EDTA forms chelates, in which a single molecule wraps a metal ion with several strong bonds; lignosulfonate forms complexes, in which a large natural polymer holds metal ions more loosely through sulfonate, carboxyl and phenolic groups. EDTA chelates are more stable in soil, especially for zinc and manganese at higher pH; lignosulfonate complexes are biodegradable, compatible with many fertiliser mixes and suited to foliar and fertigation use where very high stability is not required.
Side-by-Side Comparison
| Property | EDTA chelates (e.g. Zn-EDTA) | Lignosulfonate complexes (e.g. Zn-LS) |
|---|---|---|
| Type of bond | Chelate: one ligand, several coordinating groups around the metal | Complex: metal bound to sulfonate, carboxyl and phenolic groups of a polymer |
| Stability in soil | Higher; keeps Zn and Mn in solution over a wider pH range | Lower; one study on iron found poor mobility and stability in soil at pH 7.5 compared with EDDHA |
| Origin | Synthetic aminopolycarboxylate | By-product of sulfite pulping of wood |
| Biodegradability | Poorly biodegradable; persistence in water is a known concern | Natural wood-derived polymer; broken down by soil microorganisms over time, at a rate that depends on conditions |
| Metal content of products | Set by the chelate stoichiometry; check the data sheet | Depends on the product, e.g. Zn about 10% in a Zn-LS powder (GAC-ZnLS TDS) |
| EU fertiliser status | Chelating agents meeting CMC 1, Regulation (EU) 2019/1009 | Complexing agents; identified by EN 16109 in fertilisers |
| Typical use | Soil and fertigation where strong stability is needed | Foliar sprays, fertigation, liquid blends, NPK coating |
What the Research Shows
- In a 2016 study with navy beans on calcareous soil, ZnEDTA was the most effective of the tested sources at supplying zinc to plants; the authors proposed different delivery mechanisms for EDTA and for lignosulfonate (Frontiers in Plant Science (2016), zinc fertiliser sources including ZnEDTA and Zn-lignosulfonate in calcareous soil, doi:10.3389/fpls.2016.01767).
- In a 2015 soybean study on calcareous soil, gluconate and lignosulfonate kept relatively low concentrations of Mn and Zn in soil solution, while a DTPA-containing formulation gave the highest plant Zn (Frontiers in Plant Science (2015), Mn and Zn complexes and chelates applied to soybean in calcareous soil, doi:10.3389/fpls.2015.00752).
- Research on iron sources found lignosulfonate complexes far less stable in soil than synthetic chelates such as EDDHA, but effective when supplied directly to roots in hydroponics.
The common thread: in soil, especially calcareous soil, strong synthetic chelates keep micronutrients available longer. Lignosulfonate complexes are better matched to uses where the nutrient reaches the plant quickly – foliar sprays, fertigation and liquid blends – and where biodegradability matters.
Choosing Between Them
| Situation | Better fit | Reason |
|---|---|---|
| Soil application on calcareous, high-pH soil | Synthetic chelate (EDTA; EDDHA for iron) | Higher stability in soil |
| Foliar spray | Either; lignosulfonate is widely used | Short contact with soil; good compatibility and leaf coverage |
| Fertigation with phosphates | Test both; lignosulfonate complexes are used with polyphosphates | Compatibility depends on the blend; run a jar test |
| Programmes that avoid persistent synthetic chelants | Lignosulfonate | Natural, biodegradable carrier |
| Organic production | Check the certifier list | Status depends on the standard and the specific product |
Before any blend goes to the field, check physical compatibility with a jar test for lignosulfonate tank mixes.
Frequently Asked Questions
Is zinc lignosulfonate as effective as zinc EDTA?
Not in all situations. In calcareous soil, studies have found ZnEDTA more effective at supplying zinc. For foliar and fertigation use, lignosulfonate complexes are widely used and are biodegradable.
What is the difference between a chelate and a complex?
A chelate is a metal held by several bonds from one ligand molecule, such as EDTA. A complex is a looser association, such as a metal bound by groups along a lignosulfonate polymer.
Are lignosulfonate micronutrients recognised in EU fertiliser law?
EU rules distinguish chelating and complexing agents for micronutrient fertilisers. Lignosulfonates are treated as complexing agents, and EN 16109 is the standard method to identify them in fertilisers.
References
- Frontiers in Plant Science (2016), zinc fertiliser sources including ZnEDTA and Zn-lignosulfonate in calcareous soil, doi:10.3389/fpls.2016.01767
- Frontiers in Plant Science (2015), Mn and Zn complexes and chelates applied to soybean in calcareous soil, doi:10.3389/fpls.2015.00752
- Regulation (EU) 2019/1009 on EU fertilising products (PFC 1(C)(II) micronutrient fertilisers; CMC 1 chelating and complexing agents)
- EN 16109:2011, Fertilizers – Determination of complexed micro-nutrient ions in fertilizers – Identification of lignosulfonates
Related Pages
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