Acidic soils (pH < 5.5) cover roughly 30% of Sub-Saharan Africa. Low nitrogen fertilizer application (15 kg N ha−1 yr−1) has no effect on soil acidification in Sub-Saharan Africa (SSA). However, the effect of optimized fertilizer use on soil acidification (H+) in SSA crops remains unknown. This study intended to predict the spatial variation of H+ caused by optimized fertilizer use using data from 5782 field trials in SSA cropland. We used ensemble machine learning to predict spatial variation (H+) after measuring the inputs and outputs of major elements and their effect on H+ production. The results revealed that H+ ranged spatially from 0 to 16 keq H+ ha−1 yr−1. The most protons (H+) were produced by cassava, banana, and Irish potatoes systems with 12.0, 9.8, and 8.9 keq H+ ha−1 yr−1 , respectively. The results of the 10-fold cross validation for the soil acidification model were a coefficient of determination (R2) of 0.6, a root mean square error (RMSE) of 2.1, and a mean absolute error (MAE) of 1.4. Net basic cation loss drives soil acidification under optimized fertilizer application and climate covariates had a higher relative importance than other covariates. Digital soil mapping can produce soil acidification maps for sustainable land use and management plans. Keywords: digital soil mapping; ensemble machine learning; optimized fertilizer use; soil acidification; soil pH 1. Introduction More than a third of Sub-Saharan Africa has acid soils (pH < 5.5) [1] and soil acidity is caused by the presence of hydrogen (H+) ions, which can be generated either naturally by soil formation processes or by anthropogenic activities [2,3]. In regions where precipitation exceeds evapotranspiration, soil acidification is a persistent process that can be sped up or slowed down by the actions of plants, animals, and humans [4,5]. Soil acidification is primarily caused by the release of protons (H+) during the transformations and cycling of carbon (C), nitrogen (N), and sulfur (S) in the soil–plant–animal system [6]. The most proton (H+) and hydroxyl ion (OH¯) generating processes occur during the cycling of C, N, and S (Table S1). Soil acidification in agricultural systems is typically attributed to an imbalance in the carbon and nitrogen cycles, which leads to (1) net H+ excretion by plant roots due to excess cation uptake over anions; (2) removal of alkalinity in farm products (crop harvest and animal product); (3) accumulation of organic anions in soil organic matter; (4) mineralization of organic matter, nitrification of ammonium, and subsequent nitrate leaching; and (5) input of ammonium-based fertilizers [7]. Citation: Uwiragiye, Y.; Ngaba,