I have a bucket that I collect my runoff in, 250-500 ml from each watering of a 3G pot. When It is full after a week or so, I use my blue labs combo meter and measure that ph. Good to see any overall trends or changes averaged out over a whole week or two, and over all the plants, versus just one pot in one moment. Combo meter is 265 and you measure tds/ph in liquids. If you want to measure soil, you just soak it in water solution for a couple of hours and measure the sludge. either way blue labs is good !