I was doing slurry tests for a while, but now my medium is 99% root ball. Last time I tried I ended up taking a small root cluster with the soil, so for the sake of not damaging anything Ive just nixed the slurry tests. Plus Im not all that great at it. I never know if Im doing it right, some places say to mix equal parts water and soil mix well and wait for it to settle, some say you can just wait a few minutes then test, I just dont know what the correct method is, but I do have a cheapy probe tester, its pretty accurate and I only paid 35 bucks for it. Ive only done the initial calibration after getting it, and its been working well.