ii2 Mineral Nutrition of Plants 



sium and zinc from montmorillonite clay suspensions than from 

 kaolinite clay suspensions. These observations are in striking contrast 

 to what one would expect to happen on the basis of the carbon dioxide 

 theory. Release of potassium and zinc from the two clay minerals by 

 water, carbonic acid, and hydrochloric acid is greater for kaolinite 

 than for montmorillonite. The soil solution or, more precisely, the 

 supernatant liquid of the kaolinite suspension always contains more 

 bases than the montmorillonite sol. Accordingly, nutrient uptake should 

 be favored by kaolinite and not by montmorillonite as is actually 

 observed. 



Experiments performed by Mehlich and Col well (?2) and by Allaway 

 (2) who grew cotton and soy bean plants for several weeks produced 

 results opposite to those of Elgabaly and his co-workers. Recently, how- 

 ever, Elgabaly and Wicklander (9) confirmed the previous findings, 

 showing that barley roots accumulate more calcium and sodium from 

 montmorillonite systems than from kaolinite systems. We must con- 

 clude that the experiments of Elgabaly et al. also are valid and that 

 for their systems and techniques the soil solution theory — with or with- 

 out the carbon dioxide mechanism — is incapable of explaining the 

 results. 



Contact depletion 



Living barley roots low in potassium tenaciously hold onto their 

 content of potassium. Even intensive leaching of the roots with 380 

 liters of distilled water will not remove measurable amounts of potas- 

 sium (21). Likewise, electrolyte solutions comprising sodium chloride, 

 hydrochloric acid, sodium bicarbonate, and ammonium bicarbonate 

 will not significantly affect the potassium status of the roots. On the 

 other hand, as will be discussed on page 121, hydrogen clay, calcium- 

 hydrogen clay, sodium clay, and ammonium clay very significantly 

 deplete the root system of potassium. 



More sensitive techniques involving radioactive potassium reveal 

 similar root behavior. While some of the radioactive isotope is released 

 by the plant to water and electrolyte solution, the outgo becomes es- 

 pecially pronounced when the roots are in contact with colloidal clay 

 particles. It should be kept in mind that these experiments deal with 



