Hydroponic lettuce EC ranges generally progress from about 0.4–0.8 mS/cm for young seedlings to 0.8–1.2 mS/cm after transplanting, 1.2–1.8 mS/cm during active leaf growth, and roughly 1.4–2.0 mS/cm near harvest. These are working ranges rather than fixed rules because cultivar, water quality, temperature, humidity, and light alter nutrient demand.