Articles
Microelements behavior:
Salts vs Chelates
in system with sensitive (oligotrophic) plant species
The behavior of Fe and Mn in a system with the following factors: high oxygen (+400 mV redox), low pH (~5.0), no buffering, CO₂ saturation, and light exposure.
1. Fe/Mn Behavior: Unchelated (FeCl₃, MnSO₄) vs. Chelated (Fe-DTPA, Mn-EDTA)
Unchelated Metals (FeCl₃, MnSO₄): "Naked Ions in a Storm"
- Fe³⁺ (from FeCl₃):
- Oxidation/Precipitation: At pH 5.0 + high O₂, Fe³⁺ hydrolyzes instantly:
Fe³⁺ + 3H₂O → Fe(OH)₃ ↓ + 3H⁺
- Forms insoluble ferric hydroxide (rust-colored particles).
- Kinetics: >90% precipitates within minutes to hours at pH 5.0.
- Plant Availability: <10% remains dissolved. Plants must use inefficient Fe³⁺-reduction mechanisms.
- Colloidal Radicals: Yes! Partially oxidized Fe forms nanoscale colloids (FeOOH). These generate ROS (Reactive Oxygen Species) via Fenton reactions:
Fe²⁺ + H₂O₂ → Fe³⁺ + OH• + OH⁻ (toxic hydroxyl radical)Analogy: Throwing iron filings into seawater—they rust and sink fast, but some rusty dust (colloids) stays suspended, causing damage.
- Oxidation/Precipitation: At pH 5.0 + high O₂, Fe³⁺ hydrolyzes instantly:
- Mn²⁺ (from MnSO₄):
- Oxidation: Slower than Fe. Mn²⁺ oxidizes to Mn³⁺/Mn⁴⁺, forming MnO₂ (brown-black solids):
Mn²⁺ + ½O₂ + H₂O → MnO₂ ↓ + 2H⁺
- Kinetics: At pH 5.0 + high O₂, ~50% precipitates in hours to days.
- Plant Availability: ~30-50% remains soluble short-term.
- Toxicity Risk: Low at 50 ppb, but MnO₂ colloids can cause oxidative stress.
- Oxidation: Slower than Fe. Mn²⁺ oxidizes to Mn³⁺/Mn⁴⁺, forming MnO₂ (brown-black solids):
Chelated Metals (Fe-DTPA, Mn-EDTA): "Armored Delivery"
- Fe-DTPA/Mn-EDTA: Chelators act as protective cages, preventing oxidation.
- Stability in High O₂:
- Fe-DTPA remains soluble for weeks even at +400 mV.
- Mn-EDTA is stable but weaker (Mn²⁺ can "escape" slowly).
- pH Sensitivity:
- Fe-DTPA stable at pH 5.0–7.0 (ideal for oligotrophic system).
- Mn-EDTA dissociates below pH 6.0, releasing Mn²⁺.
- Plant Availability:
- Plants absorb intact Fe-DTPA/Mn-EDTA via transporters.
- At pH 5.0, >80% remains available long-term.
- Stability in High O₂:
2. Kinetic Comparison: "Race Against Time"
| Form | Time to 50% Precipitation/Loss | Dominant Forms in Water |
|---|---|---|
| FeCl₃ | Minutes to hours | Fe(OH)₃ (solid), FeOOH (colloids) |
| Fe-DTPA | Weeks | Soluble Fe³⁺-DTPA complex |
| MnSO₄ | Hours to days | Mn²⁺ (aq), MnO₂ (solid) |
| Mn-EDTA | Days to weeks | Soluble Mn²⁺-EDTA, some free Mn²⁺ |
3. Toxicity & Deficiency Risks
Unchelated Salts
- Deficiency: Rapid precipitation → plants starve despite dosing.
- Toxicity: Colloidal Fe/Mn generate ROS, damaging cells (e.g., leaf chlorosis).
Example: Your 100 ppb FeCl₃ dose may deliver <10 ppb available Fe but generate toxic colloids.
