No abstract

Volatilization of NH3 from NH4‐‐N salts applied to the surface of a calcareous soil was investigated. Ammonia losses from surface applied NH4F, (NH4)2SO4, and (NH4)2HPO4 were 68, 54, and 51%, respectively, of the added NH4‐‐N in 100 hours. Ammonium nitrate, NH4Cl, and NH4I lost 18, 18, and 16%, respectively, of the added NH4‐N in 100 hours. An increase in pH at the soil surface resulted in higher loss of NH3. Applications of 550 kg N/ha as NH4F and (NH4)2SO4 increased the pH from 7.6 to 8.9 and 8.1, although pH decreased rapidly with time. Soil pH decreased to 6.8 by the addition of 550 kg NH4‐N/ha as NH4NO3 and changed very little in 50 hours.Ammonium sulfate applied to soils saturated with Mg2+ and Ba2+ gave similar results. Solubility of the apparent reaction product was the major factor regulating NH3 volatilization. Formation of a Ca2+ reaction product, such as CaSO4, tends to drive the reaction to the right and favors formation of (NH4)2CO3. A proposed mechanism of the observed loss of NH3 is as follows: when an ammonium salt dissolves in a calcareous soil, (NH4)2CO3 and a Ca2+ salt of varying solubility forms. Ammonium carbonate subsequently decomposes, losing CO2 at a faster rate than NH3. This causes formation of NH4OH and an increase in pH and greater NH3 losses.

Urea in soil is a mobile compound which can be lost by leaching or ammonia (NH3) volatilization. This research examined the contributions of water additions and chemical reactions in explaining the behavior of applied urea.Urea [CO(NH2)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 NH3 losses from (NH4)2SO4 (AS) were still high when placed 7.5‐cm deep. Application of AS produced chemically equivalent amounts of Ca and Mg whereas urea and NH4OH precipitated Ca and Mg. The amount of Ca and Mg precipitated increased with decreasing NH3 losses. Precipitation of divalent cations enhanced adsorption of NH4 on soil cation exchange sites, thereby reducing upward movement of NH4 and subsequent NH3 losses. The presence of Ca from reaction of AS with CaCO3 enhanced upward movement of NH4 and increased NH3 losses.

The objective of this study was to determine the influence of soil cation exchange capacity (CEC) and depth of incorporation on NH3‐N volatilization from NH4+‐N compounds applied to calcareous soil. This study was conducted in the laboratory on soils with a wide range of CEC. An increasing CEC resulted in decreasing NH3 losses. Ammonium sulfate produced higher soil pH values and NH3 losses than did NH4NO3. The pH of the soil decreased with increasing NH4NO3 application rates. With NH4NO3, percent NH3‐N losses decreased with increasing application rate; however, with (NH4)2SO4, percent NH3‐N losses increased as the application rates increased.Incorporation of the NH4+‐compounds into the soil reduced NH3 losses. Increasing depths of NH4+‐incorporation resulted in reduced NH3 loss. Losses decreased as the CEC of soil increased. The effectiveness of soil depth in reducing NH3 loss was associated with soil water content. Decreasing the soil water increased the effectiveness of soil incorporation for reducing NH3 losses.Two regression equations were developed to describe NH3 losses with respect to CEC, soil pH, time, NH4+‐N application rate and temperature. Correlation coefficients were 0.86 and 0.81 for (NH4)2SO4 and NH4NO3 systems, respectively.