Hi Roy and welcome to GoY. Depends on the root to be honest. Potatoes like a good humus rich soil and my SO usually creates a trench which he fills with compost or well rotted manure. Carrots and parsnips like a fine sandy soil with not compost and no stones. Onions and other root veg. like some compost but are not the greedy feeders that, for example, brassicas are.

Umm Snoopy brassicas are not root veggies which Roy is specifically asking about!

What's an SO Moongrower?

Significant Other

Since when has a potato been a root crop? Thought it was an underground stem. Since when has onion been a root crop?

The turnip, which is a brassica, is classified as a root crop.
Beets, carrots, parsnips, radishes, turnips are the recognised root crops.

As to preparation, plenty of phosphates and potash for carrots, very little nitrogen as that can lead to cracks.

Scrumpy in the growing system I follow both potatoes and onions are viewed as root vegetables in terms of the way they are grown.

Well the original question was on root vegetables, not on how to grow potatoes and onions which aren't, no matter what growing system you use, even though i don't agree with your onion advice, or brassicas as you pointed out to snoopy, even though the brassica turnip is a root vegetable.
To answer the original question again, phosphates and potash are the essentials for roots and for fruiting, but more importantly as you say is the type of soil.

Peace, peace, folks! Sticking my oar in, potatoes are certainly a subterranean crop whether they are roots or not!

Thanks for the SO, Moongrower. That's the third abbreviation I've learned on here - SO, OH and OMG (which is Not oh my god in this context...)

Not all root crops are subterranean, so that doesn't make sense.

Snoopdog, I have no argument with you... and yes we can all add to the picture. I often think my ways are out-of-date - snag is we've all been growing for a very long time

Scrumpygrat you've intrigued me now - which veg roots don't grow under the ground? I only meant that potatoes do, even it they aren't roots.

Beetroot, turnips, radishes....

Can't argue with that!