Quick Glossary
- Fe - essential for chlorophyll synthesis; above pH 6.5 it precipitates as Fe(OH)₃ even in chelated form
- Mn - activator of photosynthetic enzymes; deficiency occurs above pH 6.5 and with excess Fe
- Zn - activator of auxin synthesis; suppressed by excess Cu and Fe
- Cu - component of the electron transport chain; needed in trace amounts - excess is toxic
- B - involved in sugar transport; forms insoluble compounds with Ca when Ca is in excess
- Mo - essential for nitrate reduction; better available at neutral to alkaline pH
Iron: Forms and pH Stability Ranges
| Chelate | Stable up to pH |
|---|
| Fe-EDTA | 6.0–6.3 |
| Fe-DTPA | 7.0 (optimal choice for pH 5.5–6.5) |
| Fe-EDDHA | 9.0 (for alkaline solutions) |
Chlorosis at pH 6.8 with Fe-EDTA is the wrong chelate form for that pH range - not an iron deficiency.
pH Availability Ranges
| Element | Optimal pH | Antagonists |
|---|
| Mn | 5.5–6.5 | Fe |
| Zn | 5.5–7.0 | Cu, Fe, excess P |
| B | 5.5–7.0 | excess Ca |
| Mo | 6.0–7.0 | better at higher pH |
| Cu | 5.5–7.0 | toxic above 0.5 mg/L |
Three Critical Mistakes
- Using Fe-EDTA above pH 6.3 and expecting results
- Diagnosing micronutrient deficiencies without first checking pH
- Increasing the Cu dose as a preventive measure - it accumulates and is difficult to flush out
Signs of a Healthy Micronutrient Status
- New leaves are uniformly green with no chlorosis
- Normal internode length
- pH 5.8–6.2 and the correct chelate form - most problems simply do not arise
