Lyrics

The first line, "Wave the stars and stripes high o'er us" evidently celebrates the union, that the north and south unite as one, to become the United States of America.
The second line, "let every freeman sing" seems to refer to the slaves who have been freed,
The third and fourth lines, "Old King Cotton's dead and buried; Brave young Corn is king" refers to the dethroning of the cotton king in the south, because during the war the south was depending on other countries to support them by buying their cotton; however the north threatened to cut off their food supply if they did so, and since countries such as Britain already had a surplus of cotton, the north was sending them cotton that was won from the south, and Egypt and India were increasing their cotton output, "brave young corn" was more important than cotton. Therefore, the crop of corn replaces the crop of cotton.