# White crust on the medium surface or pot edges: likely causes and fixes

> White crust on the medium surface or pot edges on cannabis is most often Salt build-up in the root zone or Hard, alkaline water pushing pH up over weeks. How to tell them apart, the fix for each and what recovery looks like.

Also called: white crust on soil, salt build up on pot, crystals on coco, white ring on pot.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=roots&s=salt-crust-medium

## Likely causes, most likely first

### Salt build-up in the root zone (Possible)

- Kind: Root zone; urgency: Deal with it this week
- Fix: Root-zone flush: water slowly with pH-adjusted, quarter- to half-strength feed (not plain water in coco) to 20-30% runoff, about 2-3x the pot volume, until runoff EC is within ~0.3-0.5 mS/cm of input. Then water to 10-20% runoff each time. This treats the medium, not the plant; it is not a pre-harvest flush, which is a myth.
- Recovery: Runoff EC tracks input at the next 2-3 waterings; new leaves stop burning.
- 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: Pour-through extraction for container pH and EC: Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.
- Confirm: Slurry (1:2) test of the medium: Take a sample from the root zone (not the top 2 cm), mix 1 part medium with 2 parts distilled water by volume, stir, wait 30 minutes and measure pH and EC of the liquid. Use it for soil and living soil where runoff is unreliable. Slurry EC reads lower than pour-through EC; compare like with like.
- Article: [Runoff testing: how to measure root-zone pH and EC and what the numbers mean](https://genetixbay.com/knowledge/ph-ec/runoff-testing.md)

### Hard, alkaline water pushing pH up over weeks (Less likely)

- Kind: Root zone; urgency: Low urgency
- Fix: Alkalinity, not water pH, drives the climb. Above about 100-150 mg/L as CaCO3 (2-3 meq/L), neutralise most of it with acid to an input pH of 5.8-6.2, or blend with RO water. Track runoff pH weekly.
- Recovery: Runoff pH stops climbing over 2-3 weeks.
- Confirm: Get a water report (or test your water): Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
- 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: Pour-through extraction for container pH and EC: Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.
- Article: [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.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.