COLLOID CHEMISTRY OF THE SOIL IX RELATION TO PLANT NUTRITION 731 



Dynamic factors 



So far we have eonsidered ehiefly the static factors in the passage 

 of nutrient cations from soil to plant root. What are the dynamic factors 

 on the soil side and how can they be assessed? 



It is clear that ions can move in the soil by two chief mechanisms: 

 by mass movement of solvent and by ionic diffusion. It appears that 

 plants do not normally depend on mass movement of water for their 

 cationic nutrition, although they probably obtain considerable amounts 

 by this route. Growth in a saturated atmosphere appears to be en- 

 tirely similar to that in an unsaturated one, although highly critical ex- 

 periments on such effects are ver\- difficult to carry out. Nevertheless it 

 is important to consider the general situation as regards plants and this 

 aqueous environment. 



The existence of a fairly well-defined wilting point sets a limit to 

 the chemical potential of water utilizable by plants. Approximately, 

 for many species, the wilting point lies near pF 4.2, which corresponds 

 to a suction or osmotic-pressure difference of about 15 atmospheres. 

 Beyond this point, the mass moxement of water over solid surfaces is 

 extremelv slow, and redistribution is effected mainly through the vapor 

 phase. 



With intermittent rainfall, plants take most of their water under 

 tensions between about one-third of one atmosphere (which corre- 

 sponds roughly to field capacity ) and 15 atmospheres. Even at one-fifth 

 atmosphere, maSs movement of water is very slow. Richards (quoted 

 by Baver, 1956) found that at this tension, capillary conductivity 

 varied from 0.002 per cent ( in a sand ) to 0.16 per cent ( in a silty clay ) 

 of the respective values at saturation. 



The existence and properties of root potentials first described by 

 Lundegardh ( 1943 ) indicate that a general resemblance bet\\'een nega- 

 tively charged colloidal membranes and roots can be established, but it 

 does not afford a detailed view of the mechanism of uptake of any 

 particular ion. If this were all that was involved, cations would be 

 more freely admitted than anions. Special mechanisms superimposed 

 upon the simple membrane picture are invoked to explain uptakes of 

 anions. But if this can be demonstrated for anions is it not possible 

 that particular mechanisms may control the uptake of individual 

 cations? This is, in effect, the basis of the Epstein and Hagan formula- 

 tion, suggesting that a small proportion of the total exchange sites on 

 the root surface are responsible for most of the uptake, through their 

 higher bonding energy for particular cations. 



Such considerations would not in any way upset the validity of the 

 Donnan relationships involving activity ratios of pairs of cations, pro- 



