The amount that each tract of land, public or corporate road or railroad shall be liable for on account of the establishment of any irrigation or drainage project under the provisions of this chapter shall bear interest from the date of the filing of the auditor’s statement in the county recorder’s office at a rate of interest not exceeding six per cent (6%) per annum until paid, such rate of interest to be fixed and determined by the board of county commissioners at the time of establishing such project: provided, that when bonds are issued by the county for the construction or establishment of such project, the same rate of interest shall be charged as said bonds so issued bear: provided, further, that in any case and at any time after the establishment of any project and in the event there is not sufficient money in the sinking fund for the purpose the board of county commissioners may upon such notice as shall be ordered and upon a showing of the necessity to provide funds to meet payments due or maturing on the bonds issued under this chapter, either change or modify any order previously made fixing and determining a rate of interest or fix the rate of interest in the case the same has not been previously fixed or change the existing rate of interest, the same to remain in effect until such time as the amount in the sinking fund is sufficient for such purpose. All interest shall constitute an additional lien on said land or lots until fully paid, which said interest when about to be paid shall be computed by the county auditor. If bonds are sold at a premium, such premium shall become a part of the sinking fund herein provided.