The county board of any county may lease to agricultural societies established and existing in its county, for such period and on such terms as it shall deem expedient, any lands of the county, including any portion of lands of the county used as a poor farm, to be used by the society for fair purposes. The society may construct on the leased land, suitable buildings, racetracks, and other improvements; provided, that in case of the leasing by the county board of any county of lands previously set aside as a poor farm, such improvements shall be constructed according to a plan previously submitted to the county board and approved by it.