((a)) The Secretary of Agriculture may enter into contracts to provide financial assistance in the form of payments to owners and operators of cropland located in counties where the soil normally freezes to a depth of at least four inches annually who remove such land from agricultural production for a period not to exceed one year for the purpose of installing enduring conservation measures which involve excavation of the soil. The payments under such contracts shall be in such amounts as determined by the Secretary to be necessary to effectuate the purposes of this subchapter but shall not exceed an amount equal to the number of acres of cropland removed from agricultural production for such purpose multiplied by 50 per centum of the typical annual rent, as determined by the Secretary, paid for similar land in the county. Financial assistance may not be provided under this section with respect to any conservation measure without the approval of the soil and water conservation district board for the district in which the land is located, and may not, in the aggregate, be provided in any year with respect to more than one-half of 1 per centum of the cropland in any county.

((b)) There are authorized to be appropriated such sums as may be necessary to carry out the provisions of this section, such sums to remain available until expended.