Iowa Code § 200.11: Item that is injurious or filler.

A person shall not manufacture, offer for sale, or sell in this state, any commercial fertilizer or beneficial substance containing any item that is injurious to crop growth or deleterious to the soil, or to use in such commercial fertilizer or beneficial substance as a filler any item that contains inert or useless plant food material for the purpose or with the effect of deceiving or defrauding the purchaser.