Geology Reference
In-Depth Information
Soil organic matter has been traditionally fractionated into a number
of subgroups (Figure 10).
Humin and humic acid are high molecular weight fractions (410,000
Da) and it is thought that humin is not extracted by alkali because it is
too strongly bound to the soil mineral component. Fulvic acid is the low
molecular weight fraction (
o
10,000 Da). There are some chemical
differences between humic acid and fulvic acid, the latter being more
acidic and having a higher oxygen content and lower carbon content
than humic acid. This reflects the greater number of carboxylic and
phenolic groups in the fulvic acid fraction. It is speculated that humic
acid forms the less reactive backbone or core of the humic material,
while the more reactive fulvic acid is present as side chains branching
from the core.
Although there is considerable variation between soils, the ratio of the
major elements, C:N:P:S, in humic material is approximately 100:10:2:1,
forming a major reservoir of N, P, and S that is made available to
plants as soil organic matter, is broken down by soil micoorganisms.
Soil organic matter
Floatation
Sinks
Floats
Humic material
Non-humic material
Extraction
with alkali
Insoluble
Soluble
Humin
Humic colloids
Acidify to pH1
Insoluble
Soluble
Humic acid
Fulvic acid
Figure 10 Fractionation scheme for soil organic matter
