(a) When a subdistrict shall have been established by order of the county commission, the owners of land within the subdistrict shall, through their officers or directors and in keeping with the provisions of the constitution and bylaws of the subdistrict, fairly, justly and equitably, insofar as possible, prorate the costs of the improvements to be made and other necessary expenses and enter same as an assessment or assessments against the benefited land of each owner of land within the subdistrict. When such assessments shall have been approved by the county commission, they shall be filed with the chairman, and each assessment shall constitute a prior lien, subject only to state and county taxes, against the benefited land so assessed. Said assessments shall be due and payable as the work progresses.

(b) If any owner of land within such subdistrict shall fail to pay said assessment or assessments when due, the chairman may issue a certificate or certificates of indebtedness in the aggregate amount of said assessment or assessments against such benefited land, which certificate or certificates shall constitute a prior lien, subject only to state and county taxes, against such land. Said certificates may be prorated over a number of years not to exceed 10 years, may bear interest at a rate not to exceed five percent per annum and may be sold to any owner or owners of land in said subdistrict or to the investing public. Failure to pay any one of said certificates when due shall immediately render all outstanding certificates of indebtedness due and payable, and the county commission may order said benefited land sold to satisfy said prior lien. Ten days notice of the time and place of said sale shall be given by posting notices thereof at three public places, one of which shall be at the courthouse door, or by publication thereof once a week for two successive weeks in some newspaper published in said county. The proceeds of such sale shall be applied to the payment of costs and expenses of said sale, to the satisfaction of any prior liens provided for in this section, and any balance shall be paid to the owner of said land. In no case shall the total assessments against any tract of land within said district be more than 90 percent of the estimated benefits to accrue to said land by reason of the improvements as shown by the engineer's report, nor shall any tract of land be held liable for any assessment against any other tract of land within said subdistrict.