Since the mid-20th century, farmers have routinely used fumigants to prepare soil for sensitive annual crops, like strawberries and vegetables, and for planting new orchards on land where previous orchards or vineyards of the same kind were grown.

Several of the most effective fumigants — such as methyl bromide, chloropicrin and Telone — work so well, scientists can’t even fully explain why the crops are so much more productive after treatment. But now the use of soil fumigants is falling out of favor.

Soil fumigation was first used in the late 1880s when the grape root aphid Phylloxera was exported from the eastern United States to Europe, with devastating effect. In desperation, farmers tried to control the pest with carbon disulfide, a readily ignitable chemical that presented explosive danger to workers applying the pesticide.

By the 1930s, 17 chemicals were available to farmers to control nematodes, tiny soil-borne roundworms that harm crops. In 1941, USDA scientists Al Taylor and C.W. McBeth were the first to report a practical application for the chemical compound methyl bromide in agricultural fields.

Methyl bromide is a naturally occurring, but toxic compound, produced by the Earth’s oceans, volcanoes and wildfires. It can also be manufactured.

It would be almost two decades before UC Berkeley plant pathologist Stephen Wilhelm and businessman William Storkan developed a method for applying the gas to an agricultural field while at the same time spreading a polyethylene tarp onto the field surface to keep the fumigant in place.

Methyl bromide treatment solves many agricultural problems. It kills nematodes, fungi, plant pathogens and weed seeds. When properly applied, methyl bromide moves freely through tiny gaps in the soil, creating an optimum environment for plant root growth and development. Treated soil permitted plants to reach their full potential, increasing yields by dramatic proportions. In strawberries, for example, yields more than quadrupled from the early 1960s to the 1990s.

However, as the turn of the millennium approached, climate scientists found that the productivity came with a price. Methyl bromide is an ozone-depleting chemical. Its production and use worldwide were severely curtailed by the Montreal Protocol and its use as an agricultural pesticide is being phased out by most countries, including the United States. The agricultural industry has petitioned for extensions of the methyl bromide ban because it lacks an equally effective alternative. Unfortunately, many chemical alternatives have been found to be highly toxic, which has resulted in strict regulations for their use — requiring wide buffer zones, careful timing, and worker safety measures. In many cases, the use of chemicals is impossible.

The reduced availability of soil fumigants in agriculture is prompting University of California scientists to look for alternatives.