Low-pH Liquid Agricultural Micronutrient Design with GLDA

Liquid micronutrient fertilizers have to keep metal ions dissolved from the blending tank through storage and the spray tank. At low pH (about 2.5–3.5), GLDA is only partly protonated, so it binds metals less strongly than EDTA. The guidance below is the full low-pH design note for GLDA-4Na liquids.

Key scientific principles

GLDA can bind metals at multiple points (hexadentate), but it is not as strong as EDTA. Because of this lower binding strength (lower stability constant, or logK), use at least a 1:1 molar ratio of GLDA to metal, and in practice often 10–50% excess GLDA.
Most metal–GLDA complexes form in a 1:1 ratio. At low pH, extra GLDA is required to keep the metal fully chelated.
At pH 2.5–3.5, stability drops because GLDA becomes protonated and some binding sites are no longer available. Hydrogen ions compete with metal ions for those sites.
Binding strength follows Fe³⁺ (very strong) > Cu²⁺ > Zn²⁺ ≈ Mn²⁺ > Ca²⁺ > Mg²⁺ (weakest). Weaker metals need more excess GLDA. Fe³⁺ stays more stable even at lower ratios.

Metal ratios at pH 2.5–3.5

Metal	Common salt	Preferred salt (low pH)	Molar ratio (GLDA:metal)	Weight ratio (GLDA 47% : salt)	Stability	Notes
Zn²⁺	ZnSO₄·H₂O / ZnO	ZnSO₄ (better solubility)	1.2–1.5 : 1	~2.5–3.2 : 1	Moderate	ZnO needs acid pre-dissolution
Mn²⁺	MnSO₄·H₂O	MnSO₄	1.2–1.4 : 1	~2.3–3.0 : 1	Moderate	Similar to Zn
Fe³⁺	FeCl₃ / Fe₂(SO₄)₃	FeCl₃ (more soluble)	1.5–2.0 : 1	~3.0–4.5 : 1	Good (best among GLDA)	Strong complex (logK ~15)
Fe²⁺	FeSO₄	FeSO₄ (fresh)	1.5–2.0 : 1	~3.0–4.0 : 1	Poor–moderate	Oxidizes to Fe³⁺
Cu²⁺	CuSO₄·5H₂O	CuSO₄	1.2–1.5 : 1	~2.5–3.2 : 1	Good	Stronger than Zn
Mg²⁺	MgSO₄·7H₂O	MgSO₄	1.3–1.6 : 1	~2.5–3.5 : 1	Weak	logK ~5.5
Ca²⁺	CaCl₂ / Ca(NO₃)₂	CaCl₂	1.5–2.0 : 1	~3.0–4.5 : 1	Weak	Easily displaced

Key observations and salt selection

At pH 2.5–3.5, GLDA does not hold divalent metals very strongly. Some of the metal can leave the complex.

Metals affected: Zn, Mn, Mg, and Ca. These complexes are only partially stable.
Use extra GLDA to compensate. A practical starting point is at least a 1.3–1.5 molar ratio of GLDA to metal.

Iron behaves differently

Fe³⁺ forms a very strong complex with GLDA and remains fairly stable even at low pH. Fe²⁺ is unstable, oxidizes easily to Fe³⁺, and can precipitate if it is not fully chelated. Prefer Fe³⁺ with GLDA, or oxidize Fe²⁺ to Fe³⁺ before or during chelation.

Calcium and magnesium are the weakest

GLDA binds Ca and Mg poorly at low pH. Calcium is moderate to weak; magnesium is weak. Below pH 3 they are very likely to dissociate, so they need more GLDA and still give lower stability than the transition metals.

Salt selection

Sulfates are the best choice: stable, cost-effective, and widely used (ZnSO₄, MnSO₄).
Chlorides improve solubility and are especially useful for iron and calcium.
Nitrates are very soluble and useful when clarity is critical, but they cost more.
Oxides such as ZnO and CuO are not recommended. They need acid to dissolve first and can destabilize the blend.

Use fully soluble salts — sulfates or chlorides — for low-pH liquids.

Stabilization strategies

1. Use excess GLDA

Because GLDA is weaker than EDTA or DTPA, extra ligand is the most important step.

Zn, Mn, and Cu: about 1.2× GLDA
Ca, Mg, and Fe: about 1.5–2.0× GLDA

2. Follow the mixing sequence

Order of addition prevents precipitation:

Dilute GLDA in water.
Adjust pH to about 4–5.
Add the metal salts slowly.
Allow chelation to finish.
Then reduce pH to 2.5–3.5.

The complex should form before the blend is acidified.

3. Use a mixed chelant when one ligand is not enough

GLDA + DTPA improves iron stability.
GLDA + EDTA improves zinc and manganese stability.
GLDA + IDS or MGDA balances performance with biodegradability.

4. Buffer the pH

Citric acid, or carefully controlled nitric acid, can hold the blend. The preferred finished range is pH 2.8–3.2.

5. Keep iron from precipitating

For Fe²⁺ systems, add a mild reducing agent or switch to Fe³⁺ with enough excess GLDA.

6. Manage the salt load

High dissolved salts can destabilize the chelates. Keep the formula balanced and avoid over-concentration.

When GLDA alone is not enough

Iron: DTPA is the stronger option.
Zinc and manganese: EDTA or EDDS.
Calcium and magnesium: GLDA can be used, but the complexes stay weak.
Multi-metal blends: GLDA plus EDTA is the most reliable combination.

Example: Zn + Mn + Fe liquid at about pH 3

GLDA 47%: 1.4–1.6 molar equivalents of the total metals.
Metal salts: ZnSO₄, MnSO₄, and FeCl₃.
Optional: replace 20–30% of the GLDA with DTPA for better iron stability.
Finished pH: 2.8–3.2.

Download the GLDA low-pH guidelines (PDF)