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ABSTRACT
Fixed ammonium was determined in 17 profiles representative of most of the great soil groups occurring within the North Central Region of the United States. The results showed that a vast reservoir of N exists in the terrestrial areas of the world as ammonium held within the lattice structure of clay minerals, thereby revealing a heretofore missing link in the N cycle.
The amount of fixed ammonium in the soil was found to depend upon the type and amount of clay minerals present. With respect to clay mineral type, the order was illite > montmorillonite > kaolinite. Drainage, type of vegetative cover, and extent of leaching of the profile by percolating water had little effect on the fixed ammonium content of the soil. Cropping decreased the amount of fixed ammonium in surface soil; however, the fraction of the N present as fixed ammonium was greater for cropped soil than for uncropped soil.
Contribution from the Department of Agronomy, Illinois Agr. Exp. Sta., Urbana. This work includes part of a thesis submitted to the University of Illinois by the junior author in partial fulfillment of the requirements for the Ph.D. degree, and was supported in part by Regional Project NC-17 on organic matter and soil structure. Published with the approval of the Director of the Illinois Agr. Exp. Sta. Presented before Div. III, Soil Science Society of America, Lafayette, Ind., Aug. 6, 1958.
2 Associate Professor of Soil Biochemistry and Research Fellow, respectively.
Received for publication August 8, 1958. Accepted for publication October 2, 1958.
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