Ammonium Fixation by High‐Charge Smectite in Selected Texas Gulf Coast Soils

The Beaumont (fine, montmorillonitic, thermic Entic Pelludert) and Lake Charles (fine, montmorillonitic, thermic Typic Pelludert) soils along the Texas Gulf Coast produce only about 2 Mg rice (Oryza sativa L.) ha⁻¹ without N fertilizer, while the Nada soil (fine, montmorillonitic, thermic Typic Albaqualf) frequently produces 5 Mg ha⁻¹. In studying differences between these soils, data showed that NaCl applied to the Beaumont soil did not reduce rice yield, but equivalent amounts of KCl did. The KCl‐induced yield reduction may have been the result of NH⁺₄ entrapment in clay minerals caused by added K. Clay mineral characterization showed that the Beaumont soil fixed more NH⁺₄ than the Nada soil because the Beaumont soil was higher in soil K, high‐charge smectite [i.e., 0.76 equivalents per (Si,Al)₄O₁₀(OH)₂] and charges in the tetrahedral sites. The 8‐wk incubation of Beaumont soil in the rice root zone resulted in partial release of added Nh⁺₄ and no release of native NH⁺₄ when the Beaumont soil had been Ca saturated. The K‐saturated Beaumont soil did not release fixed Nh⁺₄ during incubation as the Ca‐saturated soil did. The Lake Charles soil showed clay and fixation characteristics similar to the Beaumont soil, while the Nada soil did not fix beyond its native level or release any upon incubation. The presence of 2:1 layer silicates in Beaumont and Lake Charles soils with x‐ray diffraction characteristics similar to smectite, and NH⁺₄ fixation characteristics similar to vermiculite, was recognized.