All leases of state land, except mineral leases, shall be conditional upon the payment of rental, in advance, and a violation of this condition shall work a forfeiture of the lease, at the option of the state board of land commissioners, after thirty (30) days’ notice to the lessee, such notice being sent to the post office of the lessee, as given by himself to the director of the department of lands when the lease is issued: provided however, that upon the application of any person, firm, corporation or association from whom such rent is or will be owing, the state board of land commissioners is hereby given authority and power to, in its discretion, extend the time of payment of such moneys for said leases for not to exceed two (2) successive years: provided, that the applicant enters into an agreement with the said state board of land commissioners to pay the interest on said amount of rent money from January 1 of the year which the same is otherwise due, to the date of payment, at the rate per annum set by the state board of land commissioners; that this authority shall extend to amounts due on outstanding leases, leases renewed and new applications for leases. Lease rental rates established by competitive bidding may not be adjusted during the term of a lease, except that the state board of land commissioners upon a finding of a material change of circumstances from those existing at the time of auction, may, after a majority vote of those present, reduce the rental to no less than fair market value.