On or before the applicable date required under section 67.110 of each year, the board shall certify to the county commission of each county within which the district is located a rate of levy so fixed by the board as provided by law, with directions that at the time and in the manner required by law for levy of taxes for county purposes such county commissions shall levy a tax at the rate so fixed and determined upon the assessed valuation of all the taxable tangible property within the district, in addition to such other taxes as may be levied by such county commissions.