258 PLANT BIOCHEMISTRY 



variable amounts of other basic elements. Since colloidal particles 

 are usually considered to be less than 200 ni/j, (millimicrons) in diam- 

 eter, such particles will be found only in the clay fraction of soils. 

 Particles with diameters less than 0.002 mm. (2,000 m/x) are classified 

 as clay. Inorganic soil colloids are thus referred to as clay colloids, 

 although not all the clay fraction is of colloidal size. 



We have spoken of the diameter of clay colloids, but this does not 

 mean that the colloids are spherical. Actually they are composed of 

 laminated plates; that is, these particles are built up of several thin 

 sheets of crystalline minerals. Such particles are present in a wide 

 variety of shapes and sizes. Let us assume that a clay particle com- 

 posed of at least two lamellas can be represented by Figure 11-1. Be- 

 cause of its fine size and laminated structure this clay particle contains 

 a large amount of external and internal surface. This surface is elec- 

 trically charged and shows a definite electrical potential. The poten- 

 tial (zeta potential) is due to the presence of a double layer of ions 

 called the Helmholtz double layer. The inner layer of ions is a fixed, 

 negatively charged coat which forms an integral part of both the in- 

 ternal and external surfaces of the clay particle. It is the charge of this 

 inner layer of anions which determines the characteristic charge of 

 the colloidal micelle. Since this fixed inner layer has a negative charge 

 in fertile soil, these soil colloids are negative and will migrate to the 

 positive pole under the influence of an electrical cinrent. 



The outer layer of ions consists of positive ions which are readily 

 exchanged for other cations. Calcium ions and hydrogen ions are 

 present in greatest number, but potassium, magnesium, sodiimi, and 

 other ions are also present. All these cations are hydrated, the number 

 of molecules of water associated with each being a characteristic of the 



-I- + + -I- + + 



+ + + + + 



FIGURE 11-1. Charged surfaces of two lamellas of a clay particle. 



