Hans Jenny 129 



did sodium chloride, calcium chloride (0.08 N), or calcium hydroxide. 

 According to Klimenko and Syrokomskii (24), columbium precipitates 

 above pH 0.4. 



Whereas dilute acids and neutral salts are unable to release colum- 

 bium, contact will do so. The transfer is reversible; columbium will 

 move from fine clay to coarse Amberlite, and vice versa. During four 

 days of shaking of adsorbent mixtures the following amounts of colum- 

 bium were transferred from left to right: 



Cb-H-bentonite + H-Amberlite 8.6 per cent 



Cb-Na-bentonite -+- Na-Amberlite 5.8 per cent 



Cb-Ca-bentonite -f- Ca-Amberlite 1.7 per cent 



Cb-Na-Amberlite + Na-bentonite 6.0 per cent 



Cb-Rb-Amberlite -f- Rb-bentonite 10.4 per cent 



Time curves indicate that in these experiments equilibrium had not 

 yet been reached. Although the rate of transfer is slow, it is in the realm 

 of practical significance. 



Release of nonexchangeable potassium 



D. E. Williams prepared mixtures of Ramona soil and Ion-X ad- 

 sorbent. After one week of shaking the mixtures were leached with 

 ammonium acetate and potassium was determined in the leachate. 

 According to Table IV much larger amounts of nonexchangeable po- 

 tassium are liberated by contact with adsorbents than by leaching with 

 the corresponding neutral salts. The liberation of fixed potassium be- 

 comes especially noticeable when hydrogen-Ion-X or hydrogen-Amber- 

 lites are used. Conceivably the hydrogen ion swarm surrounding the 

 root likewise acts as an efficient releaser of nonexchangeable potassium. 

 It appears plausible that the contact effects involving columbium and 

 nonexchangeable potassium explain, partially at least, the root behavior 

 discussed on pages 114 and 115. 



EXTENT OF CONTACT EXCHANGE IN SOILS 



In any soil both solution and contact mechanisms will be operating. 

 As far as macronutrient cations are concerned, the soil solution mecha- 

 nism would be expected to predominate in sandy soils, whereas in clay 



