In the context of pollution-control strategies to minimize the detrimental effects of soil acidification, there is a need to know how and to what extent soils respond to acidifying substances. The purposes of this study were to assess the sensitivity of soil to acidification, in particular to a decrease in pH and in base saturation (BS), and the risk of Al toxicity for vulnerable plants using chemical indicators. These indicators were derived from soil data (pH, exchangeable cations, amount of fine earth) measured in the mineral horizons of 257 soil profiles throughout Switzerland. Based on the analysis of the distribution of pH and BS values in the soil collective, we assessed the sensitivity of soils to a decrease in pH and in BS. Soils that were considered sensitive to a decrease in pH had pH values between 4.8 and 7.0. The degree of sensitivity was estimated with the proportion of fine earth in the critical pH range to a depth of 100 cm. Soils that were considered sensitive to a decrease in BS had pH values between 3.6 and 5.5 and a BS between 10% and 95%. Since the effective cation-exchange capacity (CE_Ceff) of the fine earth might dampen the decrease in BS when acidity is added, the disposition for a decrease in BS was related to the relative amount of fine earth in the sensitive BS and to the mean CE_Ceff of this fine-earth fraction. The risk of Al toxicity for vulnerable plants was estimated using the ratio of base cations to Al at the cation-exchange sites (BC : Al_exc. A BC : Al_exc of 0.2 was taken as a threshold value below which the risk for sensitive plants increases. The degree of risk was based on the proportion of fine earth in the critical BC : Al range (<=0.2) in the soil profile. These indicators taking into account the various aspects of soil acidification are derived from usually available data and represent therefore a cost-effective tool to assess the sensitivity of soils to an input of acidity.