In the event any proposal to disestablish or to alter the boundary of an agricultural preserve will remove land under contract from such a preserve, notice of the proposed alteration or disestablishment and the date of the hearing shall be furnished by the board or council to the owner of the land by certified mail directed to him at his latest address known to the board or council. Such notice shall also be published pursuant to Section 6061 and shall be furnished by first-class mail to each owner of land under contract, any portion of which is situated within one mile of the exterior boundary of the land to be removed from the preserve.