COLLOID CHEMISTRY OF THE SOIL IN RELATION TO PLANT NUTRITION 733 



within the cyhndrical water envelope, processes of diffusion in true 

 solution will operate, the constant source of metallic cations being the 

 Donnan external phase. A quasi-steady state will be reached when the 

 rate of radical movement of the boundary between the layers is in- 

 versely proportional to the distance from the center of the root to the 

 boundary. 



But since the water layer itself is relatively thin, the layer of 

 changed composition will soon move outward into the colloidial phase. 

 The governing parameters now change drastically. We deal with ex- 

 change of ions in the colloidal phase and with ionic movements in the 

 overlapping atmospheres of individual particles. 



Thus, although the colloidal phase may not provide the immedi- 

 ate environment of the root surfaces in natural soils, ionic processes 

 within the colloidal system soon become governing as regards the re- 

 lease and movement of nutrient ions. What properties will then be of 

 greatest significance in the operation of the quasi-steady state here 

 envisaged? One is the self-diffusion of cations in colloidal systems. 

 There is evidence from the work of Bloksma ( 1957 ) that this type of 

 diffusion is less rapid than the corresponding diffusion in the equilib- 

 rium dialysate. Thus at 25° C. the coefficient of self-diffusion of the 

 sodium ion in a 6.2 per cent bentonite paste was 0.56 x 10"^, as com- 

 pared with values of around 2 x 10'^ for low concentrations of salt in 

 water. Part of this reduction was ascribed to a tortuosity factor and 

 part to the limited dissociation of the sodium ion from sodium clay. 

 The mean diffusion coefficient of the molecule Nal in the clay paste 

 was 1.08 X 10'°, as compared with about 2 x 10'^ in water alone. Thus 

 the self-diffusion of cations and of salts is considerably reduced by the 

 presence of clay. 



Furthermore, in capillary systems such as natural soils a decrease 

 in moisture content greatly decreases the rate of self-diffusion of ions, 

 whether they are in true solution as salts ( Klute and Letey, 1959 ) or in 

 the overlapping ionic atmospheres of colloidal particles (Kemper, 

 1960 ) . Thus the relationship between moisture stress and cation uptake 

 is likely to impose limitations as regards plant nutrition. But very little 

 is known about this directly. The recent work of Danielson and Russell 

 (1957) indicates a roughly logarithmic decrease of Rb^^ uptake with 

 increasing moisture tension. 



Another major factor influencing the quantity of a nutrient ion 

 moving toward the root in unit time is obviously the exchange reaction 

 between the outwardly diffusing hydrogen ion and the soil colloid. The 

 position of equilibrium temporarily attained at each distance from the 

 root will be determined by the respective activities and bonding ener- 

 gies of the hydrogen ion and the nutrient cation. In clay and soil sys- 



