Which PH reading should I believe?

Discussion in 'First Time Marijuana Growers' started by reefoid, May 10, 2011.

  1. Hi

    I got my PH pen meter through yesterday so tried to test my soil PH. I used two methods but they varied wildly so which should I take as the more accurate:

    1. Reading 1 was taken by taking some soil from 2 or 3 inches down and mixing it in with some ph7 buffer solution. The reading was ph6.9, which would suggest to me my true ph is below 7.

    2. Reading2 was taken using the runoff method. I used water with ph6.2, took the first bit of the runoff and the reading was 7.1, which would suggest my ph is in the high 7's.

    Given there is such disparity between the readings, which one would be more accurate? I'm hoping method 1 is the true reading, otherwise I've got a serious ph problem:(

  2. id go with the testing the runoff, its always the recommended method on here, if you have a PH of 7.1 thats not to bad just needs to come down a little closer to 6.5-6.7
  3. Thanks for the response, I thought that would be the case.

    Just to clarify though, if my runoff is 7.1, and the water was 6.2, doesn't that suggest my true ph is in the high 7's?
  4. dont think so, just test the runoff & thats ur PH
  5. WTF?

    Test the water going in. Thats the pH you want to adjust.

    The water coming out will tell you if you have salt build up from ferts.

    Ex. If you water with 5.8 and the runoff comes out at 4.2, you need to flush. (you should have already)
  6. Aah, that's cool, I can relax now. Will try to bring it down a tad with the next feed.

    Thanks guys.
  7. Yes, I know I need to adjust the water going in, my question was about which ph reading I should take as the most accurate which mills69 has already advised me on.
  8. yea like if your adjusting your water to say PH 6.5 going in, & the runoff is comming out at say PH 7.5 then thats your soil PH or close to(and would need to come dwn), thats my understanding anyway, if that it is wrong someone will correct me lol:smoke:
  9. 2 things.

    One, if you add dolomite lime lime to your soil, you pretty much don't need to worry about pH, or the pH of the water going in. 2tbl/gallon of mix, or 1 cup/CuFt of mix, powdered dolomite lime.

    Second, the way *I* test run off is this. Water well to saturation and decent run off, dump the run off. Let everything sit for 1/2 hr or so. Then, add a bit more water to get a little run off and pH test that run off. It's been in contact with the soil for the 1/2 hr or whatever and gives you a more accurate reading.

    Think of it as the lazy mans way of adding the soil to the tube deal.

    I only check the run off if there is a problem, but with a well limed soil and healthy micro herd, it has been months since I've checked.

  10. Your runoff is the pH you want to measure to get in range. If it is coming out 7.1 then that is the pH to be concerned with. That doesn't mean that your "true" pH, whatever that is, is in the high 7s, it means that the pH of interest is 7.1.

    The pH of interest is what your plant's roots are experiencing, which is the net pH from the interaction of your water, nutes, soil mix, amendments, etc. That is exactly what the runoff reflects.

    If that runoff pH is out of range then you need to get it in range. If runoff pH is too high then lower the pH of your input water to get a runoff result in range. If runoff pH is too low then raise the pH of your input water. Importantly you do this regardless of where the pH of your input water starts at. In other words, if you are inputting water of 7.5 and your runoff is too low, then you need to raise your input water even higher.

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