Here's what it takes to make cabbage happy: full sun, heavy soil with good drainage, and cool temperatures. Use a large pot - 18 inches or more. Cabbage have shallow roots, and for maximum growth, you don't want your prize cabbage to be cramped for space.
Cabbage like heavy, firm soil, so mix some garden soil in with your potting soil and also add some compost. Firm down the soil as you fill the pot; cabbage won't grow well in fluffy soil. Wait a month, then fertilize with a 12-12-12 fertilizer (follow the instructions for amounts) and continue monthly fertilization until the head forms.
Cabbage need a good supply of moisture, but it should not become waterlogged. Make sure your pot has good drainage and check it daily to ensure that it's moist. This will be especially critical later in the summer as temperatures get hotter. Cabbage can split or bolt (go to seed) in very hot weather; this is more common with potted cabbage. If there is hot weather in the summer, place your cabbage pot in the shade during the midday heat.
This sounds like a bit of work for a cabbage, but if you really want to win that prize, you'll have to coddle your cabbage a little. Best of luck!
