But what's wrong with the plants? I only check runoff pH if I suspect there's a problem going on.

If you're not seeing plant symptoms, I wouldn't micro-focus on runoff pH. Having said that, I think a chronically low runoff pH often accompanies over-feeding. I'd be curious about your runoff EC/ppms over time. If that measurement has been climbing, I'd back off the strength of your feed. Just an idea.

Grownsince95 said:

But what's wrong with the plants? I only check runoff pH if I suspect there's a problem going on.

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ArtfulCodger said:

If you're not seeing plant symptoms, I wouldn't micro-focus on runoff pH. Having said that, I think a chronically low runoff pH often accompanies over-feeding. I'd be curious about your runoff EC/ppms over time. If that measurement has been climbing, I'd back off the strength of your feed. Just an idea.

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I appreciate the input, but I have to disagree with ignoring a runoff pH of 4.9, especially when it was stable the previous week. My question is about how to correct it, not whether to worry about it.

I’ve heard the “don’t worry about runoff” line before—especially from Advanced Nutrients’ first-level tech support. But after speaking with a VP there, I was told directly that low runoff pH is an issue. When I circled back with first-level support, they initially tried to help, but when they couldn’t fix it, they fell back on the usual script: “Our nutrients cover a broad pH range, so runoff pH doesn’t matter.” That doesn’t sit right with me, and it contradicts what I was told higher up in the company.

If runoff pH is consistently low, something is happening in the medium—whether it’s nutrient buildup binding to the coco, depletion of buffering, or some other imbalance. I know it’s not from overfeeding because my nutrient strength is typically diluted, running around 1000-1200 PPM in bloom. If runoff creeps toward 1500, I adjust feedings gradually and, if needed, flush with RO to bring it back in line. My system is automated and closely monitored, so I can catch changes fast.

At this point, something has shifted in the coco, and I’m working to correct it. If I find a solid fix, I’ll post it here for anyone else dealing with the same issue. But logically, getting the root zone back into the correct pH range (5.8-6.2) can only benefit the plant—especially when it held steady there for six weeks before this drop.

I’ll see how today’s Cal-Mag-enhanced, pH-balanced flush works and report back.

And here’s my logic….I’ve done a little research on this topic. Most of this below is NOT my writing. It’s piece meal from various scientific sources:

There’s solid scientific reasoning that supports the idea that letting runoff pH drift too low can negatively impact plant health—regardless of what Advanced Nutrients claims about their formulas.

1. Nutrient Availability in Coco at Low pH

Coco coir behaves differently from other media like soil, but it still follows general nutrient availability trends. The optimal pH range for nutrient uptake in coco is around 5.8-6.2. When pH drops below 5.5, several key issues arise:

Phosphorus and calcium become less available, leading to deficiencies even if they’re present in the nutrient solution.

Magnesium and molybdenum uptake is severely limited, which can cause interveinal chlorosis or other deficiency symptoms.

Toxic levels of manganese and iron can be absorbed, potentially leading to toxicity symptoms like dark, crinkled leaves.

Even if Advanced Nutrients claims their formulas are “self-buffering,” that doesn’t change the chemical properties of nutrient availability. A persistent pH of 4.9 means some nutrients are getting locked out while others become overly available, which can harm the plant over time.

2. Cation Exchange & Buffering Breakdown in Coco

Coco coir naturally holds onto certain cations (like calcium, magnesium, and potassium) due to its cation exchange capacity (CEC). If pH drops too low over time, the buffering ability of coco can degrade, meaning:

The medium may start binding beneficial nutrients (like calcium and magnesium) rather than releasing them.

It may release excess potassium and sodium instead, further pushing pH out of balance.

Nutrient retention can become inconsistent, leading to fluctuations in availability that the plant cannot easily adapt to.

If Advanced Nutrients suggests that low runoff pH doesn’t matter, they’re ignoring the fact that coco doesn’t self-correct like soil—it needs proactive pH management to maintain buffering capacity.

3. Root Zone Microbiology & Stress

If the medium is too acidic for extended periods:

Beneficial microbes struggle to survive, reducing nutrient cycling and uptake.

Root stress increases, which can slow growth, reduce yield, and make plants more susceptible to pathogens.

Plants may have to expend more energy regulating ion uptake, which takes away from growth and bud development.

Conclusion: Runoff pH Matters

Even if Advanced Nutrients claims their formulas work across a broad range, science still dictates that nutrient uptake is pH-dependent. Their “ignore runoff pH” stance might be fine in the short term, but over time, an acidic root zone will cause problems.

