Simplify the following text for a Grade 7 native English-speaking person.
Original: However , where natural levels of iodine in the soil are low and the iodine is not taken up by vegetables , iodine added to salt provides the small but essential amount needed by humans .
Simplified: People only need a very small amount of iodine to prevent disease . Food crops usually get iodine from the soil , but some places around the world have low amounts of iodine .