Interpreting EC and runoff correctly
Why drainage EC relative to input EC is more informative than any single value on its own, and how to derive the right correction from it.
Balanced
Runoff EC ≈ input EC
Salt accumulation
Runoff EC > 1.3–1.5× input
Warning sign
Runoff EC clearly below input
Measurement frequency
Weekly control watering
Key points
- Not the absolute EC value of the drainage, but the RATIO to the input EC indicates whether salts are accumulating or whether the plant is taking up nutrients faster than they are replenished.
- Runoff EC clearly above the input EC (factor > 1.3–1.5) signals salt accumulation and an upcoming flush.
- Runoff EC below the input EC can indicate either high nutrient hunger in the plant or over-flushing — both require different responses.
Note
- A repeatedly strongly elevated runoff EC value without a flush can lead to chronic salt stress, which only shows up with a delay as growth stagnation or leaf-margin burn.
Definition and classification
Runoff (drainage) is the liquid that drains out of the bottom of the pot after watering. Its EC provides information about the salt status of the root zone, which the input EC value alone cannot show.
Only the comparison between input and runoff EC makes the value diagnostically useful — an isolated runoff value without a reference is not very informative.
Scientific background
EC measures the sum of all dissolved salts in the solution, regardless of their composition — a high value says nothing about WHICH nutrients are present in excess or deficiency.
If the plant takes up water faster than dissolved salts, the remaining solution in the root zone becomes concentrated — the runoff EC rises above the input EC. The opposite applies when nutrient hunger is particularly high relative to water uptake.
Interpretation rules
Runoff EC ≈ feed EC: system in equilibrium, no acute correction needed.
Runoff EC significantly higher (factor > 1.3–1.5) than feed EC: salt accumulation in the root zone — a flush with pH-corrected water is indicated.
Runoff EC lower than feed EC: either high, healthy nutrient uptake during a strong vegetative stage or over-flushing due to too large a watering volume — use plant condition and watering volume to distinguish.
pH drift in runoff
Runoff pH that deviates significantly from feed pH indicates substrate drift that a pure feed measurement conceals — especially relevant in soil with microbial buffering, which can drift over weeks.
A combination of runoff EC and runoff pH trend over several weeks is more informative than a single snapshot.
Measurement log
Measure feed EC/pH immediately before watering, and runoff EC/pH a few minutes after the drainage has fully drained.
Always measure at the same relative position in the watering cycle (e.g., a defined weekly control watering) to keep values comparable over time.
Ensure a sufficient drain proportion (10–20% of the watering volume) — with too little runoff, the measurement is not representative of the entire root zone.
Checklist
- Always document feed and runoff values as a pair, never in isolation
- Perform control watering on a fixed weekday routine
- Track the runoff trend over several weeks, not just evaluate individual values
Common mistakes
Only measuring the feed and completely ignoring the runoff — the more informative half of the information is then missing.
Deriving a nutrient correction from a single runoff value without a feed reference.
Performing runoff measurement irregularly and at different points in the watering cycle, which makes values incomparable.
Advanced considerations
In recirculating hydro systems, continuous reservoir EC measurement replaces the classic runoff logic, since there is no classic drainage — here the comparison to the last top-up is relevant.
Long-term runoff EC trends can be tracked like a simple control chart, to distinguish gradual drift from acute outliers.
Frequently asked questions
How often should I measure runoff?
What do I do if runoff EC stays persistently low despite a healthy plant?
Sources
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