A bioassay is a technique for determining if herbicide (or other chemical) residues are present and bioavailable in soil or water at high enough concentrations to adversely affect plant growth. This is a simple, economical, and direct method to determine if it is safe to seed or plant into areas previously treated with herbicides or into soil with an unknown history of herbicide use. In its simplest form, a bioassay uses susceptible plants to identify if the herbicide is present in concentrations high enough to inhibit germination and alter plant growth. However, scientists sometimes use sensitive bioassay species to estimate herbicide concentrations in soil and water, and to identify unknown herbicide residues from exhibited injury symptoms. When newly seeded or established plants show seemingly unexplained symptoms of injury, stress, or decline. Also, when seeding or planting sensitive plant species into areas previously treated with residual herbicides. Topsoil from abandoned farm land can often contain herbicide residues that can injure many plants. Another too common occurrence is the presence of herbicide residues in compost (both commercial and municipal). Additionally, if you suspect that another product may have been contaminated with an herbicide, both the product and treated soil can be tested using a bioassay.