Clay soil is soil in which clay particles, finer than two-thousandths of a millimeter, predominate. Clay particles are secondary minerals; they are formed when the forces of weathering, erosion and oxidation break down the primary particles of sand and silt, which were themselves formed under high temperature and pressure. Depending on what the primary particles chemically consisted of and how exactly they broke down, different kinds of clay minerals may occur.
Chemically, kaolinites are classified as 1:1 type layer silicates; they consist of one tetrahedral sheet containing oxygen linked to an octahedral sheet containing OH ions. Kaolinite is made up of a tightly linked stack of such 1:1 sheets. Physically, kaolinite is a nonexpanding mineral; it cannot absorb water into its layers. A soil high in kaolinites will not swell or shrink much in response to wetting and drying.
Kaolin clay is the active ingredient in a commercial barrier film spray called Surround, manufactured by Engelhard Corp. In orchards, spraying Surround, so as to thoroughly coat the fruit, prevents fruit flies from laying eggs and damaging the crop.
Montmorillonite clay has a 2:1 layer structure. An octahedral sheet of aluminum and magnesium ions is sandwiched between two silicon-tetrahedral sheets. Each sandwich layer is bound to the next loosely. Water molecules get drawn in between the layers, causing the clay to swell significantly. Thus montmorillonite is an expanding clay. A clay soil high in montmorillonites will form wide cracks from when the expanded soil dries and shrinks again.
Montmorillonite clay is used in the manufacture of some fertilizers. It is also added to ponds as a feed supplement for trout and koi.
Another 2:1 type clay, vermiculite contains octahedral sheets of either aluminum or a combination of magnesium and iron, and silicon-tetrahedral sheets with some aluminum ions substituting for silicon. Vermiculite expands upon wetting thanks to its weak layer bonding, though not to the same extent as montmorillonite clay.
Vermiculite particles are available commercially to improve aeration in soil. Spongelike, the particles will also absorb and retain water and nutrients close to plant roots.