If the earth's surface is negatively charged, why aren't objects close to the surface polarized?
Think of the electric current as a fluid, and a battery or some other source of current as a pump. Change the earth for the ocean.
I don't know about the negatively charged earth's surface, but those -500,000 coulombs are an absolute measure of charge. That charge is distributed over some surface, and the earth's surface is pretty big.