Foreign material is any material that is not the potato tuber. It is "trash" or "garbage" in potatoes. Many foreign materials originate in the field, but others enter the product stream through equipment and during storage and handling. Whether potatoes are destined for the fresh market or processing, foreign material is a great concern to everyone in the potato industry.
University of Idaho Extension has published a 16-by-24-inch poster (linked here and below) that provides examples of common foreign material found in potato production. Growers are encouraged to print and display the poster in areas to educate employees and as a reminder to remove foreign material.