Annual tax levy

After the annual estimate for the county has been filed with the board of county commissioners by the county engineer, and the board has made such changes and modifications in the estimate as it deems proper, the board shall then make its levy, for the purposes set forth in the estimate, upon all taxable property of the county, not exceeding in the aggregate two mills upon each dollar of the taxable property of said county. Such levy shall be in addition to all other levies authorized for said purposes, but subject to the limitation upon the combined maximum rate for all taxes. This section does not prevent the board from using any surplus in the general funds of the county for the purposes set forth in said estimate.