Why are soil fumigants typically covered with a plastic tarp?

Covering soil fumigants with a plastic tarp is primarily aimed at maintaining gas concentration. Soil fumigants are volatile substances that can easily dissipate into the atmosphere. By using a plastic tarp, the volatility of these gases is curtailed, allowing for a higher concentration of the fumigant to remain in contact with the soil for a longer duration. This increased concentration enhances the fumigant's efficacy in controlling pests, diseases, and weeds in the soil.

While other options like enhancing soil nutrients or preventing rain damage may sound plausible, they don’t represent the core reason for tarping in the context of soil fumigation. Similarly, reducing evaporation, while related to the preservation of the fumigant, is a secondary effect compared to the primary goal of maintaining gas concentration to ensure effective pest control.