(1) The rate of the tax shall be set by the county legislative body, but shall not exceed fifteen cents (15¢) per ton on sand, gravel, sandstone, chert or limestone severed from the ground in the county.
(2) Every interested owner shall become liable at the time the sand, gravel, sandstone, chert or limestone is severed from the earth and ready for sale.
(3) The tax shall be payable at the time of sale and delivery.
(4) The department of revenue shall use the accounting principle known as “first in-first out” in determining the tax payable on stockpiles or inventories of sand, gravel, sandstone, chert or limestone existing on the effective date of the tax in the county.