PLANT ROOT-SOIL INTERACTIONS 



677 



ions, insofar as they contribute to activity, will react directly with the 

 root. 



LagerwerflF's ( 1956 ) experiments with Rb-clay suspensions con- 

 taining RbCl led him to conclude that the two-phase effect is nil, that 

 it has no plant physiological significance, and that the soil solution 

 (ultrafiltrate) completely characterizes the ionic environment of the 

 root. 



Lagerwerff's technique differed from that of Jenny and Overstreet 

 in the addition of RbCl to a Rb-clay suspension. Since salts rapidly 

 diffuse into the free space of the root, whereas clay particles can inter- 

 act only at the outer surface, solution ions are in a preferred condition 

 for metabolic absorption, and they may overshadow contact exchange 

 at relatively low concentrations. 



Scheuring and Overstreet (1961) have re-investigated the com- 

 petition for entry of solution ions and adsorbed ions. They mixed a 1 

 per cent Na-clay suspension with variable amounts of NaCl, up to ten 

 m. eq./L, and after long standing obtained a large amount of ultra- 

 filtrate for comparison with the unfiltered suspension. According to 

 Figure 5, a two-phase effect exists at low salt concentrations but dis- 

 appears at high ones. 



Vlamis (1953) examined by double-column technique a strongly 

 acid soil to which he added fertilizer. He could not observe a two- 



5 

 <u 



E 

 o 



8 '01 



o 



a. 



5 



Q. 



3 



o 



suspension — 



2.5 5.0 7.5 10.0 



Sodium concentration of filtrate, m.e. per liter 



Figure 5. Experiment of Scheuring and Overstreet on the effectiveness of 

 sodium-ion uptake by roots from mixtures of Na-clay -|- NaCl and from their 

 ultrafiltrates. 



