The board of county commissioners shall, before awarding any contract for any such improvement, publish notice of the letting in some newspaper printed in the county, or, if there be no such newspaper in the county, said board shall cause written or printed notices to be posted in at least five conspicuous places in the county for the same length of time, which notice shall specify with reasonable minuteness the character of the improvement contemplated, the time and place at which the contract will be awarded, and invite sealed proposals for the same. Such other notice may also be given as the board may deem necessary or proper.