The board of county commissioners or the board of county supervisors of the counties of the State of Nebraska is hereby authorized and empowered to permit farmers and landowners in their respective counties to use county machinery and equipment in constructing and maintaining terraces and ditches in said respective counties; Provided, that said farmers or landowners shall first enter into a written agreement with said county commissioners or county supervisors whereby said farmers or landowners agree to pay a reasonable sum of money as rental for said equipment and machinery, which sum shall be fixed by the county commissioners or county supervisors, and to comply with any and all of the conditions and requirements in regard to said rental as made by said county commissioners or county supervisors; Provided, one of the conditions and requirements in regard to said rental shall always be that the county shall furnish an operator for said equipment and machinery, who shall operate the same, and the compensation of said operator shall be considered in determining the amount of rental in each case.