Silicate clays of soila ~ University Notes

Layered Silicate clays

Layered silicate clays are secondary minerals that have formed as the result of weathering of parent material. There are two major categories of layered silicate clays within the soil: high activity clays and low activity clays.

HIGH ACTIVITY CLAYS

Generally, soils with large amounts of high activity clays are not highly weathered. High activity clays have a high ‘cation exchange capacity’ (CEC), due to their large surface area. This means that these clays have a great capacity to retain and supply large quantities of nutrients, such as calcium, magnesium, potassium, and ammonium. Not only do these clays have a large CEC, but they will generate CEC under all soil conditions regardless of soil pH. As a result, these clays tend to produce highly fertile soils. Examples of these clays are montmorillonite (and other smectites), vermiculite, illite, and mica.

Cation Exchange Capacity or Anion Exchange Capacity?

High activity clays have a cation exchange capacity (CEC). Although high activity clays will not have anion exchange capacity (AEC), the CEC increases as pH increases and decreases as pH decreases.

Montmorillonite

Some high activity clays, such as montmorrillonite, have a shrink and swell potential. This means that the clays will shrink and crack when dry, and expand and swell when wet. With little additions of nutrients, these soils may be very productive. However, the shrink and swell potential will result in poorer drainage. And so, proper management of irrigation is required.

LOW ACTIVITY CLAYS

In contrast, low activity clays are more highly weathered. Thus, due their lesser surface area, low activity clays have a lower capacity to retain and supply nutrients. In addition to CEC, low activities clays can also have AEC, depending upon the pH of the soil. The AEC causes these clays to retain and supply nutrients, such as phosphate, sulfate, and nitrate, rather than the base cations, under acidic conditions. Yet, under neutral and alkaline conditions, these low activity clays generate a CEC.

Cation Exchange Capacity or Anion Exchange Capacity?

Under acidic conditions, low activity clays have an AEC
Under neutral and alkaline conditions, low activity clays have a CEC