Seem to remember using ground up eggshells to clear mine!
very organic!
Just out of interest I harvested a parsnip today out of my 6" pipe experiment. Pipe was 4 foot long and the roots came out of the bottom and into the soil!
After a struggle I managed to get the parsnip out and one of the allotment regulars spotted me carrying it home with some other veggies for the Sunday dinner. He told me it would have won most of the local shows and that my single parsnip should feed a family of five!