spade the ground deeper and remove rocks and obstructions . Use fertilizer , but NOT GRASS FERTILIZER .

Ask Kitazawa seeds if they can get you the right seeds for growing giant daikons.

http://www.kitazawaseed.com/

Here is a link that might be useful: Kitazawa Seed

dangould Kitazawa Seed already sells seeds for growing giant Diakon. The variety is Sakurajima Mammoth.