Peel the parsnips and trim off the crown and the thin spindly tips.
Cut the parsnips into evenly sized chunks so they heat through at the same rate.
Bring a large pot of water to a hard boil, add the parsnips, wait for the water to return to a boil, and then boil for 5 minutes. Work in batches if the pot is small.
Pack the hot parsnips into warm, clean jars, leaving 1 inch headspace.
Add salt to each jar if you are using it, then cover the parsnips with the boiling cooking liquid, keeping 1 inch headspace.
Release trapped air bubbles, top up the liquid if the level dropped, wipe the rims and threads clean, and apply two piece lids until fingertip tight.
Load the jars into the pressure canner, fasten the lid, and vent a steady stream of steam for 10 minutes before adding the weight or closing the petcock.
Process pints for 30 minutes or quarts for 35 minutes at 10 pounds pressure in a weighted gauge canner or 11 pounds in a dial gauge canner, adjusting for altitude (see notes).
Turn off the heat and let the canner depressurize on its own, then slowly remove the weight or open the petcock.
Wait 2 minutes, then unfasten the lid and open it away from your face.
Remove the jars to a towel lined counter, leaving space between them, and let them cool undisturbed for 12 to 24 hours.
Check the seals, remove the rings, label the jars, and store them in a cool, dark place.