If said district is established, the entire costs and expenses incurred under this part shall be assessed against and collected from the lands lying within such district, by the levy of a rate upon the assessable value of the land and improvements within such district, sufficient to raise the required sum; provided the board may, in their discretion, classify the land within such district and graduate the tax thereon, as provided in subchapter I . [C97, §1982; S13, §1982; C24, 27, 31, 35, 39, § 7747; C46, 50, 54, 58, 62, 66, 71, 73, 75, 77, 79, 81, §466.4 ] 89 Acts, ch 126, §2 CS89, §468.393 Referred to in §468.395 , 468.396