Difficulties with checking pH of my soil with líquid pH tester

I wouldn't worry about it, but move your plants to black/dark pots, roots need absolute darkness to develop and clear or thin white containers are a no-no. pH affects nutrient availability in the soil, but unless your soil is below 5.5 or over 7.5, 95% of nutrient availability is still possible. Anywhere between 6.2 and 7.2 is fine when growing in soil, most commercial soil mixes are perfectly adequate and unless you dump a ton of a single fertilizer on your plants, I would not even bother worrying about pH, I haven't for 25+ years! I spent the first 10 years of growing trying to get everything exact, including pH, and found it was not that critical when growing in soil that is well balanced and does not contain too much of any one single ingredient. For the last 25+ years I have refined my soil recipe to a point that I know it is well balanced and "just right" for growing cannabis, and if it is 6.6 or 7.1, (not that I check!), my plants never fail to flower exceptionally. If your plants are growing fine, then you need not get too concerned with pH when growing with soil. pH is far more critical when growing in coco or hydro, where nutrients are in a "suspension" and are designed to be "released" from the "suspension" at certain pH values. (a suspension being nutrients dissolved in water) pH can be likened to static electricity, whereby if it is extreme it "holds on" to dissolved nutrients and when in the middle range will "let go" of the nutrients so that the plants can absorb them. The middle range is wide and forgiving when growing with soil. It is far more critical to get your watering correct, evenly moist but not wet, and feeding only when needed. Feeding is better when done more often and with weaker strengths rather than once in a while with full strength, in my experience. That is to say, if your fertilizer says every 2 weeks, I would give half strength every 7 days instead. This may give you some ideas, but if you get stuck, feel free to send me a message. Hope this helps, Organoman.