Surface‐applied urea on a black chernozemic soil: Hydrolysis, nitrification, and pH change

Abstract

Abstract Hydrolysis, nitrification and changes in pH were determined in different soil layers of a Black Chemozemic soil (Udic Boroll) fertilized with surface‐applied prilled urea during incubation of soil columns for 16d. The soil columns were cut into 0–5, 5–10, 10–20, 20–30, 30–40, and 40–50 mm depths. Ura at 75 and 150 kg N/ha resulted in high concentration of urea‐N in the top 5‐mm soil layer for the initial 2 to 4d. The accumulation of NH4‐N was greatest at 4d in the top layer. Nitrification of added fertilizer N did not occur until 5d. Soil pH in the 0–5‐mm layer increased from 6.1 to 7.5 or 8.3 at 4d for the 75 and 175 kg N/ha, respectively, and it was directly related to the NH4‐N concentration in soil. The accumulation of urea‐N and subsequent hydrolysis, elevation of soil pH and NO3‐N increases did not extend beyond the 20‐mm depth. The results indicate that even in a Black Chemozemic soil which had acid pH, high organic C and CEC, the surface‐application of urea‐N at recommended rates can substantially raise pH, and accumulate NH4‐N in the thin surface layer for several days. During that period, conditions in the surface layer may favor NH3 volatilization.