no Mineral Nutrition of Plants 



sium acid carbonate in which the anion HCO a _ contained radioactive 

 carbon (C 11 ). Although the roots accumulated large quantities of 

 potassium they utilized only a small fraction, 4-5 per cent, of HC0 3 ~. 

 Jenny and Cowan (17, 18) grew soybean seedlings on pure calcium 

 clay suspensions having an initial pH of 6.30. At harvest time the 

 plants had removed from the clay 1.020 milliequivalents (m.e.) of 

 calcium. The reaction of the sol dropped to pH 4.32 and the clay 

 particles now contained 0.948 m.e. of hydrogen which they did not 



Root 



t~C0 2 +H 2 0-+H + HC0~ 3 



- Na 



K + HCO\ 



Figure i. Conventional model of liberation of ad- 

 sorbed ions by roots (C0 2 -theory). 



possess at the beginning of the experiment. While this observation is 

 in harmony with step 3 of the carbon dioxide theory, it is also in accord 

 with the contact theory. 



In this connection an experiment by Overstreet, Broyer, Isaacs, and 

 Delwiche (_?_?) is illuminating. These authors determined the uptake 

 of cations and anions by barley roots from potassium solutions and 

 potassium clay suspensions. They compared it with carbon dioxide 

 evolution and the synthesis of organic acids in the cell sap of the root. 

 The authors concluded "the excess accumulation of cations over anions 

 is roughly balanced by organic acid anions (other than HCO :{ ) which 

 are synthesized within the plant. Moreover, it is apparent that these 

 synthesized organic acids are the ultimate sources of the hydrogen 

 which replaces the adsorbed potassium on the clay, and not carbonic 

 acid." 



INADEQUACIES OF THE SOIL SOLUTION THEORIES OF PLANT NUTRITION 



In discussing his experiments on plant growth in synthetic ion ex- 

 change media, Arnon and Grossenbacher (j) concluded ". . . the data 



