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 ba ses of nitrogen availability indexes and under conditions of very low soil solution concentrations of ammonium and potassium. This informati on 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 i n 10 surface soils and 10 to 85% in their associated subsoils was fixe d, representing from 45 to 1763 mg fixed ammonium-N kg-1. These agricu lturally important U.S. soils, including Alfisols, Aridisols, Inceptis ols, and Mollisols, were subjected to analytical procedures designed t o provide chemical (i.e., autoclave-distillable N) and biological (i.e ., aerobic and anaerobic N mineralization) indexes of available N. Res ults indicated only small fractions (generally <0.1) of the fixed ammo nium were released using the indexes. Subsequent extractions of 168-da y aerobic incubated samples with sodium tetraphenylboron, which precip itates soluble ammonium and potassium in the soil solution, resulted i n 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 a n ammonium/potassium blocking effect. Consequently, the degree to whic h 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 cont aining high contents of fixed ammonium, additional information is now needed to establish the extent to which fixed ammonium is made availab le to crops so that appropriate accounting can be given to this source when calculating supplemental N needs.