PH concerns in flowering

This might help
pH imbalance remains a mystery to novice growers. The pH system is a measure of a substance’s alkalinity or acidity. The pH scale ranges from 0-14. 0 is the level of the most acidic substances on the table. 14 is the level of the most alkaline, and 7.0 is pH neutral. This is the value given to pure water.

If soil is your growing medium, the correct pH range is between 6.0 and 6.8. If you are using a hydroponics system, the best range is between 5.5 and 6.5. Your first duty is to monitor the pH of the water you use. If it is too alkaline, you encourage the growth of algae. If it is too acidic, you can ‘burn’ your crop.

Advanced growers invest in a Reverse Osmosis (RO) water system to handle that side of things. The amount of nutrients you add to your plants also alters the pH level. Nitrogen, for example, is very much on the acidic side. When you add it to pH neutral water, it should bring the pH down to the right level.

If you are using a hydroponics setup, plan out your nutrient program. Expert growers divide their feeding plans into several parts. They do this to allow for the natural changes to a plant’s diet as it grows. For instance, marijuana plants need far less nitrogen in the flowering stage than they do when in the vegetative stage.

It isn’t easy to identify lockout, primarily because it shares many of the same symptoms as nutrient deficiency. For example, yellowing or curling of the leaves is an issue associated with both. It is usual for your plants to look ‘underfed.’ This may cause novices to provide even more nutrients and exacerbate the problem.

Generally speaking, plants with nutrient lockout look weak, and growth has stagnated. If you fail to take action, the yellow color turns to brown. The leaves will also start curling up – they may even look burned!


What I would try is lowering your PH going in to maybe 5.8 or 6.0 and see where that puts you at on your runoff. I have done that in the past and it seemed to work. I have tried it with my situation this time ( ph in 6.9 ) and the runoff is still low 5.7 to 6.0. I am pretty new to this… dont want to give you bad info.

PH water going in is self explanatory. You must feed the plant in the correct PH range to prevent issues down the road. If you were getting a 7.6PH runoff, it is too high and you would need to lower the PH in the soil. Others may chime in with more knowledge than I on soil amendments, but if left uncorrected would lead to serious issues. And yes, nute lockout is a real possibility. Ask me how I know, and it was not PH related. I haven’t had any issues with either soil or Coco as far as PH, but always make sure to feed in the correct range for the medium. I am adding another article on the subject, hope they help.

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Runoff pH: Explaining the Bro Science

The internet is filled with bro science: incompletely explained scientific principles filtered through the haze of quickly posting in forums from a mobile phone. While there may be nuggets of info in these anecdotes, you have to cobble all this together to get a cohesive picture of what’s happening. I’d like to talk about a very dear topic to me, runoff pH.

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Updated April 28th, 2019


  • What should your runoff be? Ideally, within .5pH of your watering pH.
  • Is runoff pH useful? Yes, but you are generally using it as a proxy for runoff PPMs, which directly test for fertilizer buildup. If runoff EC is higher than watering EC, then you have some buildup. This is only an issue if it causes large swings in pH.
  • What happens if runoff pH diverges more than .5 from watering pH? Keep track of it to see if it continues to move away from the input pH, and correct by using fewer nutrients if it gets more acidic (lower). If it’s going up, you might have root issues. If runoff PPMs are close to input, treat with hydrogen peroxide, or a root inoculant. Otherwise, fertilize more often.

We recommend the following to do all the testing we talk about in this article:

What is runoff pH?

It’s the pH of the liquid that comes out of your plant’s pot after you water them. This liquid has a pH that correlates, but does not quite exactly measure the average pH of the grow media (Soil, coco coir, peat, etc) in the pot.

How do you take that measurement?

Put a clean catch tray under the plant that is non reactive with the nutrient mixture, water the plant until some comes out the bottom. Once it stops dripping out, measure the pH of that liquid. Our preferred mid-grade pH pen is the SX610 Waterproof pH Pen Tester.

Runoff pH being taken on a habaneroWhat does it tell you?

Directly, the pH of the runoff water. In a healthy grow, this should correspond with the fertilizer concentration of the grow medium (runoff PPMs, or runoff EC).

What does it not tell you?

The reading does not tell you much about the pH where it matters most: The surface of the roots. Because it’s an average, and the typical scenario with most soil and coco grows is that nutrients migrate down to the bottom of the pot, you can safely bet that many parts of the pot have very different pH levels.

Would it be better to take a pH reading of the soil directly?

Sure would! This is trivial at the top of the soil, Pulling out a sample at the bottom of the pot is a bit more involved. As we know, they may not have consistent pH between them. In some cases, that could be a very different reading.

