# The Final Word on How to Measure Soil pH. Method 1 or Method 2?

Discussion in 'First Time Marijuana Growers' started by Abtima, Nov 26, 2011.

1. I've been reading about pH for awhile and this is really driving me nuts that there are so many mixed messages regarding how to get an accurate measurement of Soil pH. I'm not looking for a wall of text about the science of pH (I've read those threads), and I'm not looking for any explanations for how to raise or lower Soil pH (read those threads too), and I'm not looking for people's personal opinions on the perfect Soil pH (I respect them, but it's not the purpose of this thread). All I want to know once and for all is HOW to find that magical Soil pH number.

For this exercise we want a Soil pH of 6.5 ...Assuming a Soil pH Sweet Spot Range of 6.2-6.8...

Method One-
1. Measure the pH of Input (water or water/nutrient mix) you are planning to use.
2. Water Plants
3. Measure the pH of the Runoff.
4. Soil pH = pH Input + pH Runoff divided by 2

Ex. pH Input (water or nutrient mix) (measured 7) + pH Runoff (measured 6.0)/2 = 6.5 Soil pH; basically saying that Soil pH is the mean or 'mid point' between the Input and Runoff pHs.

Method Two
1. Measure the pH of Input (water or water/nutrient mix) you are planning to use.
2. Water Plants
3. Measure the pH of the Runoff.
4. Runoff pH=Soil pH. This assumes that you would adjust the pH of your initial Input (water or water/nutrient mix) up or down so that the resulting Runoff/Soil pH is within the 6.2-6.8 Sweet Spot pH range, or 6.5 in this exercise.

Ex: Input (water or water/nutrients) has a pH of 7, the Runoff measures 5.8. If I wanted the Runoff to pH at 6.5, this would then mean that the Input's pH would need to be pH 7.7

Once and for all, which of these 2 methods is correct. Strikes me as a pretty important thing to know given that the Soil pH will come out quite different depending on which method you are using. Which is why I'm bewildered that people's technique for measuring Soil pH can vary at all. Essentially, it comes down to this:

Method One
Soil pH = the mean (or mid point) between the Input pH and the Runoff pH

OR

Method Two
Soil pH = Runoff pH
...and assumes that by raising or lowering Input pH you will raise or lower the Soil/Runoff pH proportionately.

2. The runoff pH is what you need to worry about, not how to use that calculation to determine soil pH. The runoff pH represents what your plants' roots are experiencing for nutrient uptake (or lack thereof), so adjust your input pH in order to get your runoff pH in line and don't worry about soil pH as some separate measurement.

3. #3
Last edited by a moderator: Nov 27, 2011
Thanks toastybiz! Was hoping I'd catch a clarification from you Much appreciated.

4. Bump... to see if anyone else wants to chime in.

5. toasty hit the nail on the head with this one. you want to measure runoff, due to thats the pH that your roots are eating up

6. #6
Last edited by a moderator: Nov 27, 2011
Check this out though--- for the sake of clarification. I found this from a pretty reliable poster/grower yesterday.

"I see a lot of people concerned about the pH of their soils, and concerned about the pH of their tap water, and this is a good thing, however there is a whole lot of misinformation and misconceptions floating around... The ideal pH of soil should be between 6.5 and 7. Plants can tolerate a fairly wide range before nutrient lock out will occur, so don't panic if you do have a pH fluctuation. As pH will remain stable in soil, if you use only your nutrient water or tap water and don't start experimenting with a lot of additives you may hear about.

"There are only 2 pH values that you need to concern yourself with. The pH of your SOIL is the first, and most important. Too many people are concerned mainly with the pH of the water/nutrient solution going in. As the watering solution sits in contact with the soil, the soil will change (or buffer) the pH of that solution to that of the surrounding soil. Run-off water is not really that good an indicator of the soil pH because it has “just run through” the soil, and did not have a chance to be affected by the soil.

"The soil has it's own pH value and that is the important one you need to worry about. The pH of the soil is much more stable than that of any water that runs through it. To determine the pH of your soil, You must take a sample from as close to the root zone as possible, by gently digging down along one side and obtaining 2 or 3 tablespoons of soil. Then, put the sample into a container large enough to insert your pH meter. Add enough DISTILLED water to make a loose mud or a slurry, mix it up well, and then let it set for a half hour or so. Then test the mix with your meter. This is the pH of the soil."

-quote from author J.H. posting on another forum.

So this is the problem I'm having. I get a different "soil pH" depending on whether I use the "Runoff Method" or the "Soil-based Method," which is a big issue. These are the actual results using distilled pH 7.0 water Input for testing purposes:

With the "Soil-based Method" the 'soil pH' measures at 6.8.
-This means no adjustment to my Input water is needed so long as I pH my Input close to 6.8 before adding to plants to prevent pH shock.

With the "Runoff Method" the 'soil pH' measures at 5.8.
-According to this method, if I used 7.0 pH Input water for this test I should adjust subsequent Inputs' pH to 8.0 to achieve a 'soil pH' of 6.8.

Now, one of these can't be accurate. Because they result in entirely different future watering/feeding scenarios.

"Soil-based Method" = My soil pH is fine, a tad on the high end but looking good. All I need to do, more or less, is pH my future water/nutrients to match the soil pH of 6.8 before giving to the plants.

"Runoff Method" = My soil pH is low at 5.8 and I need to adjust my future water/nutrient inputs to a pH of 8 to counter the low soil pH.

I appreciate and respect everyone's input but both of these methods can't be accurate since they result in very different outcomes. Specifically, in one case I'm fine and I'll need to feed plants with a water or water mix of pH 6.8 +/- .1. In the latter scenario, my soil pH is low and I'd be feeding the plants next time with an Input pH of around 8.0 to counter this.

Again, many thanks for your help. I'm really curious about this and not trying to be argumentative, but looking over these scenarios I'm still not confident that I'm doing this correctly.

7. You are missing something very very important with one of your methods and once i point it out your gonna be like wtf why didn't i think of that...

Your pot/containers have layers with in the soil , each layer will have different ppm and ph as you get to the bottom your PPM will get higer as salt build up from ferts as water will flush the heavey particals down to the bottom slowly compacting the soil with gravity , there fore to take a acturete reading the water must past each layer of soil to get a more accurate reading of the over all soil condition and ph ..(as most of your roots are at the bottom of all the run off in your pot/container )

I understand where your comming from but your missing the whole method and aim to get a accurate reading. by going on your method you could get any reading you wanted to get really depending on when u took the sample and how long ago you water / feed as the water you just feed the plant is ofcorse goona read the same as if u dig 1/4 way down and take some soil.