My centipede grass is full of weeds and the ph is too high. Can I correct this with ammonium sulfate?

I don't have access to a lot of chemicals for the lawn in my neck of the woods. I have found ammonium sulfate in a 4lb bag for $5 and read on the internet that if I apply 5 lbs per 1000 sq. feet this should lower the ph. My ph is 7.0. And centipede needs like 5.5. So I'm in trouble and this probably explains a thing or two. My nitrogen is also low, which means that the ammonium sulfate is the right answer correct? Cause ammonium sulfate is like 21-0-0.

As a second part, I put sod out in the back part of my lawn last September. It's not doing good either even though we received loads of rain this fall and winter. The ph in that soil is fine, though, cause I added a bunch of seed fertilizer and whatnot when I put the sod down. Weeds are trying to take over the sod, however. How can I prevent this without killing the sod until it becomes established? Thanks!