Reactions of ammonia and urea hydrolysis products with soil

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 HCO₃^- and CO₃^2- in the soil solution. Part of the CO₃^2- 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.