Recommended EC by crop
Guideline ranges for hydroponics; adjust them for growth stage, climate and how your plants respond.
| Crop | EC (mS/cm) | ppm (500) | ppm (700) |
|---|---|---|---|
| Lettuce | 0.8 – 1.2 | 400 – 600 | 560 – 840 |
| Basil | 1 – 1.6 | 500 – 800 | 700 – 1,120 |
| Spinach | 1.8 – 2.3 | 900 – 1,150 | 1,260 – 1,610 |
| Strawberry | 1 – 1.8 | 500 – 900 | 700 – 1,260 |
| Cucumber | 1.7 – 2.5 | 850 – 1,250 | 1,190 – 1,750 |
| Bell pepper | 1.8 – 2.8 | 900 – 1,400 | 1,260 – 1,960 |
| Tomato | 2 – 3.5 | 1,000 – 1,750 | 1,400 – 2,450 |
| Seedlings and cuttings | 0.4 – 0.8 | 200 – 400 | 280 – 560 |
Conversion formulas
ppm (500 scale) = EC (mS/cm) × 500ppm (640 scale) = EC (mS/cm) × 640ppm (700 scale) = EC (mS/cm) × 7001 mS/cm = 1,000 µS/cm = 1 dS/m
Keep in mind that EC measures all dissolved salts, not specific nutrients. Measure your plain water first: if it already reads 0.5 mS/cm, that part is not plant food and you need to account for it.
Frequently asked questions
Why does my ppm meter read differently from someone else’s?
Because ppm (TDS) meters actually measure conductivity (EC) and convert it with a factor that depends on the brand: 500 (NaCl scale, common on Hanna and many pen meters), 640 (European scale) or 700 (KCl scale, common on Bluelab and Truncheon). The same 2.0 mS/cm solution reads 1,000, 1,280 or 1,400 ppm depending on the scale.
Should I go by EC or ppm?
EC. It is what the meter actually measures and does not depend on the scale, so you can compare numbers with any guide or grower without confusion.
What EC does lettuce need?
Between 0.8 and 1.2 mS/cm, which is about 400–600 ppm on the 500 scale or 560–840 ppm on the 700 scale.
How do I know which scale my meter uses?
Check the manual or packaging for a "conversion factor", "TDS factor" or "0.5 / 0.7 scale". If you cannot find it, measure a calibration solution of known EC and divide the ppm reading by the EC in mS/cm.