Ammonia Loss and Associated Reactions of Urea in Calcareous Soils

Urea in soil is a mobile compound which can be lost by leaching or ammonia (NH 3 ) volatilization. This research examined the contributions of water additions and chemical reactions in explaining the behavior of applied urea. Urea [CO(NH 2 ) 2 ] initially moves undissociated into the soil with added water. Ammonia losses from surface‐applied urea thus decreased with increasing amounts of applied water. Ammonia losses from urea were minimal if injected into the soil 2.5 cm or more, whereas NH 3 losses from (NH 4 ) 2 SO 4 (AS) were still high when placed 7.5‐cm deep. Application of AS produced chemically equivalent amounts of Ca and Mg whereas urea and NH 4 OH precipitated Ca and Mg. The amount of Ca and Mg precipitated increased with decreasing NH 3 losses. Precipitation of divalent cations enhanced adsorption of NH 4 on soil cation exchange sites, thereby reducing upward movement of NH 4 and subsequent NH 3 losses. The presence of Ca from reaction of AS with CaCO 3 enhanced upward movement of NH 4 and increased NH 3 losses.