(1) All contracts for the construction of such improvements outside the corporate limits of any such city shall be let to the lowest responsible bidder who, except as provided in subsection (2) of this section, shall enter into a good and sufficient bond for the faithful performance of such contract in such amount and with such sureties as the county board may determine. All payments of such contracts shall be made by warrants drawn on the road fund of the county.

(2) If a contract, the provisions of which are limited to the purchase of supplies or materials, is entered into pursuant to this section and if the amount of the contract is fifty thousand dollars or less, an irrevocable letter of credit, a certified check upon a solvent bank, or a performance bond in a guaranty company qualified to do business in Nebraska, as prescribed by and in an amount determined by the county board of supervisors or commissioners, conditioned for the faithful performance of the contract and executed by the bidder to the county and to all parties interested in the amount of the bid, shall accompany the bid.