# Runoff EC

> Runoff EC is the conductivity of solution draining from the pot; compared with input EC, it shows whether salts are building up or being used up in the root zone.

- Canonical: https://genetixbay.com/knowledge/glossary/runoff-ec.html
- Also written: drain EC, EC of runoff, runoff ppm, leachate EC, output EC
- Glossary category: pH, EC and water · Evidence: Supported
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Runoff EC is the [electrical conductivity](https://genetixbay.com/knowledge/glossary/ec.md) of the [drainage](https://genetixbay.com/knowledge/glossary/runoff.md) from a container, in [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md). On its own it says little; its value is the comparison with what went in. It reflects the salt concentration in the medium's pore water, which is what the roots actually sit in.

## In practice

Rules of thumb for coco and [soilless](https://genetixbay.com/knowledge/glossary/soilless.md) media:

| Runoff vs input | Meaning |
|---|---|
| Within about ±0.3 mS/cm | Balanced feeding |
| Runoff higher by 0.5–1.0+ mS/cm | Salts accumulating; increase runoff or lower input EC |
| Runoff lower | Plant is using nutrients faster than supplied; feed slightly stronger |

Example: input 1.6 mS/cm, runoff 2.8 mS/cm. Salts are building up in the medium. Increase runoff fraction for a few irrigations or do a [root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) with a mild nutrient solution, then check that runoff returns towards input.

In [drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md) crop steering, growers sometimes let substrate EC climb on purpose to steer generative growth; that is a managed strategy, not neglect.

## Don't confuse with

- **Input EC**: the strength of the solution you apply.
- **Substrate EC**: measured in the medium by a probe; related but read differently.

## Myth check

GenetixBay house position: a **root-zone flush** to wash accumulated salts out of the medium is the one valid [flush](https://genetixbay.com/knowledge/glossary/flushing.md). It treats the medium, not the plant, and can be done at any stage. A **pre-harvest flush** is a *Myth* and actively harmful: it starves the plant while it is packing on its final weight and resin. See [the flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md).

## Related terms

[Input EC](https://genetixbay.com/knowledge/glossary/input-ec.md), [Runoff](https://genetixbay.com/knowledge/glossary/runoff.md), [Salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md), [EC](https://genetixbay.com/knowledge/glossary/ec.md), [Osmotic stress](https://genetixbay.com/knowledge/glossary/osmotic-stress.md)

## Read more

- [Runoff testing: how to measure root-zone pH and EC and what the numbers mean](https://genetixbay.com/knowledge/ph-ec/runoff-testing.md)

## Sources

1. 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
2. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6