THE SOIL AS A SOURCE OF MINERAL SALTS 143 



grown in the leached soil and they absorbed significant amounts of 

 rubidium. Other experiments show that plants can absorb from 

 soil potassium which is difficult to remove by leaching. Leaching of 

 a soil with carbonic acid for 10 days yielded only half as much 

 potassium as that absorbed in the same period by rye plants. 



Prolonged absorption of metallic cations in exchange for 

 hydrogen ions through contact exchange might be achieved by 

 continuous synthesis of acidic colloidal substances (for example, 

 pectic and pectinic acids) at the root surface, coupled with growth 

 to bring them into contact with fresh particles of soil. Alternatively, 

 after exchange has occurred, the adsorbed cation at the root surface 

 may be transferred elsewhere, and replaced at the adsorption site by 

 a hydrogen ion which can participate in another exchange reaction 

 with the soil. 



The presence of exchangeable hydrogen ions at root surfaces can 

 be detected either by displacing them into solution with other 

 cations, or by measurements of the electrokinetic potential difference 

 between root surface and external solution. Lundegardh (1954) 

 demonstrated with a cathode ray oscilloscope that whole wheat 

 roots immersed in a dilute salt solution have a negative electro- 

 kinetic potential of about 60 mV. The rapidity with which the 

 potential difference alters when the composition of the medium is 

 changed suggests that the site of the electrical charge concerned is 

 the root surface. The potential can be attributed to dissociation of 

 acidic substances, and it behaves essentially as a Donnan potential 

 (p. 34). In the absence of metallic cations in the medium, the free 

 acids present at the root surface cause it to behave as a hydrogen 

 electrode. The measured potential is related to the logarithm of 

 [H,.'^]/[H/] where [H,"^] and [H^"^] represent the concentrations of 

 hydrogen ions in the root surface and in the medium respectively. 

 If a neutral salt is present in the solution, other cations exchange 

 with hydrogen ions and the potential is lowered by an amount which 

 is approximately proportional to the logarithm of the salt con- 

 centration. 



The maximum number of readily exchangeable cations (including 

 hydrogen ions) held by a given weight of roots (cation exchange 

 capacity) shows wide variation between different species (Table 15). 

 In general dicotyledonous roots have higher C.E.C. values than do 

 those of monocotyledons, while species with thick mucilaginous 



