Agriculture Reference
In-Depth Information
mass is 200-1500 kg/ha (up to 900 kg/ha in vineyards), whereas the microbial
biomass is 0.5-2 t C/ha.
■
SOM
serves as a reserve of C and other essential elements for successive genera-
tions of organisms. Elements sequestered in the biomass are said to be
immobi-
lized
. Any surplus released in inorganic form is
mineralized
. For N, a substrate
C:N ratio
25 favors an increase in microbial biomass and
net immobilization
,
whereas a C:N ratio
25 favours
net mineralization.
■
Humus is conveniently fractionated into
fulvic acid
(FA),
humic acid
(HA), and
a nonextractable residue of
humins
. The FA and HA fractions are made up of
macromolecules with a “core” of substituted aromatic and aliphatic C groups. A
high density of acidic carboxyl (-COOH) and phenolic-OH groups confers a high
CEC
(150-300 cmol of charge/kg), which is pH-dependent. These acidic groups
also form stable complexes (
chelates
) with metallic cations.
■
The rate of decomposition of litter and
SOM
can be expressed in terms of the
half-life of the constituents, which ranges from
1 month to
50,000 years. The
rate of decomposition is modified by clay content, moisture, temperature, and
pH.
