Question

find the relative clause. if it is nonrestrictive, set it off with commas. if it is restrictive, submit the text without making any changes.
when adding organic materials to a compost pile, gardeners should avoid animal products and fats in favor of scraps from plants that have not been treated with pesticides.

Explanation:

First, identify the relative clause. The relative clause here is "that have not been treated with pesticides", which modifies "plants". A restrictive relative clause provides essential information to identify the noun (here, it's essential to know which plant scraps to use), so it's restrictive and no commas are added.

Answer:

that have not been treated with pesticides