The hardy cannabis plant can adapt to a lot of soil types and many successful crops have been harvested without ever paying attention to pH, but it never hurts to check. When growing plants in soil, consider the pH of the water going in and the pH of the soil itself; both of these readings can be taken with a common glass electrode meter; there’s no need for a special soil meter.
Every meter is different, so follow the instructions exactly on how to calibrate and prepare the meter. You should be given two reference solutions—normally at pHs 4 and 7—and a storage solution. It’s very important to never let your electrode dry out. If it does dry out even once, the electrode will be damaged.
A damaged electrode is still marginally functional, but is frustrating to use and inaccurate. Make sure your pH meter has a good storage system; some easily spill and don’t make a seal. You’ll need to rinse the fragile glass electrode between solutions to avoid contamination. Use the cleanest water available, preferably distilled or deionized. A little squirt bottle helps rinsing so you don’t waste the pure water.
In order to measure the pH of your soil, put about 20 mL of dry soil in a plastic cup with 40 mL of the same pure water you’ll use to rinse the pH probe, give it a swirl, then take a measurement after a half an hour. Make sure that everything has settled before dipping the meter in so it doesn’t get dirty.
Even if you don’t have the best pH meter, handling it right and doing things consistently will give you the accuracy you need. Probes that you can stick right into the ground make checking soil more convenient, but aren’t completely necessary.
There’s a lot of debate online about whether or not to check the pH of soil runoff. pH of the runoff tends to be lower than water going in, but it shouldn’t drop too much. Runoff water can give you a gauge of the soil pH, but for a truer reading, check the soil itself.
If you do choose to measure soil runoff, make sure the probe doesn’t get damaged. Runoff is a soup of different components that can coat the glass bulb and throw off future readings. You can use an electrode cleaning solution, but it’s a good idea to avoid murky waters.
A note about distilled or deionized water: don’t worry if its pH is a little low. Carbon dioxide dissolves in water and drops pH by forming carbonic acid. The deionized water is so pure that a small amount of CO2 pushes the pH down to five. Once you mix it with anything that little bit doesn’t make a difference.
