| In the Texas Gulf Coast, there are about two million acres of Beaumont and Lake Charles soils (Pelluderts) that require regular N-fertilization to yield a satisfactory rice crop. Nada (Typic Albaqualf) and other similar soils can frequently supply enough native N to produce 4,000 kg/ha or more rice grain. This study proposes to determine whether NH(,4)('+) fixation is one of the reasons why Beaumont and Lake Charles soils require more nitrogen fertilization for rice production.;Two-month incubation of Beaumont soil samples, chemically treated to increase the amount of fixed NH(,4)('+), in the root zone only showed partial release when the sample was Ca-saturated. Nitrogen isotope analysis showed that the released NH(,4)('+) came from the added fraction. The native fixed NH(,4)('+) level was little affected by either the fixation or the incubation procedures. When the Beaumont sample was K-saturated the added NH(,4)('+) was not released at all. Nada soil did not fix any more NH(,4)('+) than its native level during the NH(,4)('+) solution washings not did it release any upon incubation. Lake Charles soil showed clay and fixation characteristics similar to that of Beaumont. The presence of a 2:1 layer silicate in Beaumont and Lake Charles soils with x-ray diffraction characteristics of a smectite and NH(,4)('+) fixation characteristics of a vermiculite is recognized.;The potential for NH(,4)('+) fixation in Beaumont soil was established when a yield reduction occurred upon KCl addition to experimental plots. Equivalent amounts of NaCl did not reduce plot yields, indicating that the yield reduction was the result of entrapment of NH(,4)('+) in clay minerals when K('+) was added. Characterization of the clay minerals showed that Beaumont soil could potentially fix more NH(,4)('+) than Nada soil because of its: (1) greater content of high-charge (0.76 per formula unit) smectite, (2) greater portion of charges located in the tetrahedral sites, and (3) greater amount of exchangeable K('+). |
