Short answer: aim for an EC around 1.2 to 1.8 mS/cm and a pH around 5.5 to 6.5. UF/IFAS suggests about 1.25 mS/cm and pH 5.5 to 6.0 for Kratky growing.
What the sources say
- Oklahoma State University Extension: EC 1.2–1.8 mS/cm, pH 6.0–7.0 for hydroponic lettuce
- UF/IFAS: about 1,250 µS/cm (1.25 mS/cm), or about 800 ppm, and pH 5.5–6.0 for non-circulating systems
The ranges overlap. Starting near 1.2–1.4 mS/cm and pH 5.8–6.0 sits comfortably inside both.
Why lettuce suits Kratky
Lettuce was the crop Kratky used to develop the method. It grows quickly, uses relatively little water and is usually harvested 6 to 7 weeks after seeding, before the solution runs out (Kratky, 2009).
How much solution
Plan on about 1 to 1.5 gallons per plant from seed to harvest (UF/IFAS). One plant per gallon-sized container is a common setup.
Nutrient choice
A complete hydroponic nutrient for leafy or vegetative growth, or an all-purpose hydroponic formula, works well (UF/IFAS).
Signs the mix is off
- Pale, slow-growing plants: solution may be too weak, or nitrogen is running low near harvest.
- Brown outer leaf edges: often a solution that is too strong (UMass Extension).
- Brown tips on inner young leaves (tipburn): linked to calcium not reaching fast-growing leaves, often worsened by warm conditions (UMass Extension).
- Pale new leaves with green veins: often iron shortage caused by high pH.
Related guide: Kratky nutrient solution by crop.
Sources
- Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
- University of Florida IFAS Extension. Set It and Forget It Hydroponics (HS1488).
- Kratky, B.A. (2009). Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843: 65–72.
- UMass Extension Greenhouse Crops and Floriculture. Hydroponic lettuce tipburn.
