For a person who isn't over keen on parsnips, I can grow real whoppers. I prefer swedes but grow parsnips to add to soups. And contrary to usual advice, here's what I do.
Parsnips will get planted in the deepest soil area I have in my sloping beds (sloping only because that's how the land is). In fall I rotovate it deeply as I can, add all the organic things I can like masses of dried leaves (which shatter when rotatovated), masses of chicken manure (yes, I do manure my snips and they rarely fork contrary to popular opinion), compost if I have any left and a few handfuls of bonemeal. The works get rotovated in every fall and left until spring (mostly because the manure is fresh). come early May I direct seed the parsnips as I've found they do better than being transplanted, water every few days until I see them pop up, then leave them to it. Its important to keep them well weeded. Come fall, I must use a garden fork to dig up the monsters working their way to China. Honestly. Only a few fork because I still have a lot of stones in the soil and the snips hit the occasional stone and fork.
That's why it's important to know your garden well. Other parts of my garden hit hardpan in 12" or so, so parsnips would do badly there. My whole garden is clay with hardpan/gravel below, the clay soil added after the house was built, the septic system added, and the grounds 'landscaped', and a token veg garden added. By forking over the soil in search of rocks (and I've dug out some whoppers that took 2 people to take out), I quickly found where the deepest areas are for specific root crops, and shallower areas are that beans and sweet peppers will happily grow in.