64.890.  Building and land use restrictions — exemptions — limitations on use of powers. — 1.  Nothing in sections 64.800 to 64.905 shall affect the recovery of natural resources by strip or open-cut mining; provided, that commercial structures shall be permitted in all districts except those zoned for residential or recreational use.

  2.  The provisions of this section shall not apply to the incorporated portions of the counties, nor to the raising of crops, livestock, orchards or forestry nor to seasonal or temporary impoundments used for rice farming or flood irrigation.  As used in this section, the term "rice farming or flood irrigation" means small berms of no more than eighteen inches high that are placed around a field to hold water for use for growing rice or for flood irrigation.  This section shall not apply to the erection, maintenance, repair, alteration or extension of farm buildings or farm structures used for such purposes in an area not within the area shown on the flood hazard area map.  This section shall not apply to underground mining where entrance is through an existing shaft or shafts or through a shaft or shafts not within the area shown on the flood hazard area map.  The powers granted by sections 64.800 to 64.845 and 64.850 to 64.880 shall not be construed:

  (1)  So as to deprive the owner, lessee or tenant of any existing property of its use or maintenance for the purpose to which it is then lawfully devoted;

  (2)  So as to deprive any court of the power of determining the reasonableness of regulations and powers in any action brought in any court affecting the provisions of sections 64.800 to 64.905 or the rules and regulations adopted thereunder;

  (3)  Nor to authorize interference with the public utility services as may have been or may hereafter be authorized or ordered by the public service commission or by permit of the county commission, as the case may be.

  3.  Nothing contained in sections 64.800 to 64.905 shall affect the existence or validity of an ordinance or order which a county has adopted prior to March 4, 1991.