A nutrient bottle says "use 5 mL per liter." But that label doesn't know your water, your medium, your strain, or your growth stage. The only way to know what your plant is actually receiving — and whether it's too much or too little — is to measure it.
EC (electrical conductivity) and PPM (parts per million) are two ways to express the same thing: how concentrated the dissolved minerals are in your nutrient solution. A cheap pen meter gives you this information in seconds. Once you start measuring, you stop guessing.
EC vs. PPM: what's the difference?
EC (electrical conductivity) measures how well the solution conducts electricity. More dissolved minerals = more conductivity. Measured in millisiemens per centimeter (mS/cm) or microsiemens (µS/cm). EC is the universal standard — the same reading on every meter.
PPM (parts per million) is an estimate derived from EC, but different meters use different conversion factors:
| Scale | Conversion | Used by |
|---|---|---|
| 500 scale (TDS) | EC × 500 | Hanna meters, most US meters |
| 700 scale (CF) | EC × 700 | Truncheon, Bluelab, NZ/AUS |
This means the same solution reads differently on different PPM meters. A nutrient solution at EC 2.0 reads 1000 PPM on a 500-scale meter and 1400 PPM on a 700-scale meter. Same water, different numbers.
Recommendation: use EC. It's universal, unambiguous, and eliminates the "which PPM scale?" confusion. If your meter only shows PPM, check the manual for which conversion factor it uses.
Target EC by stage
| Growth stage | Target feed EC (mS/cm) | Target feed PPM (500 scale) |
|---|---|---|
| Seedlings | 0.4–0.6 | 200–300 |
| Early veg (weeks 1–2) | 0.8–1.2 | 400–600 |
| Late veg (weeks 3–5) | 1.2–1.6 | 600–800 |
| Transition to flower | 1.4–1.8 | 700–900 |
| Early flower (weeks 1–3) | 1.4–1.8 | 700–900 |
| Mid flower (weeks 4–6) | 1.6–2.2 | 800–1100 |
| Late flower (weeks 7+) | 1.0–1.6 | 500–800 |
| Flush (if used) | 0.0–0.2 | 0–100 |
These are guidelines, not rules. Some strains are heavy feeders (Wedding Cake, GMO) and thrive at the high end. Others are light feeders (Durban Poison, most autos) and burn at numbers that other strains love.
Start low, increase gradually. It's always easier to feed more than to fix overfeeding. Begin at the low end of the range and increase by 0.2 EC per feeding until you see the earliest sign of tip burn (a slight yellowing of leaf tips). Back off 0.2 EC from that point — you've found your strain's sweet spot.
How to measure
Feed EC (input)
- Mix your nutrient solution in a bucket or reservoir
- Stir well (nutrients can settle, giving false readings)
- Dip the EC pen into the solution
- Read the value after it stabilizes (5–10 seconds)
- Adjust: too low = add more nutrients; too high = add more water
Runoff EC (output)
This is the critical measurement most growers skip. Feed EC tells you what you're putting in. Runoff EC tells you what's actually happening in the medium.
- Water your plant until you get 15–20% runoff
- Collect the runoff in a saucer or tray
- Measure the runoff EC immediately (before it cools or evaporates)
What runoff EC tells you
| Runoff vs. Feed | What it means | Action |
|---|---|---|
| Runoff ≈ feed EC (±0.3) | Medium is stable, no salt buildup | Continue current feeding |
| Runoff EC > feed EC + 0.5 | Salts are accumulating in the medium | Feed lighter or flush |
| Runoff EC < feed EC | Medium is absorbing nutrients (common in new coco or soil) | Normal early in the grow; monitor |
| Runoff EC > 3.0 | Severe salt buildup | Flush immediately with plain pH'd water until runoff EC drops below 1.5 |
How often to check
- Hydro/DWC: check reservoir EC daily (evaporation concentrates the solution)
- Coco: check feed AND runoff EC every 2–3 waterings
- Soil: check runoff EC weekly. Soil has more buffering capacity, so it changes slower
EC management by medium
Coco coir
Coco is the medium where EC management matters most. Coco has no buffering capacity — what you feed is what the plant gets.
