Hans Jenny in 



do not require the invoking of a contact exchange mechanism to ex- 

 plain the results obtained." Allaway (2) expressed a similar opinion. 

 Indeed, why should anyone wish to consider a new theory so long as 

 the old one appears satisfactory ? 



A body of data exists which casts doubt on the omnipotence of the 

 soil solution theory. We shall briefly discuss some of the more perplex- 

 ing situations. 



Uptake of ions by living roots from salt 

 solutions and clay suspensions 



For a period of 10-20 hours, excised, low-salt barley roots rapidly 

 accumulate ions. It is of interest to compare the uptake of cations by 

 roots from salt solutions and clay suspensions having equal concen- 

 trations of cations. According to Figure 2, the uptake of radioactive 

 sodium (Na # ), at higher concentrations, is decidedly greater in clay 

 suspensions than in chloride or bicarbonate solutions (20). This obser- 

 vation is confirmed with nonradioactive sodium (^5). In the case of 

 ammonium clay versus ammonium chloride, the uptake of ammonium 

 by the roots is nearly the same for the two systems. On the other hand, 

 potassium chloride provides a better source of potassium than potas- 

 sium clay. These last two experiments were conducted by Ayers (4) 

 who used the Hoagland technique (12). 



On the generally held assumption that ion uptake is a function of 

 activities, only the potassium systems behave somewhat according to 

 expectations. With the aid of clay membranes, Marshall (jo) has shown 

 that salt solutions possess higher activities than clay suspensions having 

 concentrations of cations equal to the salt solutions. Accordingly, salt 

 solutions should provide more efficient nutrient media. It is not known 

 how far the experiments with barley roots can be generalized. Suffice 

 it to emphasize that the uptake of cations from clay suspensions bears 

 no simple relationship to ion accumulation from salt solutions having 

 corresponding cation contents. 



Significance of type of clay mineral in nutrient uptake by plants 



Several years ago Elgabaly, Jenny, and Overstreet (8) observed 

 that under comparable conditions barley roots accumulate more potas- 



