Geoscience Reference
In-Depth Information
Soils are seldom structureless (i.e. without structural aggregates) except pure
sand soils, such as dunes, or recently exposed, 'unripened', sediments, such as the
Dutch polders when they were first drained. Grassland or woodland soils of heavy
texture often show block shaped or prism-like structural units with visible cleavages
between them that allow rapid movement of air and water. At a closer view, one can
usually see that the top soil is aggregated into crumbs a millimetre or two in size.
These crumbs retain their identity for a period even after washing in water, and have
their own internal geometries. One can picture a soil crumb as a collection of fine rock
particles and amorphous organic matter held together by packs of clay platelets and
various cementing agents. The size and stability of these crumbs depend a great deal
on the microbial contribution as well as the amount of clay in the soil; a single crumb
of soil may contain a hundred million bacterial cells producing polysaccharide gums,
and five metres of fungal mycelium binding the mineral particles together. When thin
sections through soil crumbs are examined through a petrographic microscope, one
can see many of these features, and calculate that the capillary pores within them may
represent a third to a half of the total volume of the crumb.
C
LAYS AND SOIL CHEMISTRY
The clay minerals are important for their powerful influence on the physical prop-
erties of soils. One acknowledgment of this is seen in the triangular diagram of soil
textures shown earlier: almost any soil containing a clay fraction of more than 40% is
simply called 'clay', while a soil needs about 90% of sand fraction for it to be called
just 'sand'. It is their physico-chemical attributes, however, that really set the clay
minerals in a separate category from sands and silts. Their importance in the cycling
of nutrients parallels that of the soil fauna in the cycling of energy.
In most British soils, the clay fraction has been derived at second hand by release
of clay minerals from sedimentary rocks and glacial drift in which they are already
present. Ultimately, of course, the clay minerals are derived from igneous rocks such
as granite through weathering over very long periods of time. These clay minerals are
composed of two basic units each consisting of a lattice-like sheet. One unit consists
of a silicon-oxygen (SiO
4
) layer, and the other consists of an aluminium-hydroxyl
(Al(OH)
6
) layer. Owing to the way the atoms pack together, these are called silica tet-
rahedra and alumina octahedra. The two kinds of sheets are linked together by shar-
ing some of the oxygens, either as a simple pair of sheets (the 1:1 type) or as an alu-
mina layer sandwiched between two silica layers (the 2:1 type). This layer structure,
coupled with their very small size (less than 0.002mm), gives clay particles a very
large surface/volume ratio; whereas the total surface area of sand grains in a gram
