The ability of the organic and inorganic soil components to fix added ammonium was assessed using 16N as a tracer. In surface soils between 20 and 90 per cent of the ammonium retained after four cycles of alternate wetting and drying was due to organic matter. In subsoils containing little organic matter ammonium was fixed almost exclusively by layer silicate minerals.
I Present address: College of Science and Technology, Port Harcourt.fairly high indicating the possi g ility that clay minerals other than Lutz, 1965). Clay minerals of t i e I : I type or kaolin group fix little minerals such as montmoril K onite, beidellite, vermiculite, illite, and Ammonium can be fixed as a result o I chemical interaction between i l Andersson, 1943Andersson, ,1954 rf yborg, 1969). When ammonium is applied to Ip
S u m m yIn soil profiles from Nigeria native fixed ammonium was positively and significantly correlated with per cent silt, per cent clay, per cent silt plus clay, pH, exchangeable potassium and cation exchange capacity but not with exchangeable ammonium or organic matter. The results of multiple and partial regression analyses indicated that native fixed ammonium was most closely correlated with per cent clay, exchangeable potassium, and cation exchange capacity.
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