Some of it is normal. Plants take up water and nutrients at different rates, so runoff usually reads a bit above the feed, and we want a modest reserve in the pot. If runoff pH is holding (about 6.0–6.1 in coco with feed at 6.2) and the plants look good, watch the trend instead of flushing.
What we don't do is stack EC on purpose. With CropSalt we've seen excess buildup crash root-zone pH. When runoff EC keeps climbing, get more runoff through the pot. When it comes with falling pH, flush.
How high is too high?
There's no single number; read EC with pH and the trend. Real cases:
| Readings | What we said |
|---|---|
| Runoff about 200 ppm above feed on a 700-scale meter (about 0.29 EC) | Don't chase it if the plants look good and pH is in range. |
| Feed 2.2 EC at pH 5.8, runoff 3.1 EC at pH 6.0, plants good | Can be fine. Watch the trend and get more runoff if EC keeps climbing. |
| Runoff around 5 EC, pH still looks good | Don't wait for the pH to crash. Correct the buildup now. |
| Feed 2.5 EC, runoff 8.4 EC at pH 5.3; six small feeds a day, only the last gives 10–15% runoff | Heavy buildup. Correct it with plain water, then raise the irrigation volume. |
Check the sample first
Catch the last 10% of fresh runoff after a normal feed, straight from the pot into a clean cup. Water from a tray with white crust or a dirty saucer reads high. Use the same meter and scale for feed and runoff; if the numbers don't fit one ppm scale, switch the meter to EC. If the display is maxed out, get a meter that can read that high before choosing a fix. The full method is in how to test runoff pH and EC.
Why runoff EC climbs
- Not enough runoff. More feeds a day doesn't mean more runoff. We run about 25% in coco and peat; see how much runoff you need.
- Channeling. Getting runoff doesn't prove the pot wetted evenly. Water in smaller passes so it soaks in.
- Emitters. A blocked or displaced emitter leaves part of the pot dry. Catch-test each outlet, clean or move it, and test again.
- Pots drying too far. If delivery is even but pots dry too hard, start the first irrigation earlier or add a complete irrigation during the light period, not tiny shots that only wet a channel. A hotter, drier room raises water use; check it in our VPD calculator.
Bring it back down
- Restore thorough wetting and enough drainage to carry the extra salts out. Fix blocked drainage before relying on any runoff percentage. In rockwool, match drainage to the block or slab and its schedule.
- If runoff pH is also low, do a pH-6.4 tap-water flush; the full steps are in Why Is Runoff pH Lower Than Feed pH?
- Compare samples taken after the same normal feed and similar dryback. Look for EC leveling off, runoff pH back in range and healthy new growth.
If pH is stable and the plants are healthy, don't keep flushing just to make feed and runoff EC match. If EC stays high with stressed plants but the pH doesn't fit buildup, send us the full readings before trying anything else.
DWC and bottom-fed systems don't use this top-fed method; check the reservoir and system readings instead.
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