ARCHITECTURE OF THE SOIL WORLD - The Soil - page 15

amounts to a few hundredths of a square metre, the total surface area of clay particles
may lie between 50-300 square metres per gram.
Some 2:1 clays, such as the montmorillonite-vermiculite group, can take up wa-
ter molecules between the lattice layers and release them again, and are therefore
called 'expanding' minerals. Expansion and shrinkage of clays are well known phe-
nomena which can affect the stability of buildings; however, the non-expanding il-
lites, or mica-clays, are the more common 2:1 clays in British soils. The most prom-
inent 1:1 clay on a world scale, kaolinite, is also a subordinate clay in Britain. In its
industrial form it is known as china clay and is quarried in Cornwall (see chapter 10 ) .
Some of the silicon atoms in the clay crystals can be replaced by aluminium, and
some of the aluminium atoms by magnesium, producing a complex assemblage of
different clay mineral 'species'. As a result of these replacements, the clay particles
carry a negative charge and so attract positive (basic) ions such as calcium, potassium
and ammonium which are held on the surface of the platelets. Since such cations are
not tightly bound in the crystal structure they can be readily 'exchanged' with other
cations in the soil solution. Different clay minerals have different capacities for ion
replacement; thus the relative cation exchange capacity (c.e.c.) of kaolinite, illite and
montmorillonite is 1:4:10.
It is this feature that renders the clay minerals a key component of the soil world.
They act as a 'buffer' controlling throughput and release of nutrient cations in the way
that soil humus acts as an anion (negative ion) buffer retaining and supplying nitrate,
sulphate and, to an extent, phosphate ions. Soil humus has a dual role in that it also
has a cation exchange capacity as great or greater than that of montmorillonite; it can
provide virtually the entire c.e.c. reservoir in sands, and can thus be the critical nutri-
ent store. However, most soils have more clay than organic matter, and clay is a more
permanent feature of soil composition, so c.e.c. normally depends heavily on the clay
fraction.
The application of ammonium sulphate fertilizer to the soil as a source of ni-
trogen provides a practical illustration of clay chemistry. The ammonium ions attach
themselves to the clay lattice displacing calcium ions in the process. The calcium may
remain in soil solution or it may be washed out of the soil as calcium sulphate. When
the ammonium is subsequently converted by nitrifying bacteria to nitrate (see chapter
6 ) , this, in turn, can be neutralized by more calcium. Nitrate ions are then available to
be taken up by plant roots, so releasing the calcium to re-enter the lattice in an ion-
ic merry-go-round. If the nitrate is not taken up, then calcium nitrate is likely to be
leached out and lost into the ground water where it may eventually cause undesirable
effects. So long as the soil contains a reserve of lime (calcium carbonate), loss of cal-
cium through this process falls on this reserve, and not on the exchangeable calcium.