Runoff PPMs/EC

Most of the time, we test for runoff pH to gauge fertilizer building up in the soil, which causes it to get very acidic as well as exert osmotic pressure on the plant. Measuring the input and runoff EC is a much better way to tell. If the runoff has a higher EC than the input, you have fertilizer buildup. Check the runoff pH to see how bad it is and if you need to do anything about it.

How should one interpret runoff pH readings?

To start, let’s take the ideal case.

You have a grow media that you want at a specific pH for optimal growth, and you should be feeding the plants with a nutrient solution that is near that value. Typically, you’ll see 6.2 for soil, and 5.8 for hydro. In an ideal world, your first few feedings will be within 1 pH measure of that target, and after a few weeks will converge on that number. If this happens, good.

However, most people do not take regular readings when that plant is OK, because, well, it’s boring. It’s within .3 of the input pH while the plant thrives. People typically start checking on it when plants show necrotic spots, extreme yellowing, or the like.

Extreme pH readings

If you have extreme pH readings in your runoff, bad things are happening. Seeing a pH that is more than 1.0 from your input target is cause for alarm when you see any indication in the plant. This usually indicates over/under feeding, bacterial infection, large amounts of media buffers present, or some lesser-likely issues.

Coco Coir is the classic example of this. If you feed regularly, but don’t run enough solution through to dissolve and remove some build-up from past feedings, nutrients accumulate. Since these nutrients are mostly acidic mineral salts, they drop the pH of the plant. By the time you notice, the pH has hit an extreme enough level that you need to act fast.

Fixing this particular case consists of flushing the plant and giving light feeding for a little while. After the media environment stabilizes, you can increase the feed schedule. Additionally, you should run more nutrient solution through to prevent build up, or add a flush in the schedule.

Extreme pH fluctuation going alkaline is a sign that you are greatly underfeeding your plant, or could be a bacterial infection. This is far more rare, but just as deadly. The solution is to determine the cause, and attack that problem.

Readings that are off but not extreme

Let’s say you feed at 5.8 and the runoff pH is 5.2. In this case, the reading isn’t useful by itself. While it could indicate a pH problem, and certainly some of the roots are in a zone where they have lockout of some nutrients, the entire root ball is OK overall since more than some of the roots can pick up those micro nutrients.

So what can we do? Track the changes. In that previous example, if you saw consistent 5.2 runoff and the plant showed minimal or no signs of problems, then you could try to bring up the pH or… not. A gradient in pH in the soil can be beneficial for better uptake of macro nutrients. Phosphorus has better uptake in a slightly more acidic environment than nitrogen or potassium. However, if you see pH moving, then you know you have a condition that will eventually cause problems.


So how should you handle runoff pH?

  • Check pH and EC of the runoff regularly and track the changes.
  • If EC of the runoff and watering do not stay within 10% of each other, adjust fertilizing concentration to compensate.
  • If pH does not stabilize within .6 of the target pH, act to counter the effects before your plant suffers
  • Extreme reading (>1.0 from target) need to be handled quickly
  • Taking readings directly from the soil around the roots is always better. However, it’s difficult and invasive to the plant, which is why we check runoff pH.

I hope this helped you and if you have any comments, suggestions, or want to ask any questions, please comment below, or contact us. Thanks for visiting!

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great stuff @jetlag thank you. This should be a sticky. I love absorbing info

My pleasure. Understand i am pretty new as well, and like you, a voracious reader. I have also been taught by some of the great folks here that I call the “wizards”. These are a few that you can call on for excellent help. @Covertgrower @Nicky @dbrn32 @CoyoteCody, and many more. I have done almost every possible screwup you can do, and somehow my plants have survived. I love to read through the different posts and absorb the information if applicable to what I am doing. Enjoy and happy growing.


Hey there…
Great info!
I have a question and appreciate your time.
My run off PH is ~1 higher than input. I used @Docnraq 's advice for flushing(from an old thread also) and her in and out were very close afterwards…so then i fed her. She looked much better.
Today she was ready for waTer and her ppm were 265. So i fed again.
Now to my question…reading this thread, perhaps i should flush again(only been about 4 days) and not feed until her PH balances out? She is week 6 from veg and is flowering so i am concerned about the PH but also her growth and not giving nutes…
I am also concerned about flushing again so soon because she had been over watered in her youth…
Thank you both again for your input…
(Oh…in= 6.8 PH and out= 6.8 after last flush…took 20G. Now in 6.8 and out 5.9. This is back to pre- flush numbers)

Did you use ph stable water?

Hey doc. I am using RO . Is that whaT you are referring to?

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Using ro for flush too? If you did, and didnt add calmag or epsom to bring the flush water to at least 350ppm tds this is why it bounced back so fast.

I used tap for theflush… then back to RO (yes cal/mag) for feeding and watering…

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For the ppm to be 265 what was poured in had to be too low to be ph stable.