# Runoff EC much lower than input, or reservoir EC dropping fast: likely causes and fixes

> Runoff EC much lower than input, or reservoir EC dropping fast on cannabis is most often General underfeeding (whole feed too weak) or Nitrogen deficiency. How to tell them apart, the fix for each and what recovery looks like.

Also called: runoff ppm low, plant eating everything, EC dropping in res.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=roots&s=runoff-ec-low

## Likely causes, most likely first

### General underfeeding (whole feed too weak) (Possible)

- Kind: Nutrient; urgency: Deal with it this week
- Fix: Raise the whole feed in steps of about 0.2-0.3 mS/cm toward the stage target (veg 1.2-1.8, flower 1.6-2.2 mS/cm in coco and hydro). Stop any 'flush': plain water for weeks starves the plant, and pre-harvest flushing is a myth that costs yield.
- Recovery: Colour deepens and growth speeds up within a week; runoff EC tracks input.
- Confirm: Measure runoff EC against input EC: Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
- Confirm: Measure input pH and root-zone (runoff or reservoir) pH: Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
- Article: [Building a cannabis feed by stage: ppm targets per element and how to hit them](https://genetixbay.com/knowledge/nutrients/feed-by-stage.md)

### Nitrogen deficiency (Possible)

- Kind: Nutrient; urgency: Deal with it this week
- Fix: Fix roots, watering and pH first. If N is genuinely short, raise feed N by about 10-20% (not double): mineral feeds around 150-200 mg/L N in veg and early flower; one DWC flowering trial predicted a yield optimum near 194 mg/L. Coco/hydro: raise base feed EC by ~0.2 mS/cm. Soil: liquid feed at label rate. Living soil: top-dress a fast organic N source and allow 1-2 weeks. Late flower: a gradual fade is the planned N taper, not a fault.
- Recovery: Yellowing stops climbing within 5-7 days and new leaves emerge mid-green; already-yellow leaves do not recover. Runoff EC climbs back toward input.
- Confirm: Measure runoff EC against input EC: Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
- Confirm: Measure input pH and root-zone (runoff or reservoir) pH: Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
- Confirm: Look at the roots: Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
- Confirm: Watch new growth for 5-10 days after one change: Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.
- Confirm: Leaf tissue analysis: Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
- Article: [Nitrogen (N) in cannabis: nitrate vs ammonium, deficiency, toxicity and targets](https://genetixbay.com/knowledge/nutrients/nitrogen.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.