Nine in ten American strawberries come from a few valleys in California where the soil is killed with a First World War gas before every planting, and the fields are visible from the road because they are wrapped in plastic to keep the gas in. Chloropicrin was made for the trenches. It was a lung and eye agent, a vomiting gas, fired in shells to force men out of their masks. Now it is injected into the ground at Watsonville and Oxnard every year, under tarps that hold the fumes down for a week, and it does to the soil what it did to a trench: everything in it dies, the fungi, the bacteria, the worms, the insects, until the dirt is as sterile as a hospital tray. Then the strawberries go in, on raised beds of plastic, fed by drip line with everything a living soil would have given them, and the plastic and the gas go in again next year, because a dead soil has no defence against the one disease that gets through. For forty years the gas was methyl bromide, which eats the ozone layer, and when the world banned it in 2005 California's strawberry growers got themselves an exemption and kept using it for another decade, the last major crop on earth to do so. Chloropicrin was the replacement, and the state's own pesticide regulator lists it among the commonest causes of farmworker poisoning in California. The men who apply it are mostly undocumented, the drift has emptied a school in Monterey County, and the strawberry has topped the list of the most pesticide-contaminated produce in America for most of the last decade. Every year the industry calls it a heart-healthy antioxidant superfood, and every year the field it came from is gassed. The cow in the pasture next door has never once needed the soil under her sterilised. The strawberry cannot be grown at that scale without it, and the war gas is the price of a punnet in January.