Barren grounds (ground devoid of seaweed beds) in coastal areas present serious environmental problems in Japan and around the world. To restore seaweed beds on barren ground, a mixture of steel slag (hereafter referred to as slag) and compost, which included humic substances (HSs), was applied in a coastal area in Mashike (Hokkaido, Japan). Three plots on barren ground were considered. One was treated with slag, another was treated with a combination of slag and compost, and the third plot was left untreated (control plot). The types of seaweed and their fresh weights were recorded in the three subsequent years. Restoration of a seaweed bed was confirmed at the locations treated with a mixture of slag and compost, and that treated with only slag. Specifically, the amount of restored seaweed and the restored area at the former site were significantly larger than those at the latter site and control site. The concentrations of iron at the site with slag-compost mixture was significantly greater than those at the other sites. Because the pH values of seawater are known to be around 8.1, ionic species of iron are deposited due to the formation of hydroxides. Laboratory experiments showed that the elution of iron from slag is dramatically enhanced in the presence of HSs. This result was attributed to the complexation between iron on the surfaces of slag and HSs. Complex of iron with HSs can stably exist in solution. Therefore, applying a mixture of slag and compost is an effective technique to supply dissolved iron to barren grounds in a coastal area.