Agriculture Reference
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polymers (fulvic acids) have molecular weights from 1 to 10 kDa and humic acids
range from 5 to >1‚000 kDa (Swift et al.‚ 1979).
One current three dimensional model of soil organic matter molecules suggests a
complex structure with an elemental composition of and
an approximate elemental composition of 54 % C‚ 5.2 % H‚ 4.7 % N‚ 35.7 % O and
0.4 % S (Schulten and Schnitzer‚ 1997). A feature of these macromolecules is their
association with a range of simpler materials (proteins‚ polypeptides‚ amino acids‚ etc.)
trapped and occluded within the voids in the macro-molecular structures‚ or held physi-
cally or chemically at their surfaces (Schulten and Schnitzer‚ 1998).
The high molecular weights of these molecules‚ their abundant aromatic structures‚
frequent bonds with clay minerals and complex formation with metal cations make them
only slowly susceptible to microbial degradation (see review by Theng et al.‚ 1989).
Some results suggest that the constitution of organic matter in many tropical soils would
be dominated by alkyl-C‚ a particularly recalcitrant form of soil C with a polymethylene
structure. In acid soils‚ these compounds would acquire a random coil shape which may
facilitate their protection within the micropores of clay structures (Schnitzer‚ 1986;
Theng et al.‚ 1989). The aromaticities and molecular weights of humic acids do not seem
to differ significantly between temperate and tropical soils (Arshad and Schnitzer‚ 1989).
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