Hi there Sang, hey there, the "in-soil sensors" are really really inaccurate. I use the same ones as you do so i have seen this problem before. But actually "biology" absolutely does change your soil's pH. In fact many microbes help to stabilise your pH in the medium long term. Actually if you're worried about pH, maybe you can try to add some beneficial bacteria like "hydroguard" or bacilus subtilis. If you want to stabilise your pH using a chemical way, try to add humic acid . This will stabilise your pH a 6.5 (and also help the plant grow quite a lot!) Hope this helps ! 🚀