Little information exists about the extent to which fixed ammonium in soils is released under conditions imposed during analyses that are bases of nitrogen availability indexes and under conditions of very low soil solution concentrations of ammonium and potassium. This information is needed because fixed ammonium constitutes a significant portion of the total N in many soils. In this study, 5 to 45% of the total N in 10 surface soils and 10 to 85% in their associated subsoils was fixed, representing from 45 to 1763 mg fixed ammonium-N kg−1. These agriculturally important U.S. soils, including Alfisols, Aridisols, Inceptisols, and Mollisols, were subjected to analytical procedures designed to provide chemical (i.e., autoclave-distillable N) and biological (i.e., aerobic and anaerobic N mineralization) indexes of available N. Results indicated only small fractions (generally