Lettuce Nutrient, pH & EC Guide for Hydroponics
Lettuce tip burn is a calcium-transport problem driven by water temperature, not a nutrient deficiency - solution temperature management matters more than calcium concentration.
Last updated
Lettuce is the most widely grown hydroponic crop and rewards close attention to water temperature as much as nutrient balance. Its calcium physiology is frequently misunderstood: tip burn - the browning of inner leaf margins - is almost always a transpiration and calcium-transport issue rather than a true calcium deficiency. Managing reservoir temperature is often the most effective intervention available.
pH and EC Targets
| Stage | pH | EC (mS/cm) | PPM (approx) | Notes | |-------|-----|------------|--------------|-------| | Seedling | 5.5–6.5 | 0.6–0.8 | 300–400 | Keep low to avoid salt stress on emerging roots | | Vegetative | 5.5–6.5 | 0.8–1.2 | 400–600 | Ramp up gradually as root mass develops | | Mature | 5.5–6.5 | 1.2–1.6 | 600–800 | Hold at lower end in warm environments to prevent tip burn |
Note on PPM: Values above use a 500 conversion factor (EC × 500 = PPM). Some meters use 700 - multiply by 1.4 to convert.
Key Nutrients
| Nutrient | Role in This Plant | Deficiency Sign | Correction | |----------|-------------------|-----------------|------------| | Calcium | Inner-leaf cell-wall integrity; tip burn prevention depends on Ca reaching fast-growing inner leaves via transpiration | Brown-edged inner leaves (tip burn); most visible in heading types | Lower reservoir temp to 18–20°C; ensure adequate airflow before adjusting Ca concentration | | Nitrogen (NO3:NH4 ratio) | Primary growth driver; NH4:NO3 ratio must stay at or below 1:5 | Pale yellowing from oldest leaves upward | Switch to predominantly nitrate-nitrogen source; ratio above 1:5 suppresses Ca uptake | | Potassium | Stomatal regulation and water movement within leaves | Marginal scorch on outer leaves at maturity | Ensure balanced K at 150–200 ppm; check EC hasn't dropped |
Deficiency Identification
- Inner-leaf tip burn: Brown papery margins on the youngest, most enclosed leaves. Caused by insufficient calcium transport to low-transpiration inner leaves - not low solution Ca. Increase air circulation or lower solution temperature first.
- Interveinal yellowing (older leaves): Suggests magnesium or iron deficiency. Magnesium deficiency appears on mature leaves first; iron deficiency on young leaves first. Check pH is not above 6.5 which locks out iron.
- Overall pale colour with stunted growth: Likely nitrogen deficiency or excessively low EC. Raise EC by 0.2 mS/cm increments and recheck after 3–5 days.
- Marginal scorch on outer leaves: Potassium deficiency or high-salt stress at elevated EC. Check total EC and reduce if above 1.8 for lettuce.
Feeding Schedule Notes
Start seedlings at EC 0.6–0.8 and increase by 0.2 mS/cm each week through the vegetative stage. Most loose-leaf varieties perform best between EC 1.0–1.4 at maturity; heading types such as butterhead and romaine benefit from a slight EC increase to 1.4–1.6 in the final week to firm up leaf structure. Always maintain solution temperature between 18–22°C - this single factor has more influence on tip burn than any nutrient adjustment. Full reservoir changes every 7–10 days prevent nutrient ratio drift.
Research note: Above 24°C solution temperature, tip burn appears even with correct Ca levels - dropping reservoir to 18–20°C resolves it without any nutrient adjustment.