142 MINERAL SALTS ABSORPTION IN PLANTS 



takes place between hydrogen ions and adsorbed cations, as a result 

 of which the latter enter the soil solution. The released cations then 

 diffuse to the root surface where they are absorbed either by exchange 

 for hydrogen ions or in association with anions. Overstreet et al. 

 (1942) observed that cations are absorbed in excess of anions by 

 barley roots from clay suspensions and concluded that organic acids 

 synthesized in the root are the ultimate source of hydrogen ions 

 replacing other cations on the clay particles. Their observations do 

 not preclude the possibility that bicarbonate ions are taken up in 

 association with cations and converted to organic acid anions within 

 the plant, at least from alkahne soils (cf. p. 52). 



As an argument against the carbon dioxide hypothesis, Elgabaly 

 et al. (1943) demonstrated that under comparable conditions barley 

 roots absorb more potassium and zinc from montmorillonite clay 

 suspensions than from kaolinite clay suspensions, whereas release of 

 cations from the two clays by carbonic acid solutions was greater 

 from kaolinite than from montmorillonite. Nevertheless it seems 

 likely that this mechanism plays an important part in the release of 

 exchangeable ions from the soUd phase into soil solutions. 



b. Contact exchange. There is some evidence that cations can 

 exchange between roots and soil without intervention of the soil 

 solution. Devaux (1916) drew attention to the cation exchange 

 properties of roots and suggested that roots and soil particles, being 

 in intimate contact with one another, might form a single colloidal 

 system through which cations move by exchange. Jenny and 

 Overstreet (1938) elaborated the idea of "contact exchange" as 

 follows : adsorbed ions are not held rigidly at the site of adsorption, 

 but vibrate around this point, thus occupying from time to time a 

 finite volume— the so-called "oscillation volume". When the 

 oscillation volumes of two adsorbed ions overlap, contact exchange 

 can occur (Fig. 45b). 



Evidence for direct exchange of cations between roots and the 

 solid phase of soil has been demonstrated in experiments with 

 radioactive isotopes. Hoagland (1944) describes an experiment of 

 Martin and Overstreet in which radioactive rubidium ions were 

 added to the soil under investigation, and the soil was then leached 

 with a solution of a calcium saU to remove radioactivity from the 

 soil solution and from some of the sites at which rubidium ions were 

 held in an exchangeable condition. Subsequently barley plants were 



