Creating a raised veg.patch.
I am building a raised vegetable patch approx. 8' x 4'. I do not have any topsoil to fill it ready for planting so will have to use an alternative such as all purpose compost,etc., Could anyone advise me please as to the most suitable compost to use and also, will this suffice for a few seasons or do I have to "boost it up" every year with soil improvers. If so, what do you suggest?