Geoscience Reference
In-Depth Information
Large complex organic humus
molecule consisting of chains
and rings of mainly carbon and
hydrogen actoms
O
C
OH
OH
Carboxyl
group
OH
C
OH
2
O
Phenolic
hydroxyl
group
Alcoholic
hydroxyl
group
Figure 3.5
Organic humus structure (
(after
Horowitz, 1991).
3.5 SOIL ORGANIC COLLOIDS
Colloids are the most chemically active fractions of soils. They are very small: less than
2
m in diameter. They are mineral (clays) or organic (humus). They can be crystalline
(definite structure) or amorphous. Clay and humus are dynamic and very active in
charge by comparison with sand and silt that are static. Colloids can impart chemical
properties (the source of ions, source of electro-negativity and buffering capacity) and
physical properties (the large surface area per unit of mass and the plasticity) to soils.
The types of soil colloid are: (i) crystalline silicate clays, (ii) non-crystalline silicate
clays, (iii) iron or aluminium oxide and (iv) organic material (humus).
Electrical charges are carried by the surface of soil colloids, and these surface
charges are the main cause of a series of surface properties. The quantity of ions that
are adsorbed on the surface of soil colloids can be determined by the quantity of surface
charge. In addition, the surface charge properties of the soil can affect the migration
of ions in a soil, dispersion, flocculation and swelling. Therefore the surface charge
properties have a key role in soil structure. The sources of charges on colloids are:
(i) permanent or constant charges due to isomorphous substitutions (montmorillonite,
illite, zeolite etc.), and (ii) variable or pH-dependent charges due to broken edges and
OH and COOH groups (kaolinite, humus and Al or Fe oxides) (Yu, 1997; Brady and
Weil, 2007).
For soils with high organic content, humus is the most important source of variable
charge. For soils with variable surface charge, oxides, hydrated oxides of iron and
aluminium, and the edge surfaces of clay minerals of soil are responsible.
There is a large quantity of acid groups in humus. The origin of charge is the disso-
ciation of the acid groups. A large quantity of charge is carried by humus, in the range
of 200 to 500 cmol kg
−
1
. These charges come mainly from carboxyl groups. Hydroxyl
groups, including phenolic hydroxyl, quinonic hydroxyl and enolic hydroxyl groups
can also produce charge (Figure 3.5). The role of phenolic hydroxyl groups under alka-
line conditions is considerable. In soil humus, carboxyl and hydroxyl groups account
µ
