Existing data indicate that chemicals freshly added to soils are more amenable to losses, including biodegradation, than chemicals that have been in contact with soils for extended periods of time. This review presents the results of studies that indicate that increased soil‐chemical contact time increased the resistance of chemicals to desorption, volatilization, biodegradation, and extraction. Thus, results from studies conducted on chemicals freshly added to soils should not be used to predict the behavior of chemicals that have been in contact with soils for extended periods of time. In addition, a measure of the total chemical concentration present in a soil does not adequately indicate the availability of the chemicals for biodegradation or release, and does not indicate the potential for the chemical to be transported to and have an adverse effect on a human or ecological receptor.