I used to sell a fair number of them with few returns, but haven’t used one, myself. I can’t see why it wouldn’t work well for potted things. Being the geek I am, I’d have a ‘control’ pot that had the PH I was looking for to compare against.
I’ve always relied upon the old, ‘1/3 the dirt you’re stuck with, 1/3 compost and 1/3 peat’ when I’m refreshing a bed. Then I fertilize based upon what plants needs what and I don’t really worry about PH factors. So far, so good!
For pots, I use fresh soil every year, dumping the old stuff (sans root balls) into a garden bed or the compost pile. Maybe not so easy to do on your rooftop garden? ;)
I’ve either made my own (how I truly MISS having discounted broken bags of whatever soil/amendments I needed when I worked for Jung’s!) but I’ve had good results with good old Miracle Grow Potting Mix, too.