1900, column 16, 17 and 18 Titled "Citizenship". Column 16 = "Year of immigration to the United States". Column 17 = Number of years in the United States. Column 18 = Naturalization. Previous columns as where you were born, your age, and where your parents were born.
1910 US Census. Columns 15 and 16 titled, "Citizenship". Column 15 = "Year of immigration to the United States". Column 16 = "Whether naturalized or alien."
1920 US Census Columns 13, 14 an 15 are titled, "Citizenship". Column 13 = "Year of immigration to the United States". Column 14 = "Naturalized or alien." Column 15 = If naturalized, year of naturalization".
1930 US Census. Columns 22, 23, 24 titled, "Citizenship, etc.". Column 22 = Year of immigration to the United States". Column 23 = "Naturalization". Column 24 = Whether able to speak English".
1940 US Census. Column 16, titled "Citizenship". Column 16 = "Citizenship of the foreign born". They had to clear space for more questions about employment and income due to the depression.