Answer: If the soil is pure gravel, then digging out about 12" deep would be a good idea and replacing the gravel with topsoil. If the gravel is mixed with native soil then rather than digging out the soil and gravel, consider building some raised beds. These can be made by simply forming mounds of topsoil (which you will need to bring in from off site), or can be made by constructing low walls out of wood, stone, or concrete, then filled with topsoil. An 8" deep raised bed, combined with your 6" of topsoil, should give most garden annuals plenty of room..
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