Properties Of Clay

The base exchange property of clay is outstanding and of importance. Learn more!

There are two types of clays that are recognized, the silicate clays of temperate regions and the iron and aluminum hydroxide clays found in the tropics and semitropics. The great agricultural regions of the world are dominated in a large degree by clays of a siliceous nature. All clay particles are crystalline and not amorphous as was originally supposed.

Each clay particle regardless of its individual shape is made up of sheetlike molecules or units, held loosely together. Clay particles will also show considerable variation in size. The platelike molecules themselves have a lamellar organization, their sheets of atoms being two or three in number. These units are quite definite, usually changing in size only by lateral extension. A clay particle might be visualized by comparing it with a piece of mica as the flakes of the latter represent the platelike molecules or units.

Clay particles because of their fineness of division must expose a large amount of external surface. There are also internal surfaces as well, the sum of which usually greatly exceeds that of a superficial character.



It has been shown that clay particles are composed of two distinct parts, the inner, porous, and enormously larger insoluble acidoid, or micelle, and the outer and more or less dissociated swarm of cations with variable amounts of water of hydration. Since these absorbed cations are usually rather easily displaced, they are spoken of as exchangeable ions. This replacement, called ionic exchange, or more commonly base exchange, is one of the most important of all soil phenomena.

Calcium and magnesium are the absorbed metallic cations held in the largest amounts by the siliceous clays of most natural soils. Since so much of the total calcium is replaceable, its activity is assured. The main concern, therefore, is the amount present thus we use the practice of liming. With potash the total amount is often ample, but the proportion active is exceedingly small.

Two groups of clay are commonly recognized, the kaolin and the montmorillonite. The molecules of the kaolin are thought to be composed of two sheets or plates, one of silica and one of alumina. The second group, the montmorillonite, is composed molecularly of two silica sheets and one of alumina. The molecules of these clays are less firmly linked together than those of the kaolin gorup and are usually further apart.

In discussing the mineralogical nature of silicate clay, it must not be forgotten that other minerals besides the ones mentioned are present, either as mere accessories or as an important part of the coloidal complex. Of these, the hydrated oxides of silicon, iron, and aluminum should be mentioned. While these probably occur but sparingly in temperate-region soils, the latter two are especially important in tropical and semitropical regions, giving rise to what are spoken of as laterite soils. The silicate clays often contain a larger and larger admixture of colloidal iron and aluminum oxides. The red and yellow soils of our southern states are very good evidence of this transition.

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