The board of supervisors may appropriate moneys for the purpose of purchasing weed eradicating equipment and materials to carry out the duties of the commissioner for use on all lands in the county, public or private, and for the payment of the necessary expenses and compensation of the commissioner, and the commissioner’s deputies, if any. When equipment or materials so purchased are used on private property within the corporate limits of cities by the commissioner, the cost of materials used and an amount to be fixed by the board of supervisors for the use of the equipment shall be returned by the county treasurer upon the collection of the special assessment taxed against the property. In the certification to the county treasurer by the county auditor this apportionment shall be designated along with the special tax assessed under section 317.21. The equipment and its use are subject to the authorization and direction of the county board of supervisors.