Low PH in runoff

Couple of times I’ve noticed my runoff is 5.0

it goes in at 6.5 so I’m wondering why?

What can I do about it?

Is this normal?

This is only my 2nd or third soil grow ever
-thanks for the input

Your plant using what is in the mix changes the phone most of us check and correct multiple times a day

@Majiktoker what can you add buddy

Why do you want to know your runoff pH? Simple- it’s an indication of how your water, fertilizer, and grow medium are interacting chemically.

There are 3 measurements that are important to know when you are caring for plants growing in a soil or soilless medium.
The first is the pH of your source water.
Next is the pH of your nutrient solution after the fertilizers and supplements are added.
Last is the pH of your soil. But that’s the hard one! You can’t just stick a pH meter in the dirt and get an accurate reading, and the cheap metal-probed meters that show this are usually not all that precise.

So how does one determine soil pH accurately? With a runoff test! First, measure the pH of your tap water and record it. As an example, let’s say that it is 7.0, exactly neutral.

Now place the pot over a clean rinsed container and pour enough water through the soil to start dripping out the bottom. Collect about 4 ounces of runoff water. Check to see if it is discolored as well.

Now, if you only have liquid indicator, which is just fine, pour this water into a clean small tube or the test vial that came with your pH testing kit.Add a few drops of indicator solution, shake, and read the color change.
If you have a meter, simply stick the electrode in the water and read.

Let’s say that your runoff comes out at 6.5. How did that happen? The water passed through a more acidic matrix and dropped its pH. You can assume that your true soil pH is a couple tenths of a pH point lower than the runoff in this case- I’d assume about a 6.2. If it comes out HIGHER, just go in the opposite direction. If it came out 7.5, you can assume that you need to drop down from about 7.8.

You want around 5.3-5.8 for a soilless mix, or 6.3-6.8 for soil.

1- Prop up the plant over a container to catch runoff
2- POur clean water through the soil
3- For liquid indicator, pour into clean vials
4- Check the pH against the color chart
5- Adjust the flush water if necessary


Yes and no.

It is normal for the pH coming out the bottom to be a little lower than what is going in. If it was going in at 6.5 and coming out at 6.0 – I 'd say you have nothing to worry about.

In most soils with a lot of “organic” and composted material, and even peat moss, this will slowly break down naturally, especially when very wet and it has an active root system and lots of active organisms in the soil that contribute to acidity. Some acidification can be from ion exchange at the roots as the plant takes up certain nutrients, as well as the natural decomposition by microorganisms.

When you say it goes in at 6.5, what exactly is going in? Just nearly zero EC/ppm water, like distilled, de-ionized and R/o, that has been adjusted to 6.5? And if so what is used to adjust it.

If just watering between feedings, many people need to add a little cal-mag to the “water only” watering to help counter the acidification and keep everything healthy.

Also sprinkling garden lime at the surface can do about the same thing adding a cal-mag supplement would.

Happy growing,



PPM is exactly 600 + 95 from the tap
(695 total), my tapwater is 7.1 out the tap

I’m taking clones and starting the switch tomorrow on some of the plants in question

I’ll probably flush, but I would still like to understand what’s up, that seems a bit low to me

Everything looks good growing normal but I just don’t like it that low

@MacGyverStoner @Majiktoker

Tap at less than 100PPM and pH at 7.1, you probably don’t need to be downing the pH at watering if the pH coming out is so low.



I agree @MacGyverStoner correct me if I’m wrong, but I think some where through drainage there is still an acidic patch in the soil causing it to go in fine and come out acidic, maybe try to touch and reset every thing @Paranorman

Gotcha, thanks Mac

I’m going to do a flush and I’ll check the runoff after, I’ve only noticed it in two plants so far -thanks

Any time keep us posted

Okay so I have a pH question… Just transplanted into 20 gallon pots 12 days ago… I have six plants PPM ranges between 1243 and 1943… Using a mixture of fox farm ocean Forest Fox farm Happy frog an ld organic compost. My hose water is 6.4 pH… But my runoff is between 5.2 and seems like an awfully huge drop in the pH is there anything besides ph up that I can add to my water to make it less acidic???

This is a super old post. The person you responded to hasn’t posted anything since 2018. They may be inactive.

Let me tag a few people and see what they say. @Hellraiser @Myfriendis410 @MrPeat

I also have runoff PH issues. I tried flushing and could only get my runoff up to 6.0

Top dress your soil (if plants are already in it) or till in (if not) dolomite lime per the package instructions. While you’re at it get some gypsum and add that too. (Calcium)


Can’t I just increase the pH of my water…???

What do you mean by topdress my soil…??? What is that?

Applying an amendment (lime) to the top soil layer and working it into the top inch or so. Watering in will allow it to work with the rest of your media.

Ok…they are growing so fast…!! 1 Super Skunk was 47" 4 days ago and now 58 "…but I want the right pH for nutes…

Applied amendment yesterday…I will see what happens…will update runoff numbers next water…:grin:

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@Weedlvr0321 How did the Ph turn out? Did you get it to rise? Did you have to Flush?

I’m just wondering because I have the same issue and I don’t know what to do, I already added lime.