Calcium and magnesium effects on ammonia adsorption by soil clays

Anhydrous ammonia is a widely used N fertilizer and its interactions with soils and soil clays play an important role in its environmental fate. This study was conducted to determine the quantity and forms of ammonia adsorbed by clay-sized fractions of soils as a function of water content, exchangeable cation, and organic matter (OM). Fourier transform infrared spectroscopy was used to evaluate in situ the mechanisms of interaction of H ₂O, NH ₃, and NH ₄ ⁺ with the clay-size fractions of a Blount loam (fine, illitic, mesic Aeric Epiaqualfs) and a Fayette silty clay loam (fine-silty, mixed, mesic, superactive Typic Hapludalfs). Due to NH ₃ dissolution in adsorbed water more total N was sorbed at high (90%) than at low (2%) relative humidity (RH) despite decrease in the amount of NH ₄ ⁺ sorbed. At high RH, the amount of NH ₄ ⁺, NH ₃, and total N increased by 12 to 23% on the Mg-exchanged compared with the Ca-exchanged soil clays. Of the two soil clays, the smectitic sample (Fayette) sorbed more of both N species than the illitic sample (Blount). Samples with OM removed adsorbed significantly more ammonia than untreated samples. The mechanism suggested for ammonia sorption by soil clays is a combination of protonation on water associated with metal cations, coordination to the exchangeable cations and dissolution in pore water. Soil clays can retain significant amounts of ammonium in excess of the cation exchange capacity (CEC) and out of competition for exchange sites. Dissolved NH ₃ constituted the majority of N adsorbed by the sample at high RH, which is typical of field conditions.