Many soil-chemical processes affect the fate and transport of soil fumigants. Containment, degradation and soil-gas concentration (i.e., effective dosage) must be controlled to reduce emissions while maintaining adequate pest control. Unless each of these factors is controlled, unacceptable emissions or inadequate pest control will likely occur. Two important factors that may affect fumigant transformation and distribution and its ultimate volatilization into the air are depth of placement and the use of soil surface cover. These two factors have been frequently used to alter a fumigant's distribution to reach adequate control in the specified target zones. Depending on the target pest, the fumigant can be applied at the soil surface under plastic film to as deep as 60-100 cm below the soil surface. Deeper placement consistently results in deeper penetration in the soil. Abdalla et al. (1974) found that MeBr application at 76-81 cm without a soil cover resulted in gas distribution at concentrations sufficient for nematode kill as deep as 244 cm. Kolbezen et al. (1974) detected adequate dosages at 300-360 cm when MeBr was applied at 90 cm. Though these early studies were mostly designed for achieving better nematode control in deep soil layers, they demonstrate that downward diffusion is encouraged by deep application. More recent research has been directed at investigating other methods for controlling emissions.