Agriculture Reference
In-Depth Information
Specific Surface Areas According to Particle Size and Mineral Type
Table 2.1
Specific Surface Area (m
2
/g)
Mineral or Size Class
Coarse sand
0.01
Fine sand
0.1
0
0
Silt
1.0
Kaolinite
5-100
Hydrous micas (illite)
100-200
Vermiculites and mixed-layer minerals
300-500
Montmorillonite (Na-saturated)
750
a
Iron and aluminum oxides
100-300
Allophanes
1000
b
a
The maximum value for completely dispersed montmorillonite particles of
1-nm thickness
b
Includes internal and external surfaces of the mineral
tant because they too influence the reactivity of the surfaces. Both these topics are
discussed here.
Assessing Soil Texture
All soils show a continuous distribution of particle sizes, called a frequency dis-
tribution. This distribution relates the number (or mass) of particles of a given
size to their actual size, measured by the diameter of an equivalent sphere. When
the mass of particles in each size class is summed sequentially, we obtain a cu-
mulative frequency distribution of particle sizes. Some examples are given in fig-
ure 2.1. It is convenient to divide the continuous distribution into several broad
class intervals that define the size limits of the
sand, silt,
and
clay
fractions. The
class limits vary to some extent between engineers and soil scientists, and from
country to country. The major systems in use are those adopted by the U.S. De-
2.2.1
100
Cumulative distributions of
particle sizes for a typical clay,
sandy silt loam, and sandy soil
Figure 2.1
80
Clay soil
(White 1997). Reproduced with
permission of Blackwell Science
Ltd.
60
40
Sandy silt
loam
20
Sandy soil
0
Increasing particle diameter
(log scale)
