I'm trying to prepare a soil mix and I need a fairly reliable way to test my pH. I have this soil pH meter that I got from Home Depot but, it's junk. I can't trust it. I get totally different readings from the same sample when tested just a few minutes later. It's frustrating.I can test the water that I add to the soil and I can test the run off. How can I be fairly sure of what my soil pH is?