Sure, but that stuff is mostly in unexploded ordnance that has deteriorated and split open recently. Mustard agent will be broken down by sun, air, and water after being in the open air in 3 days to 3 weeks depending on temperature. The use of mustard gas was so intense in WWI that dispersed mustard agent was often churned deep into the soil by the intensity of the barrage. Buried in clay soil, it can persist much longer and there have been reports of minor mustard blistering when digging deep into such ground. There are a lot of good reasons not to be a French farmer. They cali it the "Iron Harvest" and they still recover about 900 tons of unexploded ordnance, mines, barbed wire, weapons, and trench support beams every year.
