122 



Mineral Nutrition of Plants 



EXAMINATION OF IONIC ENVIRONMENT OF ROOTS 

 AND THE MEASURING OF CONTACT UPTAKE 



In a soil pore filled with water the liquid phase contains dissolved 

 electrolyte (HoCO :; , Ca(NO.;) 2 , K 2 S0 4 , etc.) and, near the surface of 

 the solid phase, the swarm of adsorbed ions. Figure 6 schematically de- 

 picts a root segment — stripped of its electrical properties for the sim- 

 plification of drawing — immersed in a region consisting of a potassium 



Root segment 



^®©^ ® © 

 ® © © 



© ® ® ® © 



® 



^®\r ® (ci 

 ® ® ® m 



k) ®\ ■ 



\-Root segment 



Figure 6. Root segment immersed in an ionic environ- 

 ment consisting of ion swarm K (light circles) and KC1 

 (heavy circles). The ions are meant to he on the outside 

 of the root segment. 



ion swarm and dissolved potassium chloride. According to the contact 

 theory both forms of potassium are available to the plant, in fact, the 

 root would not be able to distinguish between the two potassium spe- 

 cies. The classic soil solution theory, on the other hand, would not 

 recognize the potassium of the ion swarm as a direct source of potas- 

 sium. As Mattson (_?/) recently put it: the plants feed directly upon 

 X K (K of the free salt) and only indirectly upon Z K (K in combination 

 with clay) ; the latter must be replaced by another cation before it is 

 made available. 



To experimentally ascertain the reactivity of adsorbed potassium, it 

 is necessary to separate the potassium chloride solution from the ion 

 swarm by centrifugation or ultrafiltration. If root behavior in the origi- 



