Why Is Your Runoff pH Rising?

Runoff pH above the feed usually comes from one of four things: the pots are underfed, the feed going in isn't what you think it is, fresh rockwool is still releasing alkalinity, or the water is carrying alkalinity. Find out which before you change anything, and don't chase it with a very acidic feed.

For confirmed underfeeding, we raise the complete A+B mix in steps and allow a bit more dryback, then ease back once runoff pH comes down.

Why it rises

As plants take up nitrate, the root zone tends to drift up in pH. A modest nutrient reserve in the pot holds that in check, which is why in coco we feed about pH 6.2 and look for runoff about 6.0–6.1. When there's too little reserve, the rise wins. Repeated plain-water irrigations, heavy leaching or pots that never dry back can all keep that reserve from building.

Collect the last 10% after a normal feed and compare EC and pH with the feed before deciding (see how to test runoff pH and EC).

Check the feed first

• Measure the finished feed at the dripper while it runs.
• Make sure both A and B are in at the chart ratio, the final water volume is right and each doser is pulling from the right tank.
• At chart rate the feed reads about 2.2 EC or a bit higher in RO water, and higher in tap. If it's well under, fix the mixing or injection first and feed the intended mix before going above the chart.
• If you're running water-only days, pause them while you sort out the high pH.

Fix confirmed underfeeding

1. Raise the strength of the complete Veg or Bloom A+B mix in measured steps. Keep the A:B ratio; never raise just one part.
2. Allow more dryback. That can mean a longer interval or dropping extra events when pots stay too wet. Keep wetting the whole pot at each feed, and keep coco about 50–60% dryback, never bone dry (see drybacks explained).
3. Compare feed and last-10% runoff after the next normal feeds. As the reserve builds, runoff pH should come down. Then ease strength and dryback back to a steady schedule instead of continuing to push EC.

Don't use this fix when:

• Runoff EC is already high. More dryback would concentrate it further. Runoff EC and pH can both sit above the feed; send us the readings.
• You just did a plain-water correction. Readings right after water aren't a reason to go above the chart. Get back to normal feeds and retest.
• The plants are close to finish. Tell us the flower day and harvest date first.

Fresh rockwool

New rockwool is alkaline until it's conditioned. Before planting, soak it in full-strength nutrient solution brought to pH 5.5 with RunClean. For a recirculating soak, hold 6.0 and keep adjusting back until it stops climbing. Details in preparing rockwool before planting.

If plants are already rooted in rockwool that was only soaked in plain water, and the high pH isn't from an over-concentrated root zone: increase the EC to the highest strength on the label and pH it to 5.6 with RunClean. Push a decent amount of runoff. The goal at 5.6 going in is to see 5.8 coming out in the last 10% of runoff. Then leave it and continue with normal feeds.

That's a one-time correction for that case. The 5.5 and 5.6 numbers aren't routine feed pH.

Water alkalinity

A ppm reading doesn't tell you the alkalinity, and neither does the tap water's pH. Get an alkalinity test reported as ppm calcium carbonate. If your tap is above about 250 ppm (500 scale), blend in RO to bring it down to about 250. See water pH, alkalinity and hardness and tap water, RO or a blend.