The mineral documentary tax levied above shall be in lieu of all ad valorem taxes and all nonproducing leasehold interests upon all oil, gas and other minerals in, on or under lands lying within the State of Alabama, created or assigned after October 12, 1957, and also all nonproducing interests in such oil, gas and other minerals, including royalty interests therein, hereafter conveyed to a grantee or purchaser or excepted or reserved to a grantor separately and apart from the surface shall be exempt from all ad valorem taxes levied on or after October 1, 1957, by the State of Alabama, or any county, municipality, school district, or other taxing district within the state or becoming a lien on or after said date. Any sale for taxes of the surface or of the remainder of the fee shall not in any manner whatsoever affect the interest or interests hereby exempted.

For the same purpose and with like effect there is hereby likewise exempted from such ad valorem taxation all such interests created prior to October 12, 1957, which are owned separately and apart from the surface; provided, that as a condition precedent to obtaining such exemption upon existing interests the then owner or owners thereof shall make application for exemption of the interest then owned by him or them as hereinafter provided and pay a sum equivalent to the tax herein levied by this article on instruments hereafter executed creating, transferring or reserving corresponding or similar interests. As to any existing interests, if any such sum is paid after October 1, 1957, then such exemption shall apply only to taxes becoming a lien after such sum is thus paid. The number of years remaining before the expiration of the primary term of such previously created mineral interest shall be considered as the primary term of such interest for the purpose of determining the amount of such tax.