Topsoil- Unless you are getting the organic kind, this stuff is full of feces, sewage, pesticides, chemicals, you name it. I do a lot of gardening and after talking to my kids pediatrician, he made it very clear to not let young children play with soil ( particularly topsoil, supersoil etc...) without wearing gloves. way too many pathogens. There are certain agricultural byproducts that are alkaline and some that are acidic. Definitely could have affected your pH.
I think you will be good to go without the soil.
That actually gives me a sigh of relief because I think you have your solution!
Good luck!
-Julie