Choosing Micronutrient Fertilizers

When plant tissue samples come back from the lab with low iron or zinc levels, growers must fertilize with micronutrients, but which products are most effective? What is the difference between EDTA and EDDHA? Should we use “natural” chelating agents like lignosulphonate or citrate? Over the past couple of weeks, I’ve been reading up on micronutrients and complexation chemistry to learn how to get the best fertilizer use efficiency out of micronutrient products.

First, some definitions:

What is the difference between a complexing agent and a chelating agent?

A complexing agent, also called a ligand, is an atom or molecule that forms one or more bonds with a central metal cation.

A metal cation is a positively charged atom such as iron or zinc (Fe+2, or Zn+2). The word “ligand” comes from latin, meaning claw. The claw grabs the micronutrient.

A chelating agent is a ligand that forms two or more bonds with a central metal cation. All chelating agents are ligands, but not all ligands are chelating agents.

The molecule formed by the ligand and the central metal cation is called a coordination complex.

What do we like about coordination complexes?

Coordination complexes are soluble, meaning they stay suspended in solution like salt dissolved in water. Soluble nutrients are plant available nutrients, so when we use complexing or chelating agents, the micronutrients stay in soil solution or foliar spray to facilitate easy plant absorption. In soil solution, complexed micronutrients diffuse toward plant roots. Once the coordination complex contacts the root surface, the ligand releases the cation giving it to the plant cell. The ligand then diffuses back into soil solution, where it can complex another micronutrient.

What makes one complexing agent different from another?

There are many types of complexing and chelating agents, and each of them binds to micronutrients with different strength. Weak complexing agents form unstable coordination complexes and may release the micronutrient before delivery to the plant root. Other complexing agents form very stable coordination complexes and keep micronutrients in soil solution for several weeks after fertigation.

Why are some coordination complexes more stable than others?

Factors Influencing Coordination Complex stability include:

  • Size and electric charge of atoms involved
  • Number of bonds formed between ligand and metal
  • Type of bonds formed between ligand and metal
  • Soil pH
  • The type of metal cation bound to the ligand
Metal-EDTA: the metal “M” represents cation micronutrients such as iron, zinc, manganese, etc.
Sodium-acetate: sodium can be replaced by other micronutrient cations.


Atoms with a small size and high charge form stronger bonds than large atoms with weak charge. The type of bonds formed between metal and ligand also make a difference. Chemical bonds formed by electrostatic attraction of two oppositely charged atoms are weaker than bonds formed by two atoms sharing electrons. Bonds formed by electron sharing are termed “covalent bonds,” and coordination complexes with greater covalent character are stronger than those with ionic character.

Coordination complex stability also increases with increasing number of bonds between the ligand and the metal. More bonds = more stability. Chelates that form two or more bonds are always more stable than complexes with only one bond. For example, copper (Cu) can form coordination complexes with acetate, oxalate, and EDTA. Cu-acetate has one bond between copper (the metal) and acetate (the ligand). Cu-oxalate forms two bonds and Cu-EDTA forms 6 bonds. Cu-EDTA, with the highest number of bonds, is 1018 times stronger than Cu-acetate and 1014 times stronger than Cu-oxalate.

Soil pH also affects coordination complex stability. Chelate attraction to metal cations changes depending on the acidity or alkalinity of soil solution. EDDHA forms stable complexes with micronutrients over a wide pH range from 4 to 9, while EDTA is stable from about 4-7. Other chelating agents have different tolerance to pH.

Some metal cations form stronger bonds with the ligand than others. For example, copper forms a more stable complex with EDTA than iron. If we apply iron EDTA to the soil, the iron might dissociate from the ligand and copper may take its place. Free iron left in soil solution would be prone to precipitation. The “Irving Williams Series” predicts which metals form the most stable coordination complexes. Manganese usually forms the weakest complexes, while copper usually forms the strongest:

Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+

Which micronutrient products should we choose?

There is no one-size-fits-all for fertilizer recommendations. Factors to consider include plant tissue micronutrient levels, soil micronutrient levels, soil pH, soil texture, and concentration of other macronutrients in soil solution. Fertilizer choice will also largely depend on whether the grower intends to apply the product foliarly or to soil. In general, strong chelating agents such as EDDHA or EDTA are required for soil applications while weaker complexing or chelating agents can be used foliarly. Some micronutrients work well applied together or blended with NPK fertilizer while others are best supplemented alone.

Soil organic matter contains naturally occurring complexing and chelating agents and can greatly increase micronutrient availability. Applying compost, manure, or liquid organic fertilizers will improve micronutrient use efficiency. Before choosing a fertilizer, take soil and tissue samples and consult your Certified Crop Advisor….or give me a call J

If you found this article interesting and would like to learn more, don’t hesitate to call or email:



Essington, Michael E. Soil and Water Chemistry. Boca Raton: CRC Press, 2004. Print.

Halvin, John L., et al. Soil Fertility and Fertilizers, 8th Edition. Upper Saddle River: Pearson, 2014. Print.