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2.6.1.1 Soil texture
Literature review
Soil is generally comprised of soil particles of various sizes (DONAHUE et al., 1983).
Soil texture can be described by granulometry, which is the quantification of the
distribution of these soil particles in the different size classes (LECLERC, 2003).
These size classes are called the soil separates and they can be placed in three
general classes: Sands, silts and clays (Table 2.1) (DONAHUE et al., 1983).
Table 2.1: Names of the soil separates and the particle diameters which define them (Modified
from DONAHUE et al. (1983)).
Soil separate name Diameter range (mm)
Stones > 254
Cobbles 75 to 254
Gravels 2 to 75
Very coarse sand 1.0 to 2.0
Coarse sand 0.5 to 1.0
Medium sand 0.25 to 0.5
Fine sand 0.10 to 0.25
Very fine sand 0.5 to 0.15
Silt 0.002 to 0.5
Clay < 0.002
The texture of a soil can be determined after the percentage of each separate within
a sample is known and these are then grouped into percentage sand, silt and clay
(DONAHUE et al., 1983). These percentages can then be plotted on a triangular
graph (See Figure 2.30). The description of the soil is determined by drawing three
lines each perpendicular to a side of the triangle and arranged on the axes according
to the relative percentages. The description at the point where these lines meet can
then be read off. This method can also be used to determine the percentage content
of a third separate group if the percentage content of 2 is known. Soil texture has a
large influence on plant growth due to its effect on soil water retention capacity,
oxygen capacity and thermal conductivity (LECLERC, 2003). Particles larger than 2
mm but less than 250 mm also play a large role in soil texture (DONAHUE et al.,
1983). When classifying a soil, the names of these separates precede the rest of the
name (e.g. ‘Stony’, ‘silty’ and ‘clay’).
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