I believe the safest approach for Coco Growers is what I’m doing —monitoring pH trends, adjusting when necessary, and keeping the medium within the proven 5.8-6.2 range. If AN’s nutrients were truly immune to pH issues, growers wouldn’t be reporting problems when runoff falls out of range.

Is your instrument calibrated and working correctly? Easy to overlook...

Bobcat.Branch said:

Is your instrument calibrated and working correctly? Easy to overlook...

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I get the question. And appreciate the post. And wish it was that easy. But unfortunately, my ph and ppm meter are both calibrated, within the last week. When this happened the first time I was sure it was something I must be doing wrong or a piece of equipment was off. And the first grow was done in a different brand of coco. I switched to FloraFlex this grow to hopefully be able to blame it on the brand of coco I used last time. But nope. Same results with different brands of coco.

b8kd said:

I get the question. And appreciate the post. And wish it was that easy. But unfortunately, my ph and ppm meter are both calibrated, within the last week. When this happened the first time I was sure it was something I must be doing wrong or a piece of equipment was off. And the first grow was done in a different brand of coco. I switched to FloraFlex this grow to hopefully be able to blame it on the brand of coco I used last time. But nope. Same results with different brands of coco.

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Cool... For me, I have a tendency to overlook the obvious sometimes so I thought I'd throw that on the wall to see if it would stick.

Grownsince95 said:

But what's wrong with the plants?

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Anything else going on with them besides runoff pH?

Bobcat.Branch said:

Cool... For me, I have a tendency to overlook the obvious sometimes so I thought I'd throw that on the wall to see if it would stick.



Anything else going on with them besides runoff pH?

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Nope. Nada. Trying to keep it all on track and this is the only thing concerning me. I’ve got six weeks to go and a 4.9 ph. I do hear that others still seem to grow decent flowers with the low run off so I’ll just have to wait and see. But I can’t imagine it couldn’t be anything but good for them to get the run off ph back in range.

So today’s sensi cal mag xtra enhanced flush did not work to get the ph back in range. Still coming in at 4.9. And that was with 15 gallons for four plants. I left it alone after that at about 7pm and lights out was at 9pm. Tomorrow morning at 9am first thing I’ll give them a feeding of about a gallon each at full strength, about 1200 ppm, since they haven’t had anything to eat except cal/mag all light long. After that if the ph has not risen in the run off I’ll do a 10-15 gallon per plant flush with sensi cal ph’d to 6.8 to see if I can get the coco’s ph back up in the run off. I’ll check it each time after all plants get five gallons. That way I won’t overshoot. Which I doubt will happen based on current results. If that doesn’t work I’m not sure. Might leave it alone at that point but we’ll see. I’ll definitely feed it well after the flush and leave it alone for a few days and see how they do. That will also give the coco time to level out again so I can truly see where I’m at after all the flushing.

Will post results after I finish up tomorrow night. At least I now know, after all my web and various forum searching, that I am not the only one running into the 4.9 number in run off when using advanced nutrients coco product line.

b8kd said:

Will post results after I finish up tomorrow night. At least I now know, after all my web and various forum searching, that I am not the only one running into the 4.9 number in run off when using advanced nutrients coco product line.

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Without any other bad sign or symptom, I would just feed and target pH according to stage of growth (5-6 weeks into flower), be vigilant going forward, and make adjustments based on what they're telling you. That's me, though.

In other's accounts of this in your searches, were they having other symptoms seemingly related to low pH runoff? In other words, any reports of cause and effect? My hydro brain is wired for ebb and flow, so I'm used to monitoring a reservoir over the course of several days, which could be feeding anywhere from 4 to 9 plants in Coco/perlite. I'm keen to know how you get along with this! #invested

My impression is that reports of low runoff pH in coco often involve larger pot sizes, relative to plant sizes. That might be because the plants are getting more nutrient than they're taking up, or that the pots aren't "flushing" as much with each watering. I don't know what your plants look like, so I don't know if this might apply to your situation. Just another idea to consider.

Ok. I think I got it figured out. And if you grow in coco and use Advanced Nutrients you might want to give this a read. It is possible to fix the persistently low ph that develops in your run off and root zone when using their “ph perfect” formulas. You don’t have to just live with it.

