[belto]

I am a newbie at allotments and gather it is very difficult to grow carrots and Parsnip due to the Carrot fly.

I intend to grow under a small poly tunnel.

What soil preparations needed with regard to tilth size?
Is it enough to use the spade to break up the soil or should I sieve the carrot and Parsnip soil bed?

[Trillium]

I wouldn't necessarily seive the soil unless you've got lots of stones. Carrots and parsnips don't like those - causes forking and such. They best like soil that's more sandy and loose, so you could easily add coarse (never fine) sand to your soil, along with loads of humus materials (coir, shredded leaves, etc) but very easy on the manure which causes very hairy roots. You can spade or rotovate the soil, up to you, as long as its rather loose and easy for the roots to penetrate.
If you have hard clay, then try the bathtubs, buckets, etc tricks that others use, which also puts them above carrot fly flying height.

[mashauk]

Our soil is so stony we'd never get it all sieved, just do a bit at a time, now got to find somewhere to stash the stones

[vegmandan]

Don't worry too much about getting the tilth too fine as when you water it ,it can form a crust if it's too fine once it drys out and the seedlings can find it hard to break through.

Once they've germinated they only want to go one way....downwards so dont worry about making it too fine.