The part of the constitution allows

This part of the constitution allows A. State laws to be declared unconstitutional.

The clause given establishes that all federal laws are the Supreme Law of the Land and therefore all states, federal officers and judges are bound by it, meaning that the scope of the state laws is limited by the federal's law and therefore, if there were any state law in conflict with a federal's, it must be declared unconstitutional.

crop rotation kepy different plants in the soil instead of once constintly. when switching out plants you have different nutrients being absorbed at different speeds and different roots growing differently therfore the spoil and water flow have to adapt and carry fresh nutrients to carry the constant changes that were going on in the soil.