PLANT ROOT-SOIL INTERACTIONS 669 



or, more specifically: 



.COOH COOK 



R + K2SO4 t; — ^ R + H2SO4 



COOH ^COOK 



This reaction— also repeated with the organic cation methylene 

 blue ( M.R.) as exchangeable ion on the root— disclosed strong lyotropic 

 and valency series. M.B. was displaced most eflFectively by HCl, least 

 by LiCl. The series, typical of colloidal systems, reads: 



H>>Ba>Ca>Mg>>Cs>Rb>NH4>K>Na>Li 



Significant is the excess uptake of Ca over K, as high as 6:1. Deuel 

 and others ( 1953) report high Ca adsorption by pectin resins. 



The intensity of exchange per 100 grams of fresh roots was about 

 the same whether the reaction occurred at 25° C. or at 0° C. or whether 

 the roots were dead or alive. 



Clearly these reactions are non-mctaholic exchange reactions. That 

 this is so is confirmed by the reverse behavior of these ions in meta- 

 bolic uptake. Potassium is accumulated by roots in much larger quan- 

 tities than calcium. For radish seedlings Hassan and Overstreet ( 1952 ) 

 found a K:Ca ratio of 2.8. Their ionic series for metabolic uptake 

 reads as follows : 



Li>>Rb — K>Na>Si>Cs>Ca>Ba>Mg 



In contrast to non-metabolic exchange, the divalent ions are at the end 

 of the series. 



Epstein and Leggett ( 1954), using radioactive strontium, extended 

 the work on differentiation between non-metabolic exchange adsorp- 

 tion and metabolic accumulation. In particular they showed that ex- 

 changeable strontium in the root slowly becomes non-exchangeable— 

 that is, metabolically utilized. 



Generally speaking, ion-exchange phenomena and colloidal be- 

 havior are customarily approached from two entirely diflFerent view- 

 points. The first is the electric double-layer concept of Helmholtz and 

 Gouy, and its recent developments ( Booth, 1953 ) , according to which 

 the exchangeable ions form an ion swarm or ion cloud around the 



