Here is a link to an interesting review of soluble organic nitrogen.
D. V. Murphy, A. J. Macdonald, E. A. Stockdale, K. W. T. Goulding, S.
Fortune, J. L. Gaunt, P. R. Poulton, J. A. Wakefield, C. P. Webster, W. S.
<review article> Soluble organic nitrogen in agricultural soils
Biol Fertil Soils 30 (2000) 5/6, 374-387
A few surprising (to me) quotes:
SON is soluble organic nitrogen, TSN is total soluble nitrogen
"Our data (Table 1) show,
for a wide range of soil types (0 to 30-cm layer), a KCl-extractable
SON-N content of generally 20–30 kg SON-N
ha –1 and a very constant ratio between mineral N
and SON in arable soils, with SON comprising about
"Our measurements of SON in the 0 to 5-cm
layer of a range of soils on an organic farm showed that
it accounted for 80% of TSN and ranged from 24 to
46 kg SON-N ha –1"
"It is well established
that some plants are capable of utilising SON
either directly (Chapin et al. 1993; Kielland 1994) or in
association with mycorrhiza and ectomycorrhizae (At-kin
1996; Kielland 1994; Michelsen et al. 1996)."
"It is not clear whether agricultural
crops benefit from SON either by direct uptake or
via microbial immobilisation of organic molecules and
then re-mineralisation of NH4-N."
Lauzier Agricultural Systems Educator
Washington State University Cooperative Extension
Center for Sustaining Agriculture and Natural Resources
PO Box 37, Courthouse
Ephrata WA 98823
509-754-2011, Ext. 412
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