Geoscience Reference
In-Depth Information
F
IG. 4
Identification of soil texture class according to the sand : silt : clay content, and the proportions of each
particle size class in a typical sandy clay (A) and a clay loam soil (B). (Based on the classification system
for soil textures adopted by the Agricultural Development and Advisory Service in 1984.)
Whereas fine sands are comparable with caster sugar, and silt with icing sugar
in particle size, the individual clay particles are an order of magnitude smaller still.
Less than two thousandths of a millimetre in size, they are released from sediment-
ary rocks or derived from primary minerals by chemical weathering. To a large ex-
tent, they are the engines of chemical activity in soils, as described below. Physically,
clay often dominates the textural characteristics of soils; their downward mobility in
undisturbed soils produces distinctive soil types (
chapter 2
), while their influence on
tillage is described in
chapter 8
.
The size limits of sand, silt and clay particles are thus not purely arbitrary but
relate to the observed behaviour of soils and to their feel. The triangular diagram in
Figure 4
is the latest of several schemes, used nowadays in both Britain and Amer-
ica, for classifying soil textures according to the proportions of these fractions. Any
particular soil can be defined by the percentages of sand, silt and clay that it contains.
