In order to provide for the payment of bonds at maturity a "sinking fund" shall be provided as follows: Five years from the date of the bond issue and at the same time each succeeding year until the maturity of the bonds the board of drainage commissioners shall collect from the several tracts assessed in the bond issue a sum of money which, when placed in a bank and interest paid thereon at four per cent, compounded quarterly, shall equal the total assessment against all the tracts in the bond issue at the end of twenty years. This amount shall be placed in some State or national bank of reputable standing and shall bear four per cent interest, compounded quarterly. This sum shall be collected along with the interest on the bonds and at the same time and in the same manner as State and county taxes are collected. At the date of maturity of the bonds such sum shall be paid over to the bondholders in liquidation of the bonds, such funds to be drawn out of the bank of deposit by the clerk of the court. In no case shall bonds be issued until the tax levy has been made to meet them as they come due.