"(i) That the applicant agrees that a strip forty meters wide starting from the bank on each side of any river or stream that may be found on the land applied for, shall be demarcated and preserved as permanent timberland to be planted exclusively to trees of known economic value, and that he shall not make any clearing thereon or utilize the same for ordinary farming purposes even after patent shall have been issued to him or a contract of lease shall have been executed in his favor."