- Feed to 15–20% runoff every watering. Running to waste ensures salts don't accumulate
- Runoff EC should be within 0.5 of feed EC. If runoff is consistently 1.0+ higher than feed, you're building up salts
- Fertigate (water with nutrients) every time in coco. Plain water in coco causes EC swings that stress the plant
Hydro (DWC)
The reservoir is your plant's entire water supply — EC management is everything.
- Top off with half-strength nutrients or plain water as the reservoir drops. If EC rises as the level drops, the plant is drinking more water than nutrients (normal in hot conditions) — top off with plain water. If EC drops as the level drops, the plant is eating more than drinking — top off with nutrients
- Change the reservoir weekly to prevent nutrient ratio drift. Even if EC looks stable, individual nutrients deplete at different rates
- Never let reservoir EC exceed 2.5 — concentrated solutions damage roots
Soil
Soil buffers nutrients significantly. EC management is less critical than in coco or hydro but still useful:
- Feed at a lower EC than coco/hydro — soil amendments (worm castings, lime, bone meal) already contribute nutrients
- Check runoff monthly at minimum. If runoff EC is climbing, water with plain pH'd water for the next 2–3 waterings
- Amended soil (Ocean Forest, living soil) may not need any supplemental feeding for the first 3–4 weeks. Runoff EC can start high (1.5–2.0) from the amendments alone
Your water's starting EC
Before adding nutrients, measure your base water:
| Water type | Typical EC (mS/cm) | Impact |
|---|---|---|
| RO/distilled | 0.00–0.02 | Zero minerals — needs cal-mag, nutrients provide everything |
| Soft tap water | 0.1–0.3 | Some minerals — minor cal-mag may be needed |
| Medium tap water | 0.3–0.5 | Good baseline minerals — may need less cal-mag |
| Hard tap water | 0.5–0.8+ | High mineral content — adjust nutrient strength downward |
Always subtract your base water EC from the target. If your tap water is EC 0.4 and your target feed EC is 1.6, you only need to add 1.2 EC worth of nutrients. If you add nutrients to hit 1.6 on top of the 0.4 base, your actual feed is 2.0 — potentially too much.
Choosing a meter
| Meter | Price | Features |
|---|---|---|
| Generic TDS/EC pen (Amazon) | $10–15 | Works fine, less durable, needs frequent calibration |
| Bluelab Truncheon | $80 | Industry standard, no calibration needed, extremely durable |
| Bluelab Combo Meter | $200 | EC + pH + temperature in one probe — the premium option |
| Apera EC60 | $90 | Good mid-range, replaceable probe, auto-calibration |
| HM Digital COM-100 | $30 | Reliable budget option, EC + TDS + temperature |
Calibrate monthly with calibration solution (usually 1.413 mS/cm or 2.764 mS/cm). A meter that reads 10% off is worse than no meter — it gives you false confidence.
Common mistakes
- Not measuring at all — "I follow the bottle directions" works sometimes. Measuring works every time. A $15 pen pays for itself with the first overfeeding you catch
- Only measuring feed, never runoff — feed EC tells you what you're putting in. Runoff EC tells you what the plant is experiencing. You need both
- Mixing PPM scales — if your nutrient chart says "800 PPM" and your meter reads 800 PPM but they use different scales, you could be 40% over or under. Use EC to eliminate this problem
- Not accounting for base water EC — adding nutrients to 1.6 EC on top of 0.5 EC tap water gives you 2.1 EC total. Your plant doesn't care where the minerals came from
- Chasing exact numbers — EC ±0.2 from your target is fine. Plants are flexible. The goal is to stay in range, not hit a precise number
For nutrient schedules and feeding strategies, see our feeding guide. For diagnosing whether you're over or underfeeding, see our deficiency chart. For nitrogen toxicity from overfeeding, see our nitrogen toxicity guide.