Agriculture Reference
In-Depth Information
International or Atterberg system
Clay
Silt
Fine sand
Coarse sand
Gravel
0.002
0.02
0.2
2.0
U.S.D.A. system
Ver y
Fine
sand
Medium
sand
Coarse
sand
Clay
Silt
coarse
sand
Gravel
0.002
0.05
0.01
0.5
1.0
2.0
Soil Survey of England and Wales, British Standards and Massachusetts Institute of Technology
Fine
sand
Medium
sand
Coarse
sand
Clay
Silt
Gravel
0.002
0.06
0.2
0.6
2.0
Diameter (mm) (log scale)
Figure 2.2
Particle size classes most widely adopted internationally (White 1997). Reproduced with
permission of Blackwell Science Ltd.
partment of Agriculture, or USDA (the Natural Resource Conservation Service),
the British Standards Institution, and the International Society of Soil Science.
These are illustrated in figure 2.2.
2.2.1.1
Particle Size Analysis
Particle size distribution can be measured accurately in the laboratory or estimated
“by feel” in the field. The principles of the laboratory analysis are outlined in box
2.1. The outcome of the laboratory analysis can be summarized in a textural tri-
angle of the kind shown in figure 2.3. Depending on the proportions of sand, silt,
and clay, the soil is assigned to a textural class. For example, a soil with 40% sand,
40% silt, and 20% clay is classed as a loam in the USDA system. The main dif-
ferences between the International System, which was adopted in Australia, and
that used in the United States lie in the upper size limit for silt and in the num-
ber of subdivisions in the sand fraction (see fig. 2.2).
2.2.1.2
Field Texture
In the field, a soil surveyor assesses soil texture by moistening a sample with wa-
ter until it glistens. The sample is then kneaded between fingers and thumb un-
til the aggregates are broken down into individual particles, which must be thor-
oughly wet. The proportions of sand, silt, and clay are estimated according to the
following qualitative criteria:
Coarse sand grains are large enough to grate against each other and can be
detected individually by sight and feel.
Fine sand grains are much less obvious, but when they comprise more than
about 10% of the sample they can be detected by biting the sample
between the teeth.
 
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