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!

5 years ago