Lime or lemon will do it. You could also try vinegar.
- 2 tbs white vinegar to 1 gal h2o
or
- ¼ tbs lemon/lime juice to 1 gal h2o
Use to flush. Use the whole bottle. Do it only once, wait 24 hours - recheck soil pH. If soil pH is still not below 7, repeat, if below 7 don't.