Ammonia volatilization from soils after surface application of urea or inorganic N was reduced by Ca and Mg nitrates or chlorides. Sulfates were only slightly effective. Suppression of NH 3 loss was a result of two chemical reactions: (i) precipitation of CO 3 by Ca, thus preventing permanent (NH 4 ) 2 CO 3 ·H 2 O (AC) formation, and (ii) Ca depression of soil pH by depression of the dissociation of the CaCO 3 ‐Ca(OH) 2 buffer system. Soil pH values as low as 6.5 were recorded in calcareous soil and 3.9 and 4.5 in the acid soils after addition of Ca. Soil pH values in the presence of Ca reached approximate neutrality in 24 hours even in acid soils. Soil pH without Ca but with AC increased to > 9.0 in every soil. Addition of AC plus Ca reduced the pH of calcareous soil to 5.3 and raised it to 5.7 in acid soils. Calcium carbonate precipitation eventually occurred in acid soils, preventing high NH 3 losses. Evidently, CaCO 3 precipitated on the surface of acid soils after the pH reached a certain critical value and, as expected, the pH was maintained near that of the pure CaCO 3 system.
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Fenn et al. (1981) studied this question.