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Most nutrient deficiencies people chase in hydroponics are not deficiencies. They are EC problems. Feeding a lettuce crop at tomato strength will burn it, and running tomatoes at lettuce strength will starve them, and in both cases the leaves look wrong in ways that mimic a missing element.
Below is a target range for forty-plus common crops. Bookmark it, because the single most useful habit in hydroponics is checking this number twice a week.
What EC actually measures
Electrical conductivity measures how well your solution conducts electricity, which scales with how much dissolved mineral salt is in it. More nutrient means more conductivity. It is reported in mS/cm (millisiemens per centimetre), and it is the closest thing hydroponics has to a fuel gauge.
What it does not tell you is which elements are present. An EC of 1.4 could be a balanced solution or it could be almost entirely calcium left behind after your plants stripped out the nitrogen. That is why you rebuild the reservoir periodically instead of endlessly topping it up.
EC, PPM, and why two meters disagree
If your meter reads in PPM rather than EC, it is applying a conversion factor, and there are two common ones. On the 500 scale (sometimes labelled NaCl, common on Hanna meters) an EC of 1.0 reads as 500 ppm. On the 700 scale (the 442 or Truncheon convention) the same solution reads as 700 ppm.
That is a 40% difference in the number for identical water. It is the single most common reason two growers following the same guide get different results. Find out which scale your meter uses, or switch to reading EC directly and skip the problem entirely.
EC and pH targets by crop
Ranges are for established plants in a recirculating system. Seedlings and fresh transplants go at 0.4 to 0.8 regardless of what they will eventually want.
Leafy greens
| Crop | EC (mS/cm) | pH |
|---|---|---|
| Lettuce, loose leaf | 0.8 – 1.2 | 5.5 – 6.0 |
| Lettuce, butterhead | 0.8 – 1.2 | 5.5 – 6.0 |
| Romaine | 1.0 – 1.4 | 5.5 – 6.5 |
| Spinach | 1.8 – 2.3 | 6.0 – 7.0 |
| Kale | 1.2 – 1.8 | 5.5 – 6.5 |
| Swiss chard | 1.8 – 2.3 | 6.0 – 6.5 |
| Arugula | 0.8 – 1.2 | 6.0 – 7.5 |
| Bok choy | 1.5 – 2.0 | 6.0 – 7.0 |
| Mustard greens | 1.2 – 2.4 | 6.0 – 6.5 |
| Watercress | 0.4 – 1.8 | 6.5 – 6.8 |
Herbs
| Crop | EC (mS/cm) | pH |
|---|---|---|
| Basil | 1.0 – 1.6 | 5.5 – 6.5 |
| Chives | 1.8 – 2.4 | 6.0 – 6.5 |
| Cilantro | 1.3 – 1.8 | 6.5 – 6.7 |
| Dill | 1.0 – 1.6 | 5.5 – 6.4 |
| Mint | 2.0 – 2.4 | 5.5 – 6.0 |
| Oregano | 1.8 – 2.3 | 6.0 – 7.0 |
| Parsley | 0.8 – 1.8 | 5.5 – 6.0 |
| Rosemary | 1.0 – 1.6 | 5.5 – 6.0 |
| Sage | 1.0 – 1.6 | 5.5 – 6.5 |
| Thyme | 0.8 – 1.6 | 5.5 – 7.0 |
Fruiting crops
| Crop | EC (mS/cm) | pH |
|---|---|---|
| Tomato | 2.0 – 3.5 | 5.5 – 6.5 |
| Cherry tomato | 2.0 – 3.5 | 6.0 – 6.5 |
| Bell pepper | 1.8 – 2.2 | 6.0 – 6.5 |
| Chili pepper | 2.0 – 3.0 | 6.0 – 6.5 |
| Cucumber | 1.7 – 2.5 | 5.5 – 6.0 |
| Eggplant | 2.5 – 3.5 | 5.5 – 6.5 |
| Strawberry | 1.8 – 2.2 | 5.5 – 6.5 |
| Melon | 2.0 – 2.5 | 5.5 – 6.0 |
| Zucchini | 1.8 – 2.4 | 6.0 |
| Beans | 2.0 – 4.0 | 6.0 |
Roots and brassicas
| Crop | EC (mS/cm) | pH |
|---|---|---|
| Radish | 1.6 – 2.2 | 6.0 – 7.0 |
| Carrot | 1.6 – 2.0 | 6.0 – 6.5 |
| Onion | 1.4 – 1.8 | 6.0 – 6.7 |
| Leek | 1.4 – 1.8 | 6.5 – 7.0 |
| Celery | 1.8 – 2.4 | 6.3 – 6.7 |
| Broccoli | 2.8 – 3.5 | 6.0 – 6.8 |
| Cauliflower | 1.5 – 2.0 | 6.0 – 7.0 |
| Cabbage | 2.5 – 3.0 | 6.5 – 7.0 |
| Peas | 0.8 – 1.8 | 6.0 – 7.0 |
| Seedlings, any crop | 0.4 – 0.8 | 5.8 – 6.2 |
Why your EC climbs when you are not looking
Plants drink water faster than they eat nutrient. Over a week, a healthy crop removes proportionally more water than salt, so the solution left behind gets stronger. This surprises people who assume it should fall as the plants feed.
The trap is topping up with full-strength nutrient solution to replace what evaporated and transpired. Do that repeatedly and you ratchet EC upward until the plants show salt burn: crispy leaf margins, wilting despite a full reservoir, and stalled growth.
Top up with plain pH-adjusted water. Only mix fresh nutrient at a full reservoir change. If EC is still climbing on plain-water top-ups, your solution is too strong for the crop and needs diluting.
Fixing an EC that is out of range
Too high: dilute with plain pH-adjusted water until you land in range. Do it gradually over a day rather than in one hit if the plants are already stressed. In severe cases dump and remix.
Too low: add nutrient in small increments, remeasure after ten minutes of circulation, and repeat. Nutrient does not disperse instantly, and measuring too soon is how people overshoot.
Always adjust EC first and pH second. Adding nutrient shifts pH, so setting pH before EC means doing it twice.
For the pH half, a two-part up and down set such as the Humboldts Secret pH Up and Down kit is the standard approach. Add it drop by drop with the solution circulating, because both directions overshoot easily.
A note on meter accuracy
EC readings drift with temperature. Most meters compensate automatically to a 25C reference, but cheap ones do not, and an uncompensated reading taken from a 30C reservoir can be meaningfully off. Check whether yours says ATC on the housing.
Calibrate against a known standard solution monthly. An uncalibrated meter is worse than no meter, because it gives you confidence in a wrong number.
For home growing you do not need a lab instrument. A VIVOSUN digital pH and TDS/EC meter set covers both readings with automatic temperature compensation and calibration, which is all this job requires.