The board of supervisors of the county in which a road district organized under this chapter is situated shall have plenary power of supervision and control over all of the official acts of the road commissioners and the road superintendent, and shall refuse to ratify or confirm any contract or report of any action of such commissioners or superintendent, when the same is reported to them, which is not for the best interest of such district. They shall require the letting of contracts and the purchase of teams, materials, lumber, supplies, and all other property for the use of the district to be made after advertisement and competitive bidding when such method is deemed likely to best subserve the interest of the district and be most economical.