Two sewage sludge ash (SSA) samples were analyzed using X-ray fluorescence spectrometry and X-ray diffractometry, and a chemical mass balance method was employed to estimate the contribution of soil-derived components. Plagioclase in the SSA samples, which originates only from soil, was used as a fingerprint for the crystalline phase to determine the relative contribution of soil components in each sample. Specifically, an average of 18% of total mass was found to be plagioclase in soil samples collected at 35 sites in the drainage collection district, and this value was used to estimate the contributions of other SSA soil components. The contributions of the soil to SSA A and B were approximately 50 and 80 mass%, respectively. The crystalline phases of the two SSAs were found to consist of different components such as phosphate crystalline phases. These results suggest that the coagulant composition may influence SSA content, including the soil-derived components.