Seaweed bed depletion in coastal areas resulting in barren ground is a serious problem in Japan and around the world. Focusing on the lack of dissolved iron as one of the possible factors causing barren ground, we have developed a method for restoring seaweed beds using steelmaking slag and compost containing humic substances. This method is useful for effective utilization of a byproduct and unused biomass for improvement of the coastal environment. Its effectiveness has been confirmed in a field test in the Sea of Japan in Hokkaido, and fundamental studies have been conducted for practical use. In the previous works, the effect of organic matter on the acceleration of iron elution from steelmaking slag was investigated through iron elution tests using steelmaking slag, compost, and bamboo. In this study, we evaluated the effect of organic matter through an iron elution test with nine samples consisting of steelmaking slag, compost, bamboo charcoal, and magnesium ammonium phosphate (MAP) supported on bamboo charcoal. We also evaluated the characteristics of phosphorus elution from the samples. It was found that mixing steelmaking slag with organic matter was effective for accelerating iron elution, and the amount of organic matter eluted and its structural characteristics were important for utilizing this method. We also confirmed the elution of phosphorus from the fertilizer. However, this method was expected to be useful for iron fertilizer rather than phosphorus fertilizer.