Root vegetables absorb every chemical that is put into the soil around them. Most of the time we eat either the leaves or the 'fruit' of a plant. Even if they are treated with fertilizers and/or chemicals, the majority of the bad stuff is stored in the plants' roots. In the ground. Where we don't EAT IT.
When you eat a potato - or carrots, turnips, radishes, or yams - you are eating the ROOT of the plant. The very part of the plant that absorbs all of those nasty chemicals. There have been studies that have shown that root vegetables that have been grown with all of those chemicals actually have incredibly high concentrations of them INSIDE the vegetable. No amount of washing or peeling is going to fix that! As a result, I spring for organic on the rare occasion that we eat them.
There are several other foods that we don't touch. This article touches on a few of them (and freaked me out about a few more). I suggest a quick read.