If you have an old farmhouse on your rural property, it's highly likely that it was painted with lead-based paint at some point in its history. Likely many times, in fact. A lot of the soil contamination comes from lead paint flaking off as it weathers. Fortunately(ish), this is mostly confined to the adjacent areas.
Given that you likely have more space in which to free range your chickens, the problem is now reduced to free ranging them away from the house. Arranging this is left as an exercise to the reader :-)
Given that you likely have more space in which to free range your chickens, the problem is now reduced to free ranging them away from the house. Arranging this is left as an exercise to the reader :-)