So how did I fix it? I flushed about 80 gallons of RO water with half a cup of Sensi Cal Mag per five gallons of water ph’d to about 6.8 through the four (4) five gallon pots using my floraflex matrix system in batches of about 10 gallons at a time and then letting it rest for 15 min in between. The PPM of the full strength cal mag solution was about a 1000. I dropped the Sensi Cal Mag to 1/4 cup per five gallon of water towards the end and then just plain RO water for about the last ten gallons ph’d to 6.8. At this point my run off finally came out at 5.5 ph with 210 ppm. It stayed at 4.9 till the very end when it just didn’t anymore. Tomorrow morning I’ll start back with regular feedings probably starting at half strength and getting back to full strength in a couple days.. Need to let them dry out tonight.

Pics of plants, along with a video, below.

b8kd said:

Ok. I think I got it figured out. And if you grow in coco and use Advanced Nutrients you might want to give this a read. It is possible to fix the persistently low ph that develops in your run off and root zone when using their “ph perfect” formulas. You don’t have to just live with it.

So how did I fix it? I flushed about 80 gallons of RO water with half a cup of Sensi Cal Mag per five gallons of water ph’d to about 6.8 through the four (4) five gallon pots using my floraflex matrix system in batches of about 10 gallons at a time and then letting it rest for 15 min in between. The PPM of the full strength cal mag solution was about a 1000. I dropped the Sensi Cal Mag to 1/4 cup per five gallon of water towards the end and then just plain RO water for about the last ten gallons ph’d to 6.8. At this point my run off finally came out at 5.5 ph with 210 ppm. It stayed at 4.9 till the very end when it just didn’t anymore. Tomorrow morning I’ll start back with regular feedings probably starting at half strength and getting back to full strength in a couple days.. Need to let them dry out tonight.

Pics of plants, along with a video, below.


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Did you say 80 gallons?
What's your plan going forward to prevent this chore again? I'm glad you got the results you were looking for, but dang that's like, real work lol.

Bobcat.Branch said:

Did you say 80 gallons?
What's your plan going forward to prevent this chore again? I'm glad you got the results you were looking for, but dang that's like, real work lol.

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Yep, 80 gallons. I’ve heard that when flushing coco, you need at least 3–5 times the pot volume to fully remove built-up salts and reset the medium. The key is using pH-adjusted water, with a little cal-mag to buffer the solution—a little higher than your target ph, but not so high that it shocks the roots—until the runoff reaches the desired range. If the pH isn’t budging, just keep going. It will get there.

On my last grow, I tried flushing but never used the full recommended water volume. I’d run 5–10 gallons, see no change, get frustrated, and give up. Turns out, I just needed to stick with it.

Today, I fed at 5.7 pH, and for the first time in weeks, my runoff matched at 5.7. I’m honestly amazed—it actually worked. I fully reset the medium, and it’s even better than the 5.5 I got yesterday.

Moving forward, I’ll probably work in proactive flushes during my next grow to prevent pH from getting locked in the first place.

b8kd said:

Yep, 80 gallons. I’ve heard that when flushing coco, you need at least 3–5 times the pot volume to fully remove built-up salts and reset the medium. The key is using pH-adjusted water, with a little cal-mag to buffer the solution—a little higher than your target ph, but not so high that it shocks the roots—until the runoff reaches the desired range. If the pH isn’t budging, just keep going. It will get there.

On my last grow, I tried flushing but never used the full recommended water volume. I’d run 5–10 gallons, see no change, get frustrated, and give up. Turns out, I just needed to stick with it.

Today, I fed at 5.7 pH, and for the first time in weeks, my runoff matched at 5.7. I’m honestly amazed—it actually worked. I fully reset the medium, and it’s even better than the 5.5 I got yesterday.

Moving forward, I’ll probably work in proactive flushes during my next grow to prevent pH from getting locked in the first place.

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Results are all that matters... Nice job! All of my coco experience is with ebb and flow, and never needed to flush. I guess it's self-flushing? Never really thought of it that way til now.

I can see where more frequent flushes would be easier to manage, using many gallons less per flush as you go along. Interesting stuff!

I had the same thing going on. Don’t panic about it but it could be a sign of root rot. I had the low ph and also had a rot smell here and there. I started adding hydro guard and it cleared up in about 2 weeks. No more rot smell and I never even check my run off again. It was the first week of flower that I smelled the rot. They’re in the last week of flower and everything looks perfect.

Root rot would really suck. Were you in coco? I’ve found it’s hard to overwater when I’m using coco and running an automated watering system that hits the plants 4-5x times a day. I will keep an eye out for it though, but I’m sure that’s not it this time. Plants are thriving, bulking up, and the only smell in the room is the one I want.