Chelated micronutrients are widely recognized as more efficient and plant-available forms compared to their non-chelated counterparts. EDTA (ethylenediaminetetraacetic acid) and similar chelating agents have been extensively used to stabilize metal ions such as iron (Fe²⁺), zinc (Zn²⁺), copper (Cu²⁺), manganese (Mn²⁺), calcium (Ca²⁺), and magnesium (Mg²⁺), ensuring better solubility, reduced precipitation, and improved uptake by plants.
However, boron (B) and molybdenum (Mo) stand out as exceptions — they cannot be effectively chelated in the same way. Understanding why requires looking deeper into their unique chemical forms in solution.
1. Chelation Mechanism and Why It Works for Metals
Chelation occurs when a chelating agent, such as EDTA, surrounds a metal cation with multiple binding sites, forming a stable ring-like complex. EDTA carries negatively charged carboxyl groups and nitrogen atoms with lone electron pairs, making it highly effective at binding positively charged metal ions.
This strong coordination prevents metals from reacting with other anions in the soil (like phosphate or carbonate), thereby keeping them soluble and available for plant uptake.
2. Boron: Present as Boric Acid, Not a Cation
In soils and aqueous solutions, boron does not exist as a free cation. Instead, it is typically present as boric acid (H₃BO₃ or B(OH)₃), a neutral molecule and a weak Lewis acid.
-
Boric acid lacks the positive charge necessary for chelation with EDTA.
-
Instead of forming coordination complexes like metals, boron prefers to interact with hydroxyl groups or form esters with polyols (such as sugars in plants).
-
This explains why chelating agents designed for metals are ineffective for stabilizing boron.
3. Molybdenum: Present as an Anion
Molybdenum behaves differently but is also incompatible with EDTA chelation. Under normal soil and plant-relevant pH conditions, molybdenum exists as the molybdate anion (MoO₄²⁻).
-
EDTA itself is negatively charged in solution (EDTA⁴⁻).
-
According to electrostatic principles, two negatively charged species strongly repel each other, making stable chelation impossible.
-
As a result, molybdenum remains in its soluble anionic form and does not require chelation for bioavailability.
4. Practical Implications for Agriculture
-
Chelation is critical for cationic micronutrients (Fe, Zn, Cu, Mn, etc.) to avoid precipitation and ensure plant availability.
-
Boron and molybdenum remain available in soil solutions without chelation due to their chemical forms (boric acid and molybdate).
-
For boron supplementation, farmers often rely on borax, boric acid, or specialized boron carriers.
-
For molybdenum, sodium molybdate and ammonium molybdate are the most common forms used in agriculture.
The inability of boron and molybdenum to form chelates with EDTA or similar agents is not a limitation of chelation technology but rather a reflection of their unique chemical nature. While most micronutrients benefit from chelation to improve plant uptake, boron and molybdenum are already soluble and bioavailable in their natural forms.
For effective crop nutrition strategies, it is essential to understand these differences and apply each micronutrient in its most suitable form.
