Reactions of Ammonia and Urea Hydrolysis Products with Soil
- D. E. Kissel ,
- M. L. Cabrera and
- R. B. Ferguson
A quantitative description of pH changes in N fertilizer application zones is needed to predict the degree of NH3 formation in the soil solution. In particular, the pH changes following surface application of urea determine in part the degree of NH3 formation and possible loss by volatilization. The objectives of our study were to compare the rise in soil pH following addition of urea and NH4OH to soil, and evaluate the importance of inorganic C reactions to any differences in the rise in pH caused by the two respective N sources. Titration curves with urea or NH4OH as a base were used to determine differences in the patterns of pH change. Up to pH 8.2, soil pH was raised identically by NH4OH and urea that had been completely hydrolyzed. Above pH 8.2, urea was less effective than NH4OH in raising soil pH. The lower effectiveness of urea was due in part to the urea-C not forming CO2 but remaining as HCO−3 and CO2−3 in the soil solution. Part of the CO2−3 precipitated as CaCO3 when enough urea was added to raise soil pH to 8.8. At a lower application rate of urea, which resulted in a soil pH of 7.15, no CaCO3 was detected. These results demonstrate the importance of inorganic C from urea hydrolysis in providing resistance to a pH increase above pH 8.2 in concentrated fertilizer application zones.
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