Clay Minerals in soil


Soil structure is recognized as one of the most important property of soil mass since it influence aeration, permeability, water capacity etc. Soil structure refers to the arrangement of soil particles both primary and secondary. Clays are the secondary minerals they are hydrated alumina silicates.

The clay fraction

In the process of decomposition relatively stable new minerals are formed from the products of weathering which constitutes largely the clay fraction. The clay particle forms are crystalline and composed of sheets of hydrated alumina and silica linked by oxygen atoms. The nature of clay properties depends on the type of minerals that predominately composed the clay. The clay fractions are included

  1. clay minerals which are crystalline layers silicates

  2. amorphous silicates,

  3. oxides of iron, silicon and aluminum and

  4. clay-sized primary minerals.

Following are three main groups: -

  1. Kaolinite

  2. A unit kaolinite crystal lattice consist of one sheet of silica and one sheet of alumina and called as 1:1 layer silicate these two sheets are held by mutually shared oxygen atoms. It has a low specific surface and low cation-exchange capacity, plasticity, cohesion, shrinkage and the swelling properties.

  3. Montmorillonite

  4. A unit montmorillonite crystal lattice consist of two sheet of silica and one sheet of alumina held together by mutually shared oxygen atoms known as 2:1 layer silicate. There is isomorphic substitution of iron or magnesium in the alumina sheet; montmorillonite crystal can expand hence cations and water molecules are able to move in between the crystal units. It has high specific surface and cation-exchange capacity, plasticity, cohesion, shrinkage and the swelling properties.

  5. Illite

    It has the same general structural organization as montmorillonite about 15% of silica in the silica sheet is replace by aluminum and potassium atoms and they supply the additional connecting linkages between the crystal units. It has properties intermediate between kaolinite and montmorillonite.

Soil colloids

The most active portions of the soil are those which are in the colloidal state. The colloidal state has to phase system; the dispersed phase- fine clay and humus dispersed in the dispersed medium-water. In soils the mineral and organic colloids exists in heterogenous admixture. The clay particles less than one micron in diameter posses colloidal properties and these properties increases with a decrease in the size of the particles. The colloidal material forms a thin gelatinous film around coarser particles serving as a binding material. Soils with a high amount of colloidal clay compete with plant roots for water and mineral nutrients especially at low levels of their availability. The soil colloids have a high exchange capacity, which increases with silica sesquioxide ratio.

Functions/ Properties

  1. Clay fractions are highly reactive, being very small.

  2. Form the seat of ion exchange in soil.

  3. Controls and regulates adsorption retention.

  4. Release of many plants viz. potassium, calcium, magnesium and phosphorus.

(Soil Magt.)