Most of the larger bareroot nurseries fumigate their seedbeds with a poisonous gas, such as methyl bromide/chloropicrin, to sterilize the soil. Fumigation is expensive, but eliminates all the common nursery pests: pathogenic fungi, insects, nematodes, and weed seeds. The fumigant is injected into the soil and then immediately covered with a plastic tarp, allowing the gas to permeate throughout the soil. After several days, the tarp is removed, the gas dissipates, and the soil is ready to plant. Currently, the use of methyl bromide is being curtailed because of potential damage to the ozone layer.