobility of nutrients and help prevent their unwanted loss due to processes such as leaching or nutrient precipitation. Chelates have been likened to both an orange peel and a lobster’s claw in their form and function. The word “chelate” derives from the Greek word chelé, which means lobster’s claw.

The action of chelates can be likened to organic molecules encircling a metal nutrient like a claw embraces with its pincers. This marriage of organic molecules and metal nutrients is called a “ligand” or chelator. This embrace protects the metal nutrient from combining with other elements or from being lost through absorption. Like an orange peel, the ligand shields the nutrient within until it is ready for use.

What are Chelates and Chelating Agents?

To take this a step further from the above description, chelates are more like a ring system. This ring system is made up of a metal ion encapsulated by a “ring” of varying components derived from organic matter decomposition. The most commonly chelated metal ions are:

• Calcium (Ca)
• Magnesium (Mg)
• Iron (Fe)
• Cobalt (Co)
• Zinc (Zn)
• Manganese (Mn)
• Europium (Eu)

Once these metals have been swept up in this organic chemical process, their cationic characteristics have been compromised. They can be absorbed by plants since the chelate releases the metal ions slowly enough for plant uptake. This characteristic of chelated metals makes them useful in horticultural and agricultural applications as many metal ions would be prone to loss through leaching, runoff, or other chemical reactions. The chelated metals will essentially stay put and become useful to plants.

Read also: Pumping Iron: Why Plants Need Iron in Their Diet

The chelating agents, or those components that spring forth from the naturally occurring decaying and decomposing processes of organic matter are numerous. These organic molecules can create several different types of bonds with a single metal ion. Natural chelating agents are organic substances either applied by or produced by plants or microorganisms. These include:

• enzymes
• organic acids
• amino acids
• ligninosulfonates
• ligninipolycarboxylates
• sugar acids and derivatives
• phenols
• poly flavonoids
• hydroxamate siderophores
• phytosiderophores (phyto: plant; siderophore: iron carrier)
  
  

Amino acids are often favored as chelating agents due to the numerous benefits they provide to plants and animals including ease of absorption. Essential amino acids work together by aiding structure function and enzymatic function as well as improving reproduction. Amino acids are thought by many sources to increase plant health, growth, and produce greater yields. They are therefore often used in agricultural applications.

There are numerous synthetic chelating agents as well. These synthetic and often proprietary chelating agents include such difficult to pronounce formulations as:

Egtazic acid, also known as EGTA (ethylene glycol-bis (β-aminoethyl ether))
Pentetic acid, also known as DPTA (diethylenetriaminepentaacetic acid)
EDTA (Ethylenediaminetetraacetic acid)

And numerous others in existence and numerous more being developed…

Both classes of these chelating/complexing agents increase micronutrient (the metal ions) solubility and therefore usefulness to plants. It should be noted, however, that this process can occur naturally without any human intervention.

In the soil, plant roots can release natural chelates through their exudates.

Mugineic acid, (a non-protein amino acid) is a type of natural chelate called phytosiderophore. This is produced by various species of graminaceous (grassy) plants as they exhibit stress due to low-iron stress conditions. The exuded chelate then works by helping plants absorb nutrients in the root-solution-soil system. Such root-excreted chelates form a metal complex (i.e., a coordination compound) with a micronutrient ion in soil solution and approaches a root hair. The chelated micronutrient near the root hair releases the nutrient which finds its way to the root hair. The chelate is then free and becomes ready to complex with another micronutrient ion in the adjacent soil solution, amazingly having the ability to restart the cycle.

Read also: Amino Acid Chelates: Giving Your Plants a Helping Hand

Nutrient Management and Fertilizers

Plant nutrients are, of course, one of the environmental factors essential for crop growth and plant development. The management of those needed nutrients then becomes crucial for optimal yields in commercial crop production on any scale.

