Iowa Code § 468.37: Contracts.

All agreements and contracts for work or materials in constructing the improvements of a district shall be in writing and be signed by the chairperson of the board of supervisors for and on behalf of the district and the parties who are to perform the work or furnish the materials specified in the contract. The contract shall specify the particular work to be done or materials to be furnished, the time when it shall begin and when it shall be completed, the amount to be paid and the times of payment, and contain such other terms and conditions as to details as are necessary to a clear understanding of the terms of the contract.