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 [COLOR=#009900 !important]supplements[/COLOR] are added.
Last is the pH of your soil. But that's the hard one! You can't just stick a [COLOR=#009900 !important]pH meter[/COLOR] 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 6.0-6.5 for a soilless mix, or 6.3-6.8 for soil.