The origins of H^+ in forest ecosystem have hardly been discussed in Japan, although it is obvious that soil acidification includes not only anthropogenic processes such as acid deposition but also natural formation in the soil (pedogenesis). The concept of a H^+ budget was used in a study of acid deposition in Sweden to evaluate the relative importance of H^+ sources. The H^+ budget can integrate most ionic nutrients, but mainly biogeochemical processes consisting of H^+ production and consumption. To identify H^+ sources and sinks, published data on nutrient cycling were reanalyzed in terms of H^+ budget. Usually H^+ production and consumption processes are heterogeneous in time and space; biochemical processes are active at the surface soil horizon, while at the deeper soil horizon geochemical processes like weathering predominate. For forest ecosystems, it is desirable to calculate the H^+ budget for a watershed unit and the sepecific soil horizon. A distinct difference in H^+ budget was found between Japan and European countries. In European forests, soil acidification is caused by external H^+ input (acid deposition, especially SO_2 and NH_4^+), whereas in Japanese forests, it is caused mainly by internal H^+ production (cation uptake and nitrification in the surface soil horizon).