When a pesticide or any other additive, other than plant nutrients, is incorporated in a commercial fertilizer, the fertilizer mixture must be registered and guaranteed with respect to the kind and percentage of each of these additives as well as to claimed plant nutrient elements, all of which must be determinable by accepted laboratory methods. The labeling shall, in addition to other required information, state in a prominent manner the purpose for which the combined fertilizer and additive is recommended. It must be shown by scientific data that each additive is present in sufficient quantities to impart a distinctive characteristic to the product.