# Runoff testing: how to measure root-zone pH and EC and what the numbers mean

> Runoff (leachate) pH and EC tell you what the roots are actually sitting in, which is often different from what you fed. Use the pour-through method: water to full saturation, wait 30 minutes to 2 hours, then pour a measured amount of water on top and collect about 50 mL. Compare runoff EC with input EC: higher means salts are building up, much lower means the plant is hungry or you are over-leaching. Track the trend over weeks rather than reacting to one reading.

- Canonical: https://genetixbay.com/knowledge/ph-ec/runoff-testing.html
- Category: pH and EC · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Soil, Coco, Rockwool, Drip, Living soil
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**How do I test runoff [pH](https://genetixbay.com/knowledge/glossary/ph.md) and [EC](https://genetixbay.com/knowledge/glossary/ec.md)?** To test runoff pH and EC, irrigate the pot to full saturation, wait 30 minutes to 2 hours, then pour enough clean water evenly on top to collect about 50 mL from the bottom. Measure pH and EC promptly, because leachate pH can change within about 2 hours. Also measure your irrigation water, and sample the same 3–5 plants each time.

**What should my runoff EC be compared to my input?** Cannabis runoff EC is normally about equal to the input EC, up to roughly 0.5 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md) higher. Runoff more than 0.8–1.0 mS/cm above input and rising means salts are accumulating, while runoff well below input means uptake exceeds supply or you are leaching heavily. These targets are grower practice for soilless media, consistent with general greenhouse practice.

**Why is my runoff EC higher than what I feed?** Runoff EC climbs above feed EC because cannabis takes up water faster than some ions, so salts concentrate in the medium between irrigations. If runoff is more than about 0.8–1.0 mS/cm above input and rising, lower feed EC by 10–20 % or increase runoff volume. Sampling a dry pot with a small splash exaggerates the reading.

**Why is my runoff pH different from my feed pH?** Runoff pH differs from feed pH because chemistry changes inside the pot: nitrate uptake pushes pH up, [ammonium](https://genetixbay.com/knowledge/glossary/ammonium.md) pushes it down, alkaline water raises it over time, and peat and fresh coco have their own chemistry. Runoff rising more than 0.5 above input over weeks calls for acidified water or a small ammonium share; runoff falling below about 5.5 suggests ammonium, acidic organics or root problems.

**What is the pour-through method?** The pour-through method is an extension technique for measuring root-zone pH and EC in containers. After irrigating to [container capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md) and waiting 30 minutes to 2 hours, you pour a set volume of water on top, for example about 350 mL for a 12 L pot, which pushes roughly 50 mL of the pot's held solution out of the bottom for testing. It is established in container horticulture.

**How much runoff should I get in coco?** In [drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md) coco and [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md), a common target is 10–20 % of the applied volume as runoff each day, increased in hot conditions or when runoff EC is climbing. That figure is grower practice rather than a cannabis-specific trial result. Constant heavy leaching wastes fertiliser and, with phosphorus, pollutes, so fix the feed rather than over-leaching every watering.

**How do I flush salt build-up out of my medium?** To flush salt build-up, run about 2–3 times the container volume through slowly in several passes: pH-adjusted plain water at about 6.3–6.8 for soil, or a light solution at EC 0.4–0.8 mS/cm with calcium and magnesium, pH 5.8–6.0, for coco and rockwool. Stop when runoff EC is within about 0.3–0.5 mS/cm of the input, then resume normal feeding.

**Should I flush before harvest?** No, you should not flush cannabis before harvest. Pre-harvest flushing is a myth that costs yield and resin by starving the plant during its final weeks, so feed through to harvest. The only valid flush is a root-zone flush that washes accumulated salts out of the medium when runoff EC is high or lockout appears, and it can be done at any stage.

**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]:

1. **Irrigate thoroughly** as normal, to container capacity.
2. **Wait 30 minutes to 2 hours.**
3. **Place a saucer** under the pot.
4. **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 |

5. **Measure pH and EC promptly**; leachate pH can change within about 2 hours [2].
6. **Measure your irrigation water too** and subtract its EC to see what comes from fertiliser [1].
7. **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](https://genetixbay.com/knowledge/glossary/ph-drift.md) 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](https://genetixbay.com/knowledge/glossary/nutrient-burn.md) | [Salt stress](https://genetixbay.com/knowledge/glossary/sodium.md) | Root-zone flush (below); see [burnt tips](https://genetixbay.com/knowledge/diagnosis/brown-burnt-tips.md) |

**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](https://genetixbay.com/knowledge/glossary/living-soil.md):** 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.

1. Record input and runoff pH and EC.
2. **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.
3. Run about **2–3× the container volume** through slowly, in several passes.
4. 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.
5. Go straight back to normal feeding, and fix the cause (feed EC, runoff volume, [water hardness](https://genetixbay.com/knowledge/glossary/general-hardness.md)).

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](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)).

## 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](https://genetixbay.com/knowledge/ph-ec/meter-calibration.md)).
- 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].

## Evidence

- Pour-through gives representative substrate EC and pH: *Established* in container horticulture [1][2].
- Interpreting runoff EC relative to input EC for cannabis: *Grower practice*; consistent with general greenhouse practice [5].

## Sources

1. NC State University / NC Department of Agriculture and Consumer Services. The pour-through extraction procedure: a nutrient management tool for nursery crops. https://ncagr.gov/ncsu-pour-through-extraction-procedure/download
2. Cavins TJ, Whipker BE, Fonteno WC, Harden B, McCall I, Gibson JL. Monitoring and managing pH and EC using the PourThru extraction method. NC State University Horticulture Information Leaflet 590. Cited via MSU Extension: https://www.canr.msu.edu/uploads/resources/pdfs/measuring_ph_and_ec_of_crops_grown_in_large_containers.pdf
3. Lopez RG, Mickelbart MV, Pasian C. Purdue Extension HO-242-W. https://www.extension.purdue.edu/extmedia/HO/HO-242-W.pdf
4. Westmoreland FM, Bugbee B. 2022. Frontiers in Plant Science 13:1015652. https://doi.org/10.3389/fpls.2022.1015652
5. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6