Chelated Forms
- Deficiency Risk: Low (high stability).
- Toxicity Risk:
- Overdosing: Chelated Fe/Mn resist precipitation but can be over-absorbed.
- ROS from Light: Under PAR light, Fe-DTPA can undergo photoreduction, releasing free Fe²⁺ that fuels Fenton reactions:
Fe³⁺-DTPA + light → Fe²⁺ + DTPA⁻ (then Fe²⁺ + H₂O₂ → OH•)
Analogy: Armored truck (chelate) protects cargo, but sunlight can crack the armor, releasing hazardous material.
- Deficiency: Rapid precipitation → plants starve despite dosing.
- Toxicity: Colloidal Fe/Mn generate ROS, damaging cells (e.g., leaf chlorosis).
Example: Your 100 ppb FeCl₃ dose may deliver <10 ppb available Fe but generate toxic colloids.
- Deficiency Risk: Low (high stability).
- Toxicity Risk:
- Overdosing: Chelated Fe/Mn resist precipitation but can be over-absorbed.
- ROS from Light: Under PAR light, Fe-DTPA can undergo photoreduction, releasing free Fe²⁺ that fuels Fenton reactions:
Fe³⁺-DTPA + light → Fe²⁺ + DTPA⁻ (then Fe²⁺ + H₂O₂ → OH•)Analogy: Armored truck (chelate) protects cargo, but sunlight can crack the armor, releasing hazardous material.
4. Chelates vs. Salts: Which is Safer for Sensitive (Oligotrophic) Species?
Hyper-accumulation behavior of some [oligotrophic] plant species changes the risk calculus:
| Factor | Unchelated Salts (FeCl₃/MnSO₄) | Chelates (Fe-DTPA/Mn-EDTA) |
|---|---|---|
| Bioavailability spike | Short, intense burst (minutes-hours) | Sustained low-level release (days-weeks) |
| Plant response | Sudden metal flood → rapid overload | Continuous trickle → gradual accumulation |
| Toxicity trigger | Acute poisoning (necrosis in days) | Chronic buildup (toxicity in weeks) |
| Worst for… | Species with fast-responding transporters | Species lacking excretion mechanisms |
Key Mechanisms Driving Toxicity:
- For salts: Plants detect the brief soluble Fe/Mn spike → activate high-affinity transporters → massive metal influx before precipitation occurs.
Example: FeCl₃ dosed at 0.1 ppm may deliver 0.08 ppm soluble Fe for 1-2 hours—enough for oligotrophic plants to absorb 2-3 days' worth.
- For chelates: Slow degradation (pH/light-driven) releases Fe/Mn steadily. Plants absorb metals faster than chelates degrade → sustained uptake.
Example: Fe-DTPA degrades 5% per day, but plants absorb 10% of available Fe daily → net accumulation.
- For salts: Plants detect the brief soluble Fe/Mn spike → activate high-affinity transporters → massive metal influx before precipitation occurs.
Example: FeCl₃ dosed at 0.1 ppm may deliver 0.08 ppm soluble Fe for 1-2 hours—enough for oligotrophic plants to absorb 2-3 days' worth.
- For chelates: Slow degradation (pH/light-driven) releases Fe/Mn steadily. Plants absorb metals faster than chelates degrade → sustained uptake.
Example: Fe-DTPA degrades 5% per day, but plants absorb 10% of available Fe daily → net accumulation.
5. Why Chelates May Be More Dangerous in System with Sensitive (Oligotrophic) Species
- Continuous "Appetite Stimulation":
- Chelated Fe/Mn remain detectable by root sensors → high-affinity transporters stay activated.
- Salts precipitate → signal dissipation → transporters downregulate.
- Colloidal Middlemen:
- Degraded chelates form FeOOH/MnO₂ colloids that oligotrophic plants efficiently scavenge (via specialized reductases).
- Bicarbonate Synergy:
- HCO₃⁻ displaces chelators → releases metals and forms bioavailable bicarbonate complexes.