Excess soil acidification caused by fertilizers is a major factor in world‐wide soil deterioration. Fertilizers, particularly nitrogen, acidify soil mostly when too much is used (in excess of crop needs). Acidity is otherwise caused by differential cation‐anion uptake by plants which varies with species. A crop can acidify the soil whether or not commercial fertilizers are used, like if the nitrogen came from the soil organic matter or from symbiotic nitrogen fixation. When the exact amount of nitrogen that is needed is applied to land, little acidification results unless nontillage is practiced to give soil surface acidification. In that case the acidification can equal the theoretical. For ammonium‐N, the theoretical is twice the value given in fertilizer handbooks and if there are no plant roots in the soil surface, the full acidification effect is expressed. There are plant species and cultivar differences on soil acidification caused by differential cation‐anion uptake. Legumes acidify soil considerably.