The word "Chelate" is derived from the Greek word for "Claw". Metallic chelates are a complex of a metal ion bound to an organic molecule (ligand).
Metal ions are important minerals for plants. They are required by plants in small amounts, and therefore are being refered to as 'Micronutrients. Their deficiencies result in yellowing of leaves, retarded growth and general low quality crops.
Chelated compounds are more stable than non-chelated compounds. Therefore, metallic chelates are widely used in agriculture as micronutrient fertilizers to supply plants with Iron, Manganese, Zinc and Copper. The most common chelates used in agriculture are EDTA, DTPA and EDDHA.
Metal chelation is important because it makes metal ions more available for uptake by plants. Positively charged metal ions, such as Zn+2, Mn+2, Cu+2 and Fe+2, readily react with negatively charged hydroxide ions (OH-), making them unavailable to plants. OH- ions are abundant in alkaline or neutral soils and soil-less media.
The ligand (a molecule that binds to a central metal atom) coats the metal ion, protecting it from the surrounding OH- ions. The complex can then be easily absorbed by the plant, where it is being degraded and consumed as micronutrients.
The strength of the chemical bond between the ligand and the metal ion depends on the type of ligand, the type of ion and the pH. The stronger the bond, the more stable the metallic ion and each chelate has a characteristic "stability diagram".
Below are examples for stability diagrams for a Copper chelate and a Zinc chelate. It is obvious that in specific pH levels, the complexes are not stable, i.e. the ligand tends to separate from the metal ion.
When other positively charged ions, such as Calcium and Magnesium, are present in high concentrations, they compete with the metal ion for binding to the ligand. The metal ion might then be replaced, making the chelate ineffective in delivering the metal ion to the plant.