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.
89 Acts, ch 126, §2
CS89, §468.393