<;P>;PROBLEM TO BE SOLVED: To provide a soil-improving material containing bottom soil deposited in seawater, and suitably usable as the soil-improving material for plant cultivation. <;P>;SOLUTION: The soil-improving material is produced by adding a compost containing plant waste as a raw material to the bottom soil deposited in seawater. Plant culture soil contains the soil-improving material. <;P>;COPYRIGHT: (C)2011,JPO&INPIT