I believe the test was not good.
I put some soil in de mineralized water, mixed then checked yes, it's litmus paper.. but then I checked this water only and it's not around 7.
Tap water is 8 and I guess it's right since it's a lot of limescale and limescale comes from calcium (alkaline)
I guess I need a more accurate test!
Edited to add: demineralized water is not neutral, just did a quick research and it's a common mistake. It's a bit acidic, around 5.8 so my test is really not useful.
