Quick Glossary
- Osmotic stress — the osmotic pressure of the solution approaches or exceeds the internal potential of root cells, preventing water uptake
- Turgor — internal cell pressure that maintains tissue firmness
- EC — measures salt concentration; the foundation of osmotic pressure
Why a "Normal EC" Does Not Guarantee Stress-Free Conditions
Salt accumulation between irrigations — the plant absorbs water, but Na⁺, Cl⁻, and SO₄²⁻ accumulate. EC at the root zone can run 0.5–1.5 mS/cm above reservoir EC.
High solution temperature — at 27–30°C, the internal osmotic potential of root cells decreases. EC 2.0 at 28°C causes stress; the same EC at 20°C does not.
Uneven substrate — salt accumulation zones develop with irregular irrigation.
Diagnosis: Osmotic Stress vs. Similar Conditions
| Symptom | Osmotic Stress | Root Rot |
|---|---|---|
| Wilting | During the day; recovers at night | Not time-dependent |
| Roots | Normal, no slime | Brown, slimy, with odour |
| Drain EC | Significantly above input EC | May be normal |
Temperature and Osmotic Stress
"EC 2.5 at 20°C is a normal concentration for tomato. EC 2.5 at 28°C is potential osmotic stress."
Reduce EC below the standard recipe during summer.
Three Mistakes That Cost the Most
- Diagnosing root rot and applying fungicides — if turgor recovers overnight and roots are clean, the cause is osmotic stress
- Raising EC during heat — during heat stress this worsens the situation
- Ignoring drain EC — "drain EC exceeds input EC by 1.5+" confirms salt stress and requires a flush
Signs of Correct Control
- Consistent turgor throughout the day with no wilting
- Drain EC stable at 0.3–0.5 above input EC
- When temperature exceeds 24°C, EC reduced by 0.3–0.5 below the standard recipe