Turnips belong to the Brassica family of crops (cabbages, cauliflowers etc.) rather than the Roots and Onions family (carrots, leeks etc.).
So - if you use crop rotation on your allotment plot - they should be sown and grown in the Brassica bed.
They can be sown in Spring or Summer - for a succession of crops through to late Autumn.
Harvest when small and sweet - or grow on to full size and harvest for use in Winter soups and stews.
Prepare the ground by hoeing out any weeds and levelling with a rake.
Mark out the rows with string - with rows about 8 inches apart. Use a hoe or trowel to draw a shallow drill - about half an inch deep.
If the weather is dry - water the drill before sowing.
Sprinkle the seeds thinly along the drill and backfill - add a plant lable with variety and date sown.
It might be worth repeating the watering after a few days if you happen to have a dry spell of weather and you have (like me) well draining soil. Keep the plants well watered as they grow on
The seeds should germinate in two to three weeks - depending on the weather and the season.
They will need thinning out once established - leaving a gap of four to six inches between each turnip plant.
You can start harvesting them six to ten weeks after sowing.