EC Targets by Stage

What EC actually measures

Electrical conductivity (EC) measures how well a solution conducts electricity, which scales with dissolved ionic salts — primarily the fertilizer ions you've added (nitrate, ammonium, potassium, calcium, magnesium, sulfate, phosphate) plus whatever was in your source water ^[1] Strong evidence.

EC is reported in millisiemens per centimeter (mS/cm) or microsiemens per centimeter (µS/cm). PPM (parts per million) is a conversion from EC using a scale factor — commonly 500 (NaCl/Hanna), 700 (KCl/European), or 640. The same solution reads as different PPM numbers on different meters, which is why serious growers quote EC, not PPM ^[2] Strong evidence.

EC tells you how much salt is in solution. It does not tell you which salts. A solution heavy in sodium from poor source water can read the same EC as a clean balanced feed, but the plant response is very different ^[1] Strong evidence.

Why growers use staged EC targets

Cannabis nutrient demand changes through its life. Seedlings have small root systems and are easily burned. Vegetative plants build leaf and stem mass and want more nitrogen and potassium. Flowering plants shift demand toward potassium and phosphorus and away from nitrogen. Matching feed strength to demand reduces two failure modes:

• Underfeeding: pale leaves, slow growth, premature senescence.
• Overfeeding (salt stress): tip burn, clawing, lockout, reduced water uptake, root damage ^[3] Strong evidence.

Staged EC targets are a heuristic for staying in the safe zone. They're not a yield-maximization protocol — controlled trials specifically optimizing EC schedules in cannabis are scarce. Most published numbers come from grower practice and extension guidance adapted from tomato and greenhouse vegetable research ^[4] Weak / limited.

Typical EC targets by stage

These are widely used ranges for soilless (coco, peat) and hydroponic systems, assuming reverse-osmosis or low-EC source water. Add your source water EC to these input targets if you're feeding tap.

| Stage | Input EC (mS/cm) | Notes | |---|---|---| | Seedling / fresh clone | 0.4 – 0.8 | Very light; some growers feed only plain water plus calmag for the first week | | Early veg | 0.8 – 1.4 | Ramp as roots fill the pot | | Late veg | 1.4 – 1.8 | Peak nitrogen demand | | Transition (first 2 weeks of flower) | 1.6 – 2.0 | Stretch phase, high demand | | Mid flower | 1.8 – 2.4 | Heaviest feeding window | | Late flower | 1.4 – 1.8 | Demand tapers as senescence begins | | Flush (optional) | 0.0 – 0.4 | See Flushing Before Harvest — contested practice |

Sources for these ranges are primarily extension publications and commercial nutrient manufacturer guides ^[4][5] Weak / limited. There is no peer-reviewed consensus EC schedule for cannabis specifically. Treat the numbers as starting brackets, not setpoints.

For soil with organic amendments, ignore most of this — you're feeding the soil biology, not the plant directly, and meter readings of the runoff are not very informative Anecdote.

How to do it: step-by-step

1. Calibrate your meter. Use fresh two-point calibration solution (typically 1.413 mS/cm and either 0.0 or 12.88 mS/cm) every 1–2 weeks. An uncalibrated meter is worse than no meter Strong evidence.

2. Measure your source water EC. If it's above ~0.4 mS/cm, factor it into your input targets, or switch to RO. Hard tap water can push you over target before you add any nutrients.

3. Mix your nutrient solution to the input EC for the current stage. Add nutrients, stir, let temperature equilibrate, then measure. Adjust pH after EC — pH adjusters change EC slightly but EC adjustments change pH a lot.

4. Feed and collect runoff. Aim for 10–20% runoff in coco and hydro. Measure runoff EC.

5. Read the runoff.

• Runoff EC ≈ input EC: plant is taking up water and nutrients in balance.
• Runoff EC > input EC (e.g. input 1.8, runoff 2.6): salts are accumulating. Lower input EC or increase runoff volume.
• Runoff EC < input EC: plant is eating faster than you're feeding, or media is buffering. Consider raising input EC.

6. Watch the plant. Tip burn, clawed leaves, or dark glossy green = back off. Pale lower leaves with no other deficiency cause = step up Strong evidence.

7. Log everything. EC trends matter more than single readings. A simple spreadsheet beats memory.

Common mistakes

• Chasing a number instead of reading the plant. Two gardens with the same EC can have wildly different outcomes due to light intensity, VPD, root zone temperature, and genetics. High-light rooms can run higher EC because plants transpire more and uptake more nutrients Weak / limited.
• Confusing PPM scales. A grower quoting "1000 ppm" on a 500-scale meter is at EC ~2.0; on a 700-scale meter that same EC reads ~1400 ppm. Always confirm scale ^[2] Strong evidence.
• Ignoring source water. Adding 1.6 mS/cm of nutrients to 0.6 mS/cm tap water gives a 2.2 mS/cm feed, not 1.6.
• Not calibrating. Cheap pen meters drift fast. Calibration solution is cheap insurance.
• Over-flushing. Running plain water for weeks before harvest based on the belief it improves flavor or removes residuals is not supported by the available controlled research ^[6] Disputed.
• Treating organic soil like hydro. Runoff EC in living soil reflects little about what's bioavailable. Meter readings are mostly noise there Anecdote.

Related techniques

• pH Management in Coco Coir — EC is meaningless without pH in range.
• Reading Runoff — interpreting what comes out the bottom.
• Flushing Before Harvest — the contested last stage.
• VPD Targets by Stage — transpiration drives nutrient uptake, so VPD and EC interact.
• Calcium and Magnesium Supplementation — often needed when running RO water at any EC.

Sources

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