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Cation-Anion Balance in Crops as a Factor in Determining the Effect of Nitrogen Fertilizers on Soil Acidity¹

The quantitative effect of crops on the acidity produced in soils from nitrification of NH,NO3 fertilizer was determined by growing oats and. buckwheat in the greenhouse and finding the amounts of CaCO₃ necessary to bring the soils back to their original pH. Three crops of each plant species were grown, and the plants were analyzed for total N, nitrate-N, and excess base. Determinations of pH were made both in 0.01 M CaCl₂ and in H₂O, with modifications to eliminate errors due to soluble salts.

The acidity developed from NH₄NO₃ in uncropped soil was almost equal to the theoretical amount that should be developed by nitrification. Without the removal of soluble salts erroneous values were obtained. The increase in acidity due to N₄NO₃ when oats was grown lower than the theoretical value in the absence of a crop by 27%, and the increase in acidity with buckwheat was higher than the theoretical value by 87%. Analyses of the crops showed that the deviations from the theoretical increase in soil acidity from NH₄NO₃ fertilizer was quantitatively explained by the numbers of chemical equivalents of N and excess bases taken up by the crops, excess base being defined as total cations (Ca⁺⁺ Mg⁺⁺, K⁺ and Na⁺) — total anions (C1^-, SO₄^-, and H₂PO₄^-).

Key Words: Excess base • Excess base-N ratios • Salt effect on pH • pH in 0.01 M CaCl_2-

² Professor of Agronomy, Student Assistant, and Graduate Assistant, respectively.

Received for publication July 26, 1969.