Agriculture for Engineers 4 (3+1)

Lesson 8. Charges on Soil Colloids

8.2. In organic colloids

8.2.1 Sources of negative charges on silicate minerals

There are two ways to account for the negative charges associated with silicate clay minerals.

(1) Dissociation of OH ion at the exposed crystal edges

This involves the unsatisfied valences at the broken edges of silica and alumina sheets. The external surfaces of mineral (kaolinite) have some exposed oxygen and hydroxyl groups which act as exchange sites. These groups are attached to Si and Al atoms within their respective sheets. At high pH, the H of these OH radicals dissociates slightly and colloidal surface is left with a –ve charge carried by the oxygen. The loosely held H is readily exchangeable (Fig. 8.1). This situation may be represented as follows:

Fig. 8.1 : Dissociation of OH^- at crystal edge

The presence of such group gives the clay a negative charge. The phenomenon accounts for most of the adsorbing capacity of 1: 1 type of colloidal clays. These are pH dependent charges of inorganic colloids. The magnitude of pH dependent charges varies with the type of colloids. It accounts for most of the charges of the 1: 1 type minerals and up to one forth of that of the 2: 1 types.

 (2) Ionic substitution (isomorphous substitution)

The overall –ve charge carried out by clay crystal is the substitution of one atom by another similar size in a crystal structure of the minerals is known as ionic or isomorphous substitution. In 2: 1 type minerals magnesium atoms have substituted for the aluminum atoms in the alumina sheet. The substitution is on the basis of one Mg++ atom for each Al+++ atom replaced. Therefore, each substitution is results in an unsatisfied negative valence because a three valent atom is replaced by a two valent one. This is shown below.

Alumina sheet                                                                        Alumina sheet

(No substitution)                                                           (Mg substituted for Al)

O^{– -}Al⁺⁺⁺OH^-                                                                        O^{– -}Mg⁺⁺OH^-                  

No net charge                                                              one excess negative charge

Similarly, in minerals such as beidellite and illite the substitution of three-valent atom such as Al for one of the four-valent silicon atoms in silica sheet leaves an unsatisfied negative valance. This represents below.

Silica sheet                                                                     Silica sheet

   (No substitution)                                                                  (Al substituted for Si)

     O^{– -}Si⁺⁺⁺⁺O^--                                                                     O^{– -}Al⁺⁺⁺O^--

 No net charge                                                             one excess negative charge

The charges resulting from ionic substitution are not pH dependent. So they are referred to as permanent charge.

The source of negative or positive charge for kaolinite is the same and hence there is little difference in CEC and AEC. In kaolinite, pH dependent is the only source while in montmorillonite negative charge is mainly due to ionic substitution and only ¼ by dissociation of OH ion and therefore there is a wide difference between CEC and AEC in the montmorillonite .

The silicate clay minerals thus, posses negative charges due to isomorphous

Substitution and dissociation of OH ion. 'They can attract positively charged cations like Ca⁺², Mg ²⁺, K⁺, Na⁺, H⁺. NH₄⁺. These Cations are adsorbed on the clay surface. They are in exchangeable and available to the plants.

The cations and water are absorbed on the clay surfaces. In general the cations adsorbed on the surface of silicate clays are in the order of:

                      H⁺ > Al₃⁺> Ca⁺² > = Mg ²⁺> K⁺ = NH₄⁺> Na⁺

Certain cations are especially prominent under natural conditions.

(i) Humid region soils                            Al₃⁺> H⁺> Ca⁺²= Mg ²⁺> K⁺ > Na⁺

(ii) Well drained and semi arid soils      Ca⁺²>Mg ²⁺> Na⁺ > K⁺ > H⁺

(iii) Sodic or Alkali soils                               Na⁺ > Ca⁺²>Mg ²⁺> K⁺ > H⁺

8.3 Organic Colloids

Organic colloids are chiefly due to presence of humus in soil. Humus is the product of decomposition of plant and animal residues. Humus colloids are composed of carbon, hydrogen, oxygen and nitrogen, instead of silicon, aluminum and oxygen, as in clay colloids. Organic soil colloids have higher adsorptive properties for water and cation exchange capacity than inorganic colloids. Humus is a temporary intermediate product left after considerable decomposition of plant and animal residues. Temporary because the organic substances remain continue to decompose slowly. The humus is usually referred to as an organic colloid and consists of various chains and loops of linked carbon, hydrogen atoms. Humus is amorphous and the size of the individual micelle is variable. Humus has three components viz., fulvic acid, humic acid and humin. The charge on humus colloids is pH dependent. Under strongly acid conditions hydrogen is rigidly bound and not easily replaceable by other cations. Humus colloid, therefore, exhibits a low negative charge.