How can I measure the pH of soil/soilless runoff?

  Added by: snoofer    Viewed: 764 times  Rated by 9 users: 9.33/10
Contributed by: bald1
Submitted: January 9th, 2005

How to test the pH of your soil mix

Measuring the pH of soil is just as important as with hydro applications, but few people know how to test soil pH to see if it is within the optimum range for growing robust healthy plants. Here I will try to explain my method of testing any soil / soilless mix, enabling me to spot any problems and correct them if necessary.

Firstly, wait till your soil has dried out and is due for its next watering schedule. Then take some plain water that you usually water your garden with, and adjust the pH to 7.0. You must make sure that you know the exact pH of the water going into your soil, and the neutral 7.0 is best, but anywhere from 6.5 – 7.0 will suffice.

Then place your pot into a bowl of some sort to catch the runoff water, and then start to water your soil slowly (with your pH- corrected plain water) till the water starts to drip from the bottom.
It’s the first drops of water that will give you the best reading of your soil, so make sure to water slowly till you see the first droplets. Then remove the pot from the bowl to eliminate excess water entering the bowl. Then perform the pH test on the runoff and compare it too your initial test.
The results of the runoff test will likely be lower than your starting value of 7.0. If this is the case, a small drop of 0.5 pH to 6.5 pH (example) would be ok and your soil needs no further alterations at the moment. But that’s not to say that it won’t need any future tests at all, just not at this time.

[Editor’s note: It may be beneficial to obtain an initial sample, as well as a ‘full flush’ sample in seperate bowls. In addition, test several plants in the garden just to verify your results]

What if the pH is off?
If your results prove to have dropped considerably, say to around 5.5 (which can happen in late stages of flowering), you will need to add some lime into your soil to help buffer the pH back up again.

Remove the first inch or so of soil, taking care not to damage any roots whilst performing this task. Then sprinkle the lime into the pot, nice and evenly at a rate of 1 teaspoon (5 ml) of lime per gallon of soil. Then replace the soil you removed earlier, and saturate the soil good to wash in the lime.

Do the same test next time your plants need watering just to check that everything is fine, if more lime needs to be added then just repeat the process again till you reach close to 6.5 – 7.0 with the runoff.

Ensuring that your pH is correct should be done throughout the life cycle; this will help eliminate any nutrient lockout that may occur. I recommend doing this once a month just to keep the PH in check, and you should never have a problem with deficiencies caused by pH lockout.
  Last modified: 01:24 - Jan 16, 2005 

faq:1696 "How can I measure the pH of soil/soilless runoff?"