This is a common question that comes up when troubleshooting, so I thought I'd post up a little summin summin.
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 6.0-6.5 for a soilless mix, or 6.3-6.8 for soil.
1- Propping up the plant over a container to catch runoff
2- POuring 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