THE SOIL AS A SOURCE OF MINERAL SALTS 



145 



there is preferential absorption of univalent ions from the cation 

 exchange sites. 



Several objections have been raised to the contact exchange 

 hypothesis, but none have seriously weakened it. It has been 

 suggested, for example, that the surface area of soil particles in 

 contact with the absorbing region of a root is too small to permit 

 absorption by contact exchange to an appreciable extent. However, 

 at the microscopic and sub-microscopic level, colloidal clay particles 

 present a large surface for cation exchange in close proximity to the 



-0-5/^ 



RCOO' 



RCOOH 

 RH 

 R" 



ROH 



R' 



R.COO' 



R' 



RCOOH 



ROH 



Cytoplasm Cell woll Soil particles 



Fig. 46. Diagrammatic representation of cross section of a root hair cell 



wall showing relationship with cytoplasm and soil particles and direction of 



movement of cations (modified from Frey-Wyssling, 1945). 



root surface. Jenny (1951) calculated that 1 mm^ of root surface 

 can make contact with 10* clay particles each carrying 6000-7000 

 exchangeable cations. A second objection is that the presence of an 

 inert cellulose cell wall between the clay particle and cytoplasmic 

 surface prevents the two systems from making intimate enough 

 contact with one another for direct exchange to occur. However, 

 the cell wall has a definite exchange capacity, and together with 

 excreted mucilage, it can effectively bridge the gap between exchange 

 sites in soil and cytoplasm (Fig. 46). 



3. Micronutrients 



It is a somewhat surprising fact that in spite of the extremely 



