Pesticides and Fertilizers
The first thing to do when planning the disposal of pesticides and fertilizers is to identify what you have. Ask yourself where you acquired the chemical in the first place. If you bought it from a local nursery or a hardware store, it is likely a garden grade chemical intended for private homeowners. If it was purchased at a farm supply store or in much greater quantities, it may be an agricultural or commercial product. Read the label on the container and contact the manufacturer for verification if you are unsure.
Also, check the label for banned substances. Some pesticides that were once widespread are now banned by state and federal regulations. Chemicals such as dichlorodiphenyltrichloroethane (DDT), ethylene dibromide (EDB), endrin, dinoseb, and chlorodane are all banned or suspended-use substances and must be disposed of properly. Please see the "Agricultural and Commercial Grade Chemicals" section below. If the chemicals are in unlabeled containers, assume that they are not permitted and follow the process for agricultural and commercial grade chemicals.
Garden Grade Chemicals
If you have leftover garden grade chemicals, check with local organizations (gardener's clubs, 4H groups, community gardens, etc.). Some groups may be very appreciative of free product. If there is only a small amount of the product left in the container, dilute it with water and use it for its intended purpose (e.g., spray herbicide on weeds; the ones along pavement or in cracks would be an excellent use). If the chemicals are too diluted, they may not have any effect, but they also are no longer in the container. Read the label. If it specifies a specific disposal method, follow that instruction. If not, dispose of the container with your regular household waste.
If the above options are not desirable or viable for any reason, the Benton County Public Works Department does collect garden grade pesticides and fertilizers at its household hazardous waste (HHW) events. Please check the HHW page for more details. You may also call the distributor from which you purchased the product and ask if they have a take-back program. Please note that even if they do, you will likely be required to deliver it to the location (postal regulations on the transport of such chemicals tend to be very strict).
Agricultural and Commercial Grade Chemicals
Agricultural and commercial grade chemicals are typically much more potent or contain compounds that may require specialized disposal. This grade of chemicals also includes legacy chemicals, such as DDT and its derivatives, which can be harmful to the environment. There are a couple of options for disposing of these chemicals. First, contact the manufacturer and ask if they have a take-back program. As with garden grade chemicals, you will likely be required to deliver the materials to the manufacturer's collection location as postal regulations are quite strict in this respect. If this option is not viable, the Washington State Department of Agriculture (WSDA) operates a Waste Pesticide Program. This program only applies to agricultural and commercial grade chemicals, not to garden grade chemicals. A link to the program can be found here. From their website, you can apply for pesticide disposal, which occurs at events held around the state. Events are scheduled in part by need, so the more people who participate, the greater the likelihood of an event nearby. Please note that this program is for pesticides only. The program will not accept fertilizers.