No superphosphate containing less than eighteen percent of available phosphoric acid may be sold or offered for sale in this state. Specialty fertilizers, except manipulated animal and vegetable manures, guaranteeing less than five percent total plant food shall contain on the label specific directions for use, and prior to registration, the department may require proof of the efficacy of the product when used as directed.

Construction—Severability—1987 c 45: See notes following RCW 15.54.270.