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Definition: Fertilizers have either neutral, acidic (lower soil pH), or basic (increased soil pH) effects when added to the soil. This effect is commonly expressed in terms of the amount of pure calcium carbonate that would be required to either offset the acid-forming reactions of 100 pounds of fertilizer materials or the amount of calcium carbonate required to equal the acid-neutralizing effects of 100 pounds of fertilizer.

Properties: The acid- and base-forming nutrients and their equivalent acidities or basicities, in terms of calcium carbonate (CaCO₃) per 100 pounds of product are shown in Table 3 in Fertilizer Introduction (C6).

Agronomics: Most of the acid-forming effects of fertilizers are due to the activities of soil bacteria which convert ammonium-nitrogen to nitrite and nitrate in the process of nitrification. The theoretical values shown in Table 3 in Fertilizer Introduction are based on acidity produced during nitrification. These values, however, have been substantiated by research. Ammonium nitrogen from legumes, animal wastes, and sewage sludge produces exactly the same effect as commercial fertilizers. Oxidizable forms of sulfur (elemental S or thiosulfate S) also contribute to soil acidification.