FIXED AMMONIUM AND NITROGEN AVAILABILITY INDEXES

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 <0.1) of the fixed ammonium were released using the indexes. Subsequent extractions of 168-day aerobic incubated samples with sodium tetraphenylboron, which precipitates soluble ammonium and potassium in the soil solution, resulted in most of the fixed ammonium being released in 7 days. Obviously, then, much of the fixed ammonium in soils is available in the absence of an ammonium/potassium blocking effect. Consequently, the degree to which plants can extract soil fixed ammonium will depend on the extent to which they lower ammonium and potassium concentrations in the vicinity of the roots and, thereby, remove the blocking effect. For soils containing high contents of fixed ammonium, additional information is now needed to establish the extent to which fixed ammonium is made available to crops so that appropriate accounting can be given to this source when calculating supplemental N needs.