"EC 1.8 means the solution has the right amount of nutrients." This is a misconception that leads to chronic plant stress. EC does not show how much fertilizer is in the water - it shows the osmotic pressure of the solution. And osmotic pressure determines whether the root can absorb water at all. When EC is too high, the plant takes up water with more difficulty and pays for it in growth, even if the root system is healthy and the setup is working correctly.
Quick glossary: EC (Electrical Conductivity) - measures the total concentration of dissolved salts in water, in mS/cm. Osmotic pressure - the force with which a solution holds on to its water; the more dissolved salts it carries, the more effort the plant spends to take that water.
Why EC Is About Water, Not Fertilizer
Roots take up water by osmosis: water moves toward the side with more dissolved solutes, which means from the solution into the root cells. Every mS/cm of EC adds about 0.36 atmospheres of osmotic pressure (the force with which the solution holds its water), and the higher it is, the more effort the plant spends to take the same water.
The popular figure of a root "internal EC" of 4-5 mS/cm does not survive the arithmetic. A solution at EC 4-5 has an osmotic pressure of 1.4-1.8 atmospheres, while the contents of a plant cell commonly hold about 5 atmospheres (0.5 MPa). At that EC, water still flows into the root. The plant closes the gap by raising the pressure in its own cells, and that adjustment costs it growth.
That is why yield drops long before the root stops drinking. In the FAO table (Maas and Hoffman data), tomato gives full yield as long as the soil saturation extract stays at or below EC 2.5 mS/cm, loses a quarter at 5.0 and half at 7.6. These thresholds were measured in soil, so for a reservoir they are a reference point rather than a limit. What an overloaded plant looks like, and how it differs from root rot, is covered in the article on osmotic stress.
That is why the "correct" EC for a solution depends on the crop, temperature, growth stage, and root health. The same EC of 2.5 can be normal for a tomato in the fruiting stage and stressful for seedlings in their first week.
Working EC Ranges: Guidelines, Not Rules
Working EC ranges are guidelines, not fixed rules: seedlings and microgreens sit at EC 0.8-1.2, lettuce and herbs run 1.2-2.2, and tomatoes and cucumbers climb to 2.0-3.5 depending on the phase.
Seedlings and young plants - EC 0.8-1.2. The root system is still weak and sensitive to osmotic load.
Lettuce - EC 1.2-2.0. Short cycle, fast turnover, higher nitrogen demand relative to other elements.
Microgreens - EC 0.8-1.2. Fed mostly from seed reserves, so feeding matters less here than water and substrate.
Basil and herbs - EC 1.4-2.2. More sensitive to fluctuations than lettuce.
Tomatoes and cucumbers - EC 2.0-2.5 vegetative, 2.5-3.5 fruiting. Potassium demand rises sharply in fruiting.
These are guidelines. The actual working EC is determined through drain EC - if it consistently runs 0.3-0.5 above the feed EC, the system is in balance.
How to Account for Source Water EC When Mixing
Tap water already has its own EC, and the meter reads it together with the fertilizer. If the target for lettuce is 1.8 mS/cm and the water gives 0.5, the nutrients should add only 1.3. A dose calculated for pure water will read about 2.3 on the meter, 28% more load than you planned.
How to do it in practice:
- Measure the EC of the water going into the reservoir: 0.5 mS/cm.
- Work out the nutrient share: 1.8 - 0.5 = 1.3 mS/cm.
- Add stock solutions A and B in equal portions, mix and measure after each one until the meter reads 1.8.
- Write down how many milliliters went into 10 L. With the same water the dose stays the same next time; when the water source changes, work it out again.
Those 0.5 mS/cm of source water are mostly calcium, magnesium and bicarbonates. The plant will use the calcium and magnesium if the recipe accounted for them, while bicarbonates only push pH up. If the water's EC comes from sodium and chloride, that part is pure ballast, and a water test will tell you what to do about it.
There is a second way to count. The Cornell hydroponic lettuce handbook sets the target as 1.15-1.25 mS/cm above the source water EC, meaning the increment rather than the total. Pick one method and write it next to the number: "EC 1.2" means different solutions under the two approaches.
Three Mistakes That Cost the Most
Raising EC to "accelerate growth." Higher nutrient concentration does not equal better nutrition. Above the optimal EC, the plant spends more energy fighting osmotic pressure and less on growth. The result is the opposite of what was intended.
Not accounting for source water EC when mixing. A nutrient recipe is calculated for "clean" water with EC ~0.0. If your water has EC 0.4-0.6, that difference must be factored in - reduce the amount of nutrients accordingly.
Ignoring drain EC. Drain EC is the only way to know what is actually happening in the root zone. If it steadily rises from one irrigation to the next, salts are accumulating and a flush is needed. If it is lower than the feed EC, the plant is actively consuming nutrients or the substrate is not yet stabilized.
How to Know EC Is Under Control
The feed EC matches the crop's growth stage. Drain EC consistently runs 0.3-0.5 above feed EC and does not climb week over week. Plants are turgid, with no signs of osmotic stress - no wilting under adequate irrigation, no leaf-edge burn.
If you want to go deeper: Drain EC: How to Read the Output Solution and What It Tells You About the System - the next step once you understand the logic of EC in the reservoir.