The soil, as a medium for decontamination, offers a large number of processes by which organic substances can be destroyed. As such, progressive accumulation of organic pesticides would appear to be unlikely. Unfortunately, the chemical and physical properties of certain insecticides and herbicides afford them a degree of stability against the natural destructive processes in soils. The stability of these compounds is best illustrated in a recent summary of persistence data on 12 major classes of pesticides in a number of soil types (Fig. 1) (Kearney et al. 1969). Persistence values are expressed in months and each bar represents one or more classes of herbicide or insecticide. Each open space in the bar represents an individual pesticide falling within the larger chemical class of compounds. The length of each bar depicts the time for each class of pesticide to decrease 75 to 100 percent of the amount applied. These values are based on normal rates of application. As anticipated, the organochlorine insecticides are the most persistent pesticides. The organic herbicides persist for a few days or for more than 12 months depending on their respective properties. Only the major herbicides that persist for a month or longer are shown in Figure 1. The phosphate insecticides do not persist for long periods in most soils. A more detailed picture of organochlorine pesticide persistence is shown in Figure 2. Chlordane and DDT usually persist for several years while heptachlor and aldrin extend their activity through the formation of their respective metabolites, i.e., heptachlor epoxide and dieldrin.