to check pH of soil, you can dig up some from as close to center of plant root mass as you're willing to go and mix it up with some ro or distilled water, then check the pH of that solution. No need to dig up 20gal pot.. LOLOLOLOLOLOLOLOLOL!!!!!!!!!!!!!!
I would stop focusing on pH and just be careful not to overfeed fertilizers, as the excess fertilizers is what drives pH fluctuations and imbalances and in a healthy soil, with added dolomite lime or not, with acidic soil or not, the soil and microbes will buffer the pH for proper plant uptake and eventually with good watering practices and not over fertilizing the soil will take care of itself. You should read my first few posts to you in here. I mention pH information, but your problem is not pH, I did not mean for you to focus on pH only now, this is not the cause, this is the effect, the cause is fertilizer applications.