As soon as practicable after the end of each calendar year, the Secretary shall pay to the county in which any land is held by the Secretary under this subchapter, 25 per centum of the net revenues received by the Secretary from the use of the land during such year. In case the land is situated in more than one county, the amount to be paid shall be divided equitably among the respective counties. Payments to counties under this section shall be made on the condition that they are used for school or road purposes, or both. This section shall not be construed to apply to amounts received from the sale of land.