Soil acidification is caused by both external and internal acid load in ecosystems. As a natural pedogenetic process, it is important to evaluate ecosystem internal acid load such as excess uptake of cation over anion by plants, nitrification, formation of organic acid and dissociation of carbonic acid. As these processes are closely related to organic matter cycles, the soil acidification rate should be understood quantitatively with organic matter dynamics. Soil acidification rate was calculated from the budget of ions constituting ANC (Acid Neutralizing Capacity) at three forest plots in Japan, one forest and one cropland plots in both Thailand and Indonesia. The ion budget was calculated from the difference between input supplied by throughfall and output as plant uptake (sum of wood increment and litterfall) and leaching of soil solution collected by tension free lysimeter. In the forested ecosystems, acidity originated mainly from nitrification and formation of organic acids in O horizon. Biologically produced anions, which work as a driving force of cation movement, were bicarbonate ion in the moderately acidic forest soil in Thailand and organic acids in the highly acidic forest soil in Japan and Indonesia. Acid load contributed to cation movement in the surface soil, and accelerate nutrient availability to plant. After all, soil was acidified mainly by plant uptake, which included excess cation accumulation in both litter and wood increment. Since acid load by excess cation accumulation in litter was neutralized by litterfall returned to forest floor, net soil acidification rate was high at the plots where excess cation accumulation in wood increment was high. In cropland, nitrate ion was produced by nitrification in both the moderately acidic soil in Thailand and highly acidic soil in Indonesia. Base cations were leached out with nitrate ion, resulting in irreversible soil acidification. It is shown that soil acidification will occur at the rate of 0.01-0.02 mol_c/ha/yr along with net primary production by 1molC/ha/yr. This fact supports the presence of strongly weathered soils in tropical regions from the viewpoint of biological activity, reflecting higher primary production and decomposition rate of organic matter than in the temperate regions. Reciprocal dynamics between organic matter and biological acid load should be taken into account to evaluate soil acidification rate along with the change of organic matter dynamics in time and space, not assuming that ecosystems are in steady state.