1. It shall be the duty of the county commission in making assessments, as provided in this chapter, to take into account the maturing bonds and interest on all bonds, and to make ample provisions in advance for the payment thereof. In case the proceeds of the assessment made under the provisions of sections 243.550 to 243.553 are not sufficient to pay the principal and interest of all bonds issued, then the county commission shall make such additional levy or levies upon benefits assessed as are necessary for this purpose, and under no circumstances shall any assessment be made that will in any manner or to any extent impair the security of the bonds or the fund available for the payment of the principal and interest of the same.

2. The funds derived from the sale of the bonds or any of them shall be used for the purpose of paying the cost of the drainage works and improvements and such costs, expenses, fees and salaries as may be authorized by law and used for no other purpose.