Hydroponic Micronutrients Reference — Fe, Mn, Zn, B, Cu, Mo, Cl

Quick answer

Seven micronutrients are essential for plant growth: iron, manganese, zinc, boron, copper, molybdenum, and chlorine. All are required in concentrations below 5 ppm, but every one is non-substitutable. Iron deficiency is the most common in hydroponics (caused by high pH); boron toxicity is the easiest to trigger by accident [OSU-NUT-01].

Iron (Fe)

Role: chlorophyll synthesis, electron transport. Required in higher quantities than other micronutrients — roughly 1–3 ppm in solution.

• Deficiency: interveinal chlorosis on new leaves, sharp yellow with green veins. Most common cause: pH above 6.5 locks Fe out of solution.
• Form: chelated as Fe-EDTA, Fe-DTPA, or Fe-EDDHA. Chelation keeps iron soluble across a wider pH range.
• Fe-EDDHA works up to pH 9.0; Fe-DTPA up to pH 7.5; Fe-EDTA only up to pH 6.5 [OSU-NUT-01].

See /troubleshoot/iron-deficiency for diagnostic photos and recovery steps.

Manganese (Mn)

Role: photosynthesis (oxygen-evolving complex), enzyme activation.

• Deficiency: interveinal chlorosis on new leaves, similar to iron but the contrast between vein and tissue is less sharp. Bands of yellow between veins rather than a clean pattern.
• Excess: small dark spots on older leaves, similar to disease lesions.
• Form: manganese sulfate or chelated Mn-EDTA [OSU-NUT-01].

Fe and Mn deficiency look similar. The differentiator: Fe deficiency has sharper vein contrast; Mn is muddier. ICP analysis is the only reliable diagnostic.

Zinc (Zn)

Role: protein synthesis, auxin (growth hormone) production.

• Deficiency: small, narrow leaves ("little leaf"), shortened internodes, distorted new growth. Common in high-pH solutions.
• Excess: stunted, dark green plants.
• Form: zinc sulfate or Zn-EDTA [CORN-CEA-01].

Boron (B)

Role: cell wall synthesis, calcium uptake, reproductive development.

• Deficiency: hollow stem in brassicas, brittle new growth, distorted flowers, "corky" spots on fruit. Tomato and broccoli are particularly sensitive.
• Excess: the most easily triggered toxicity in hydroponics. Yellow margins on older leaves, then necrotic edges, then plant death. Toxic above 2 ppm.
• Form: boric acid or sodium borate [OSU-NUT-01].

The boron working range is roughly 0.5–1.5 ppm — narrower than any other nutrient. Source water already containing boron plus a formula adding more easily pushes the system into toxicity. Test source water if recurring boron problems appear.

Copper (Cu)

Role: electron transport, enzyme function.

• Deficiency: wilting, dark blue-green leaves, dieback of growing tips. Rare in modern hydroponic formulas.
• Excess: root toxicity, stunted growth, eventual root death. Common cause: copper plumbing fittings or copper algicides.
• Form: copper sulfate or Cu-EDTA [OSU-NUT-01].

Avoid copper-bearing pipe in any hydroponic plumbing run.

Molybdenum (Mo)

Role: nitrate reductase enzyme — converts nitrate to amino acids.

• Deficiency: pale, twisted leaves; symptoms mimic nitrogen deficiency because the plant can't process nitrate without Mo. Most common at very low pH (under 5.0).
• Excess: nearly impossible to reach in normal hydroponics.
• Form: sodium or ammonium molybdate. Required at extremely low concentration (0.05 ppm) [OSU-NUT-01].

Chlorine (Cl)

Role: osmotic regulation, photosynthesis.

• Deficiency: essentially never seen in hydroponics. Tap water provides plenty of chloride; municipal water often contains 20–100 ppm.
• Excess: can occur with heavily chlorinated tap water or repeated top-ups with sodium chloride contamination. Leaf bronzing, edge burn.
• Form: chloride ion (Cl-) [USDA-NUT-01].

Use municipal tap water dechlorinated by sitting for 24 hours, or use RO water and ignore chloride entirely.

pH and micronutrient availability

The single biggest factor affecting micronutrient availability is pH:

This is why hydroponic pH targets sit at 5.8–6.5 — it's the compromise that keeps every nutrient available [OSU-NUT-01].

Micronutrient drift in reservoirs

Micronutrients deplete faster than macronutrients per unit of EC. A reservoir reading "correct" EC after 14 days can have:

• 40–60% of original iron consumed or oxidized
• 30–50% of manganese
• Most other micronutrients still adequate

The 14-day full reservoir reset addresses this — top-ups don't replace consumed micronutrients without re-dosing the full formula [OSU-NUT-01].

What we recommend

Use a complete A+B formula that includes all seven micronutrients in chelated form for iron and manganese. Hold pH at 5.8–6.3 to keep micronutrients available. Refresh the full reservoir every 10–14 days rather than relying on top-ups. Test source water for boron, sodium, and chloride before scaling up — these can quietly push systems out of balance. For deficiency symptoms, cross-reference visual diagnosis with the /troubleshoot/<element>-deficiency pages and confirm with leaf-tissue analysis on commercial crops where the cost matters.