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>>>> true soil likes it around 6.5; inputs ideally should be around this as well.... There are a lot of pH influencing factors happening in a pot, so dialing in your feeds and, if applicable, your water's pH helps take the burden off the soil's buffering capacity alone,... proper supplements in the soil, like lime help, so can the soil herd if it's healthy and robust enough....
Some broad strokes here: really hard water can become a problem sooner or later because of the large amount of cumulative minerals (CaCO3 mainly) building up in there, which will drive pH up.. This mineral is what gives water some buffering capacity--- well, the carbonate (bicarbonate) part actually, free Ca++ isn't directly involved.... Also, too much Ca will start to screw with the uptake of certain other nutes.... More extreme, if you're trying to get the pH down after that build-up, CaCO3 can stay in it's compounded form, not free ions,....this is what sucks the acidity adjustment up by reacting to the H+ and dissolving, freeing up that carbonate to combine with a free H+ and form bicarbonate... If you're fighting a high pH caused defc. for example, it'll be a bitch to fix because CaCO3 is not especially soluble in water, flushing may not do the trick like it does under overly acidic conditions,...
At the opposite end, using straight RO/Di/very low ppm water, stripped of this mineral, offers no help at all so pH can potentially slip down more readily... Much depends on the specific amendments added in to deal with this though! ..... Your Ca-Mg inputs (if they have carbonate in them) can help boost this too...
My gospel: by monitoring your actual in-pot pH with a quality pH probe, and watching your inputs pH (and ppm's), you can help prevent any pH snafu's and spot them faster, treat and fix sooner and better... I have hard water to deal with in my grows inside and out... tap can be 170-200+ppm, so I lix in some RO water to get the ppm's down to low 100's, enough to offer buffering but not cause excessive build-up over time,... watering enough to get some run-off helps keep the ion/ppm load in there lower too, but it's not always necessary and constantly soaking wet pots are not wise either!