In spring 1919, farmers across Belgium and northern France ploughed their fields for the first time in five years. Over the past four years their fields had been subjected to repeated heavy barrages, estimated in total at one tonne of high explosive per square metre of the battlefield; the soil had been impregnated by poisonous gases including phosgene, mustard and chlorine gas; the farmers’ fields had been extensively trenched; mines had been dug, packed with explosives and then detonated; woodlands and orchards had been destroyed, the trees blown to shattered stumps and then, with the advent of tanks, farmland had…