Some vegetable plants such as Beetroot and Turnip dislike the root disturbance that transplanting causes, so grow better if sown directly in to the position they are to mature.
The problem with a heavy soil is that it is usualy colder and wetter than lighter soils, so seeds struggle to germinate, and those that do find it difficult to get established.
To combat the effects of my heavy clay soil I first cultivate the area to produce as finer soil as I can, then take out an over deep seed drill approx 2 inches (50 mm) deep.
This is then filled with potting compost and lightly firmed down. The veg seed is then sown into the compost at their normal depths.
This gives the seedlings enough room to germinate and establish a stronger root system before coming into contact with the harsher heavier soil.
The compost used to fill the over sized seed drill does not need to be of the best quality, and I quite often use the compost saved from last years pots and tubs.