EC and PPM for Kratky Nutrient Solution
Solution strength is measured with a conductivity meter. Dissolved nutrient salts carry an electric current, so the more fertilizer in the water, the higher the reading (Oklahoma State University Extension). This guide explains the units and the ranges to aim for.
EC: the base measurement
EC stands for electrical conductivity. It is usually shown in millisiemens per centimeter (mS/cm). Some meters show microsiemens (µS/cm): divide by 1,000 to get mS/cm, so 1,250 µS/cm is 1.25 mS/cm.
PPM and why meters disagree
Many inexpensive meters show PPM (parts per million) or TDS. They still measure conductivity and then multiply it by a conversion factor. Different makers use different factors (Bluelab):
- 500 scale: 1.0 mS/cm reads as 500 ppm
- 700 scale: 1.0 mS/cm reads as 700 ppm
The same solution at 2.0 mS/cm reads 1,000 ppm on one meter and 1,400 ppm on another. Check which scale your meter uses before following a PPM chart, or work in EC, which means the same thing on every meter.
Typical strength ranges
Oklahoma State University Extension lists these optimum EC ranges for hydroponic crops:
| Crop | EC (mS/cm) |
|---|---|
| Lettuce | 1.2–1.8 |
| Basil | 1.0–1.6 |
| Sage | 1.0–1.6 |
| Pak choi | 1.5–2.0 |
| Parsley | 1.8–2.2 |
| Spinach | 1.8–2.3 |
| Strawberry | 1.8–2.2 |
| Cucumber | 1.7–2.0 |
| Peppers | 0.8–1.8 |
| Tomato | 2.0–4.0 |
For Kratky leafy greens, UF/IFAS gives about 1,250 µS/cm (1.25 mS/cm), or about 800 ppm, as a target, checked against the nutrient label. These ranges come from hydroponic production in general. In a Kratky container the reading usually changes as the plant takes up water and nutrients at different rates, so many growers start near the middle of the range rather than the top.
Calibrating a meter
Meters drift. Calibrate with a standard solution, commonly 1.41 mS/cm, following the meter's instructions (Oklahoma State University Extension). Rinse the probe after use. A meter with automatic temperature compensation gives steadier readings.
Taking a good reading
- Stir the solution first and wait for the number to settle
- Measure your plain water too, so you know how much of the reading is already from the water
- Write the reading down with the date; it makes problems easier to diagnose later
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).
- Bluelab. What are the different conductivity scales?
