Fixed ammonium in soils developed on some Queensland phyllites and its relation to weathering

Abstract

Analyses for native fixed ammonium and for other constituents are reported for four soil profiles developed on Permian phyllite. While total nitrogen contents decreased with depth, fixed ammonium nitrogen increased and reached very high values in each subsoil (one contained 7.7 m-equiv. fixed ammonium nitrogen per 100 g dry soil, or 0.108 % N). Nitrogen-15 enrichments of the fixed ammonium nitrogen were not significantly different from those of N2, in the atmosphere. Fixed ammonium nitrogen in one surface soil was unavailable as a source of nitrogen for Setaria sphacelata. Total potassium also increased with depth and was highly correlated with fixed ammonium nitrogen. On the basis of these results and from clay mineral analyses presented it is suggested that the fixed ammonium nitrogen is present in interlayer sites in illite formed front mica in the parent material. The ammonium/potassium molecular ratio differs for each profile but does not change during the weathering sequence mica -> illite -> vermiculite in any one profile.