Short answer. Runoff (leachate) pH and EC tell you what the roots are actually sitting in, which can be very different from what you fed. Use the pour-through method: irrigate to full saturation, wait 30 minutes to 2 hours, pour a measured amount of water on top, and collect about 50 mL from the bottom [1]. Compare runoff EC with input EC: higher means salts are building up; much lower means the plant is hungry or you are leaching hard. Watch the trend over weeks, not one reading.
What's happening
In any container, water and salts are not uniform. Plants take up water faster than some ions, so salts concentrate between irrigations. Water draining from the bottom after a fresh pour displaces the solution held in the bottom of the pot (the perched water table) and gives an average reading of what the roots experience [1].
pH also shifts in the pot: nitrate uptake pushes it up, ammonium down; alkaline water pushes it up over time; peat and fresh coco have their own chemistry [3].
The pour-through method
From NC State's nursery procedure [1]:
- Irrigate thoroughly as normal, to container capacity.
- Wait 30 minutes to 2 hours.
- Place a saucer under the pot.
- Pour clean water (or your normal feed, but be consistent) evenly on the surface, enough to push out about 50 mL. Guide volumes:
| Container | Pour | Collect |
|---|---|---|
| 4 L (1 gal) | ~120 mL | ~50 mL |
| 12 L (3 gal) | ~350 mL | ~50 mL |
| 19 L (5 gal) | ~600 mL | ~50 mL |
| 27 L (7 gal) | ~825 mL | ~50 mL |
| 38 L (10 gal) | ~1,200 mL | ~50 mL |
- Measure pH and EC promptly; leachate pH can change within about 2 hours [2].
- Measure your irrigation water too and subtract its EC to see what comes from fertiliser [1].
- Sample the same 3–5 representative plants each time and log it.
Drain-to-waste coco and rockwool growers usually just catch the drain from the irrigation itself. That works, but the first runoff after a dry-back is saltier than the last; mix the whole catch or always sample the same way.
Reading the numbers
These ranges are grower practice for cannabis in soilless media; extension ranges exist for ornamentals but not cannabis.
| Runoff vs input | What it usually means | What to do |
|---|---|---|
| Runoff EC ≈ input to +0.5 mS/cm | Normal | Nothing |
| Runoff EC >+0.8–1.0 mS/cm above input, rising | Salts accumulating; plant drinking more water than nutrients | Lower feed EC 10–20% and/or increase runoff volume |
| Runoff EC well below input | Plant uptake exceeds supply, or heavy leaching | Raise feed EC slightly; check runoff volume |
| Runoff pH >0.5 above input, rising over weeks | Nitrate uptake and/or alkaline water | Acidify water; slightly lower input pH; small ammonium share |
| Runoff pH falling below ~5.5 | Ammonium-heavy feed, acidic organics, or root problems | Raise input pH; review N form; check roots |
| Runoff EC very high with burnt tips | Salt stress | Root-zone flush (below); see burnt tips |
Runoff volume in coco and rockwool drain-to-waste: 10–20% of applied volume per day is common grower practice; more in hot conditions or when EC is climbing.
Soil and living soil: pour-through still works, but organic soil EC is lower and pH more stable; use it to catch trends and sodium or salt build-up rather than to steer daily.
When runoff says flush the medium
A root-zone flush washes accumulated salts out of the medium so you can start fresh. It treats the medium, not the plant, and can be done at any stage. Use it when runoff EC is far above input (roughly +1.0 mS/cm or more and rising), when salt crusts the surface, or when you see lockout symptoms despite a full feed.
- Record input and runoff pH and EC.
- Soil: pH-adjusted plain water (pH ~6.3–6.8). Coco, rockwool, hydro media: a light solution (EC ~0.4–0.8 mS/cm, with Ca and Mg) at pH ~5.8–6.0; plain water strips Ca and Mg from coco.
- Run about 2–3× the container volume through slowly, in several passes.
- Check runoff EC on the last pass: stop when it's within ~0.3–0.5 mS/cm of the flush input and runoff pH is in range.
- Go straight back to normal feeding, and fix the cause (feed EC, runoff volume, water hardness).
This is not a pre-harvest flush. Flushing before harvest is a Myth that costs you yield and resin; feed through to harvest (see the flush debate).
Common mistakes
- Testing runoff from a dry pot with a small splash: you get the most concentrated salts and panic.
- Changing feed in response to one reading.
- Measuring hours later after the sample has sat in the sun.
- Using an uncalibrated meter (see meter calibration).
- Over-leaching every watering to "fix" a high runoff: one proper root-zone flush, then fix the feed. Constant heavy leaching wastes fertiliser and, with P, pollutes [4].
