About
Hydroponics nutrient strength = TDS in ppm or EC in mS/cm. Conversion: PPM ÷ 500 ≈ EC (US 500 scale). Most crops 400-2200 PPM. Adjust by growth stage: seedlings 50%, veg 75%, flower 100%. Test with EC meter weekly.
Formula
Frequently asked questions
What is the difference between EC and PPM in hydroponics?
EC (electrical conductivity, measured in mS/cm) measures how well your nutrient solution conducts electricity, which rises with dissolved salts. PPM (parts per million) restates that as a concentration. They describe the same thing: EC is the raw physical measurement, and PPM is a converted, scale-dependent estimate of total dissolved solids.
How do you convert EC to PPM?
Multiply EC (in mS/cm) by a conversion factor that depends on the scale your meter uses. On the US 500 scale, PPM = EC x 500, so EC 1.2 equals 600 PPM. On the European 640 scale, PPM = EC x 640, and on the Australian 700 scale, PPM = EC x 700. Always check which scale your meter and your nutrient chart assume.
What EC or PPM should I use for my crop?
Leafy greens like lettuce and herbs want a low strength, around EC 0.8 to 1.2 (400 to 600 PPM on the 500 scale). Fruiting crops like tomatoes and peppers want EC 2.0 to 3.5 (1000 to 1750 PPM). Start at the low end and raise gradually while watching the plants, since over-feeding causes more harm than under-feeding.
Why adjust nutrient strength by growth stage?
Young plants have small root systems and burn easily, so seedlings take about 25 to 50 percent of full strength. Vegetative plants take roughly 75 percent, and mature fruiting or flowering plants take full strength. Feeding a seedling at tomato-fruiting strength scorches the roots and stunts growth.
How often should I check EC and PPM?
Test the reservoir at least weekly, and more often in hot weather or with fast-growing plants. As plants drink water faster than they take up nutrients, EC can drift up; as they consume nutrients, it can drift down. Top up with plain water or fresh solution to hold the target range, and do a full reservoir change every one to two weeks.
About hydroponic nutrient strength
In hydroponics there is no soil to buffer nutrients, so the strength of your water determines everything about plant health. Too weak and plants starve; too strong and the high salt concentration pulls water out of the roots through reverse osmosis, scorching them. The two numbers growers watch are EC (electrical conductivity) and PPM (parts per million), and they are really two views of the same measurement: how much dissolved fertiliser salt is in the water.
EC is the honest, scale-independent physical measurement, in millisiemens per centimetre (mS/cm). PPM is a converted estimate of total dissolved solids that depends on which conversion scale your meter uses, which is a common source of confusion. Getting comfortable converting between them, and knowing the target range for your crop and growth stage, is the core skill of nutrient management.
How the conversion works
EC measures conductivity directly. To express it as PPM you multiply by a factor that differs by region and meter:
PPM (500 scale, US/Hanna) = EC x 500 PPM (640 scale, EU/Eutech) = EC x 640 PPM (700 scale, AU/Truncheon)= EC x 700 EC (mS/cm) = PPM / scale factor
- EC is the raw reading in mS/cm. It does not depend on any convention, which is why many growers prefer to work in EC.
- Scale factor (500, 640, or 700) reflects which standard your meter assumes. Mixing scales is the most common nutrient mistake.
- PPM is the converted concentration. A 600 PPM reading on the 500 scale is the same water as 768 PPM on the 640 scale.
Worked example
Suppose you are growing tomatoes in the vegetative stage and your nutrient chart calls for EC 2.0 at full strength, but you want 75 percent strength for vegetative growth, using a US 500-scale meter:
Target EC = 2.0 x 0.75 = 1.5 mS/cm Target PPM = 1.5 x 500 = 750 PPM
Crop EC and PPM reference
Typical full-strength target ranges by crop (PPM shown on the 500 scale):
| Crop | EC (mS/cm) | PPM (500 scale) |
|---|---|---|
| Lettuce, leafy greens | 0.8 to 1.2 | 400 to 600 |
| Herbs (basil, parsley) | 1.0 to 1.6 | 500 to 800 |
| Strawberries | 1.2 to 2.0 | 600 to 1000 |
| Cucumbers | 1.7 to 2.5 | 850 to 1250 |
| Tomatoes | 2.0 to 3.5 | 1000 to 1750 |
| Peppers | 2.0 to 3.0 | 1000 to 1500 |
Common pitfalls
- Mixing up PPM scales. A 700-scale meter reads ~40 percent higher than a 500-scale meter for the same water. Always know your scale before comparing to a chart.
- Feeding seedlings at full strength. Young roots burn easily; start at 25 to 50 percent and ramp up by growth stage.
- Ignoring source-water EC. Tap or well water already carries dissolved salts. Measure it first and add your nutrient EC on top of that baseline.
- Chasing PPM instead of plant signs. Leaf tip burn means too strong; pale slow growth means too weak. The meter guides, the plant decides.
- Forgetting pH. EC tells you how much nutrient is present, not whether plants can absorb it. Keep pH around 5.5 to 6.5 so the nutrients stay available.