1. If the board of supervisors deem it necessary, the board shall, without unnecessary delay, levy a tax of such portion of said readjusted assessed benefits on all lands, railroad and other property in the district to which benefits have been assessed, as may be found necessary by the board of supervisors to pay the costs of the completion of the proposed works and improvements as shown in the supplemental plan for reclamation adopted by the board of supervisors pursuant to section 242.230 and in carrying out the objects of said district, and plus ten percent of said total amount for emergencies. The tax levied pursuant to this section shall be apportioned to and be levied on each tract of land or property in said district in proportion to the readjusted assessed benefits, provided that the amount of such tax levied pursuant to this section, when added to any taxes previously levied and remaining unpaid at the time of the levy provided for in this section, shall not exceed the total amount of the readjusted assessed benefits.

2. The tax shall be levied in the manner provided in sections 242.450 and 242.460.