Flood irrigation bonds issued as provided in this chapter shall bear interest at a rate or rates resulting in an average net interest cost not exceeding twelve percent on those issues which are sold at private sale. There is no interest rate ceiling on those issues sold at public sale or to the state of North Dakota or any of its agencies or instrumentalities. Interest and principal may be payable under the amortization plan over a period of not to exceed twenty years, or the principal may be divided into such amounts and made payable at such periods, not exceeding twenty years, as the board of county commissioners may determine.