Runoff EC Drift: Reading Your Medium’s Health

Runoff EC drift is the medium’s audit trail. Field capacity is locked per medium — coco 75%, rockwool 80%, soil 45%, perlite 35%, clay 50% — with the wilting point below it (coco 15%, rockwool 10%, soil 15%, perlite 5%, clay 10%), and moisture reads as 100 minus the dryback you steer. The moment runoff stops, every fed ion stays in the bank and EC drifts at ΔEC = substrate EC × (1 − runoff ÷ volume) × 0.05 per event — so a no-runoff coco pot fed at 1.8 mS/cm stacks roughly 0.09 mS/cm on every dry feed and climbs toward a reading that looks like a salt problem, when the real problem was the flow.

Runoff EC Drift by Medium: Reading the Water Bank Before Salt Stacks

1. Pick the medium and container size — Field capacity (coco 75%, rockwool 80%, soil 45%, perlite 35%, clay 50%) and wilting point (coco 15%, rockwool 10%, soil 15%, perlite 5%, clay 10%) come from the substrate; container litres turn them into a water bank you can pour.
2. Set the irrigation shot — The litres each event delivers. Runoff is honest accounting — a dry coco pot with storage room may drink the whole shot with nothing draining.
3. Slide the dryback target — The percent of the bank you want transpired before the next event. The air lifts it: dryback actual equals target across a VPD factor, so 1.4 kPa pulls more moisture out of the coupon.
4. Read moisture as 100 minus dryback — A 20% target at a 1.0 VPD reads 80% on the sensor. This is the number your probe would tell you, and the number the safety band judges.
5. Check the air the roots breathe — AFP = porosity − moisture. Roots need both water and air, so the ideal lands inside 15–30% AFP while moisture sits in 50–70% — the wet-and-breathing band.
6. Price the salt the no-runoff shot stacks — EC drift grows the moment runoff stops. If nothing drains, every fed ion stays — the moisture screen and the feed chart are the same conversation.

The Formula Behind the Result

• Actual dryback — target × the VPD factor, so dry air pulls more → DB_{actual}=DB_{target}cdot f_{VPD}
• Substrate moisture — what your sensor reads → M_{substrate}=100-DB_{actual}
• Water holding capacity — field capacity minus wilting point → WHC=FC-WP
• Field capacity in litres — the top of the water bank → FC_{L}=V_{container}cdotfrac{FC}{100}
• Wilting point in litres — the floor of the bank → WP_{L}=V_{container}cdotfrac{WP}{100}
• Runoff — the shot only drains after the storage gap is full → Runoff=maxleft(0,;I_{shot}-left(FC_{L}-M_{current}right)right)
• Air-filled porosity — pore space minus the water sitting in it → AFP=P_{porosity}-M_{actual}
• EC drift — salt stacking the moment runoff stops → Delta EC=EC_{substrate}cdotleft(1-frac{Runoff}{V_{container}}right)cdot0.05

Target Ranges: What the Numbers Should Be

• Field capacity by medium — same locked table as the irrigation timer: coco 75%, rockwool 80%, soil 45%, perlite 35%, clay 50%.
• Wilting point by medium: coco 15%, rockwool 10%, soil 15%, perlite 5%, clay 10% — the floor of the bank.
• Water holding capacity = FC − WP: coco 60%, rockwool 70%, soil 30%, perlite 30%, clay 40%.
• Moisture = 100 − dryback actual, so a 20% target at VPD 1.0 reads 80% on the sensor.
• Air-filled porosity = porosity − moisture; roots need both, and 15–30% AFP is the air band to land in.

Use the free Nutrient Lab Pro — Elemental PPM Formulator to run the numbers on your own setup.

Use the free CO2 Enrichment & HVAC Engine to run the numbers on your own setup.

Use the free Climate Controller & HVAC Sizer — VPD, Dehumidifier & CO2 Blueprint to run the numbers on your own setup.

Compare every free calculator side-by-side on the MistCulture tools hub — each one keeps your workspace in sync.

Related guides

Worked example: 0.09 mS/cm per no-runoff feed in a 10 L pot

Take a 10 L coco pot: field capacity 7.5 L (75%), wilting point 1.5 L (15%), a 6 L usable bank. A pot sitting 2 L below field capacity drinks the entire 2 L irrigation shot — runoff is zero, and the drift formula applies at full weight: ΔEC = 1.8 × (1 − 0 ÷ 10) × 0.05 = 0.09 mS/cm on that single feed. Four dry feeds in a row stack roughly 0.36 mS/cm and walk the substrate from 1.8 toward 2.16 mS/cm before the medium visibly dries. Rockwool delays the same climb: a 10 L block holds 8 L at field capacity with a 1 L wilting point — a 7 L bank — so more of the shot is absorbed before anything drains, and the moment runoff restarts the drift term falls toward zero. Air-filled porosity stays the constraint on both: roots hold only when AFP sits in the 15–30% band while moisture rides 50–70%.

Frequently Asked Questions

References & Science

• HortScience — hydroponic nutrient studies
• Purdue Extension — hydroponics guide
• Cornell CALS — hydroponic research manual

Frequently asked questions

References

• ASTM D1125 — Standard Test Methods for Electrical Conductivity and Resistivity of Water
• APHA Standard Methods 4500-H — pH value (electrometric)
• Resh, H. M. — Hydroponic Food Production (7th ed.), CRC Press

Related resources

• All Water chemistry tools
• More Plant care guides
• Open Water Quality Analyzer
• Open Ph Alkalinity Architect