Nutrients that typically occur in concentrations of ≤ 100 parts per million (ppm) in plant tissues are described as micronutrients and include the metals mentioned above as well as the nutrients boron (B), chlorine (Cl), molybdenum (Mo), and nickel (Ni). When these micronutrients are combined with ligands, they form a chelated fertilizer. Chelated micronutrients are extremely useful to growers as they are protected from such production obstacles as oxidation, precipitation, leaching out, and immobilization in certain conditions. The lobster claw-like way in which the micronutrient is bonded to the ligand changes the micronutrient's surface property and allows for a more efficient up take of applied micronutrients.

Some nutrients can become difficult to absorb as they are prone to being fixed or “stuck” outside the plant in the soil. Nutrients become “fixed” due to incompatible charges between the plant’s negative charge and the nutrient’s positive charge.

Chelated fertilizers create charge compatibility by surrounding the positively charged nutrient and neutralizing its charge. The nutrient is then free to move into the plant and absorbed much more easily. These include the micronutrients such as iron, manganese, zinc, and copper which are easily oxidized or precipitated in soil. This makes utilization inefficient. Chelated fertilizers (ligands with desired micronutrient) have been developed to combat this by increasing micronutrient utilization efficiency. For example, if the inorganic iron salt (iron sulfate) is applied to some soils, most of the iron is transformed into forms that are not readily usable to plants. It is converted to plant-unavailable forms.

Applying nutrients such as iron, manganese, zinc, and copper directly to the soil is inefficient because in soil solution they are present as positively charged metal ions and will readily react with oxygen (oxidation) and/or negatively charged hydroxide ions (OH-). If they react with either oxygen or hydroxide ions, they will 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. This problem can be overcome by using chelated fertilizers

Read also: The Chelation Effect

How Chelates Prevent Nutrient Precipitation and Leaching

Nutrient loss can occur to crop soils by precipitation and by leaching. Precipitation is not the effect of weather, but rather the action of precipitates. When nutrient precipitation occurs, nutrients can bind to one another, effectively locking them up and rendering them useless. An entirely available nutrient combines with another ion and forms an insoluble precipitate; useless to the plant. Nutrient leaching occurs when unused or surplus nutrients and micronutrients are lost due to their mobility in the soil. They may be lost through runoff or work their way down the soil profile out of the reach of plant roots. Chelates can help prevent nutrient loss to both phenomena.

Nutrient leaching is a natural occurrence but can be problematic to plants and the environment for a multitude of reasons. Many man-made conditions can also exacerbate the problem. The main concern when nutrient leaching occurs is contamination of ground water. As water makes its way through soil it takes with it plant nutrients and even pesticides. Though beneficial to plant life, as these nutrients make their way into water supplies, they create damage for the animals and people around it. Chelated nutrients are less likely and able to be leached out of soils.

Parting Notes on Chelation

Some beneficial effects of chelated fertilizers may be observed under conditions where a specific metal may not be noticeably deficient to the extent of causing chlorosis but may in fact be limiting the growth of those plants.

Soils with a high pH (pH > 6.5) often have low bioavailability in micronutrients such as iron, manganese, zinc, and copper. Micronutrient fertilizers are often needed for commercial crop production to make them more available.

Crop susceptibility to micronutrient deficiencies depends on the plant species and cultivar. Commercial crops can be categorized into three groups: high susceptibility, medium susceptibility, and low susceptibility. High- and medium-susceptibility plants often need chelated fertilizers.

Inorganic, water-soluble micronutrient application to the soil is often ineffective for correcting micronutrient disorders. Most of the fertilizer value in such amendments never ends up getting used by the plant.

Chelated fertilizers are less reactive to soil conditions and can significantly enhance nutrient uptake and utilization efficiencies.

Chelated fertilization rates range from an average of 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.

Chelation allows for the efficient use of metals in the soil by plants. Without chelation, needed micronutrients may be present in soils, but not in a form that is useful to plants. Chelation can occur naturally or artificially. Any grower experiencing a nutrient deficiency of one or more metals will be better served by applying a chelated fertilizer to correct the issue.