About Chelates

Application of chelated micronutrients has proven to be a highly effective way to increase crop yields and their quality.

The word ‘chelate’ stems from the Greek word ‘chele’, meaning ‘pincers’ or ‘crab claws’.

A chelate can be described as a cation surrounded by a chelating compound (a ligand). This complex molecule is stabilised by strong chemical bonds. There are approximately 450 well-defined different chelating compounds. The most effective fertilisation takes place when the nutrients are chelated; chelation protects nutrients from being adsorbed or oxidised by different soil particles and reagents. This is especially true when dealing with micronutrients, but also when dealing with cations such as calcium and magnesium.

A micronutrient chelate is produced by a chemical process, in which a chelating compound is reacted with micronutrient cations (e.g. Fe, Mn, Zn or Cu) under very specific reaction conditions.

Chelates are formed in the process of chelation

Chelate products are most effective when 100% of the cations are actually chelated (complete chelation). However, since it is a technical challenge, the current fertiliser registration acts permits that a product can be termed a ‘chelate’ even if only 80% of the micronutrient cations are really complexed (partial chelation). This means that when the product is applied under unfavourable conditions (e.g. high soil pH), the non-chelated cations may quickly become unavailable to plants. This is why it is worth reading the product label carefully. In contrasts, chelates produced by ADOB® are comprised of 100% chelated nutrients, which ensures complete effectiveness of the applied fertiliser.

Chelation can only take place when the nutrient is a cation. However, boron and molybdenum do not form cations; as a result they cannot be reacted with chelating compounds. Therefore, these micronutrients occur in fertilisers as inorganic salts exclusively.

Mechanism of chelation by a zinc cation
EC Directive 2003/2003

The EC Directive 2003/2003 states that only a handful of chelating compounds are approved for agricultural use in the European Union. These chelating compounds were selected, amongst other reasons, for having high stability constants which guarantee their positive effect on crop plants. The European Commission published the concrete registration status of chelating agents for micronutrients in the ‘Official Journal of the European Union’. The most recent version includes the following chelating agents: EDTA, DTPA, EDDHA, HEEDTA, EDDHMA, EDDCHA, EDDHSA, IDHA and HBED. It should be pointed out that IDHA and HBED were developed and patented by ADOB®.

Stability constant pK

One of the most important characteristics of a specific chelate is its specific stability constant (pK). The higher the stability constant of a chelate, the higher the pH it can endure without disintegrating and losing its nutritional capacity. For instance, citric acid has a pK of 6 for iron, but EDTA has a pK of 15.2 and HBED has a pK of 39.7 for the same cation. This shows the necessity of using Fe-HBED under challenging cultivation conditions.


Nowadays, the attention of the developed world is rightly focused on environmental friendliness. Fast decomposition of fertilisers is a desirable feature that will be gradually enforced in worldwide agriculture. An important factor in this decomposition is the biological role of water and soil microorganisms like bacteria and fungi. The present OECD Directive states that a product can be declared “biodegradable” if at least 75% of its mass decomposes within 28 days. Currently, the only biodegradable chelating agent used in agriculture is ADOB’s IDHA.