ABSTRACT
It has been known since the early part of the twentieth century that some types of soils have
the ability to bind ammonium (to certain types of clay minerals, predominantly vermiculite
and mica types) such that it is not readily recovered by extraction with dilute acid or alkali
(McBeth 1917). This form of ammonium is referred to as fixed or nonexchangeable
ammonium (NEA). Barshad (1951) proposed that fixed ammonium should be defined as
ammonium that is not displaceable with prolonged extraction or leaching of soil with
potassium salt solution. The proportion of soil N as NEA usually does not exceed 10% in
surface soils, but it can increase with depth of soil to over 50% in some subsoil horizons
(Hinman 1964; Bremner 1965). Sources of NEA in the soil include (i) NH þ4 produced by mineralization of organic matter, and added through ammoniacal-N fertilizer material, and
(ii) indigenous or native fixed ammonium found in parent rock materials. There is consid-
erable interest in quantifying the NEA pool because the amount in the soil through the
rooting depth can be considerable, and its availability to plants and microorganisms has been
demonstrated in many studies (Kudeyarov 1981; Scherer 1993; Green et al. 1994; Scherer
and Werner 1996; Soon 1998). Soderland and Svensson (1976) estimated that there is as
much fixed NH þ4 -N as there is plant biomass N in the global soil-plant system. The NEA pool in the soil has been found to be a slow-release reservoir of available ammonium when
the exchangeable NH þ4 levels become depleted (Drury and Beauchamp 1991). Ammonium fixation and release must be characterized and quantified especially in soils with a high
ammonium fixation capacity (i.e., soils with a high vermiculite or mica content) in order to
efficiently manage N use in soils for agronomic and environmental reasons.
